MARINE BIOLOGICAL LABORATORY. Received June 27* 1939 Accession No. 50458 Given by Amer loan Book Co* Place, New York City *,t*flo book OP pamphlet is to be removed ttom the Iiab- oratopy tuithout the pepmission of the Trustees. BIOLOGY The Story of Living Things GEORGE WILLIAM HUNTER Lecturer in Methods of Science Teacliimj Department of Education Claremont Colleges HERRERT EUGENE WALTER Professor of Biology, Brown Unirersity GEORGE WILLIAiVI HUNTER, III Assistant Professor of Biology, Wesleyan Unirersity "^1 c AMERICAN BOOK COMPANY NEW YORK CINCINNATI BOSTON ATLANTA DALLAS CHICAGO SAN FRANCISCO Copyright, 1937, by AMERICAN BOOK COMPANY All rights reserved COLLEGE BIOLOGY, H. W. & H. W. P. 2 MADE IN U. S. A. This hook is gratefully dedicated to our wives, to whom much of the credit and none of the blame is due. 1>11EFACE Here are a few chips left over from the authors' workshop. First of all we do not pretend to ha\'e presented herein the last word in a field already overcrowded by worthy ri\'als. The "last word" has an undesirable mortuary connotation quite out of keeping in a book about living things. The authors have been teaching biology for a total of ninety-four academic years, in addition to over sixty seasons of strenuous service in summer field work with classes at marine and fresh-water labora- tories, and they can truthfully and enthusiastically say that they have enjoyed this experience. If what they would pass on to other students of biology appears from the table of contents to bear the familiar marks of old stuff, the reason is that it represents, in their minds at least, what remains after many years of trial and elimination at the hands of an army of different teachers and scholars. The fact that much material that has been worked over before it was retained does not necessarily prevent, it is hoped, some degree of freshness in its presentation. Any text- book, the authors hold, should be somewhat like a dish of uncracked nuts, accompanied by a good substantial nutcracker. It is desirable that the reader should have some of the fun of wielding the nutcracker, for no pedagogical cellophane can preserve nuts already shelled in an entirely fresh and satisfactory condition for a very long time. An inevitable handicap that the textbook method of presentation of any subject is bound to suffer, is the fact that between the covers of a book the whole banquet is set upon the table at once in a more or less complete array. It is the part of the instructor to break up the feast into courses and to serve them in digestible portions. Perhaps the method of suspense employed in magazine serials woidd furnish a better way of arriving at the desired end than presenting the matter all at once in l^ook form, since sufficient time shoukl always be pro- vided between the planting and harvesting of intellectual ideas to allow for unforced sprouting and growth. In the use of any textbook it is well to remember that the pages may be turned backward as well as forward, and that it is no crime either to skip or to reread. Every studious and effective reader, moreover, is wary about vi PREFACE accepting witliout question whatever he may come across in print, for even textbooks are often known to l)o incomplete and liable to error. Again, if the art of reading between the hnes has not been culti- vated, it does not greatly avail simply to scan the printed hnes themselves. Every opening that induces the reader to seek further should be gratefully prized. Goethe once said: "Wer nicht mit der Bewunderung anfangt, werdet nie in das innere Heiligthum eindringen." Wonder is truly the mother of wisdom, for once the capacity for wonder slips away, one is prone to become blase, imcomfortably sophisticated, and intellectually slothful. With this explanation of the way it is hoped that this book will be used, the authors unite in cordially inviting the reader to join them in exploring the following pages. ACKNOWLEDGMENTS The authors wish to make grateful acknowledgment to all who have aided them in the preparation of a college textbook in biology. In particular, mention should be made of the members of the Biology Department of Wesleyan University who so willingly collaborated in trying out the ecological approach to a study of general biology for several years prior to publication of this book. Grateful acknowledg- ment is also made to them for innumerable suggestions as well as for their willingness to include certain successful features of the course in the text. Their help and advice has frequently been sought and willingly given. Special mention should also be made of the tireless effort and willing help of Wanda S. Hunter and Alice Hall Walter, both of whom read the manuscript and proof and contributed much to whatever success this book may attain. It is impossible here to enumerate all who have aided in the production of this book, but the following names must be men- tioned : Dr. E. C. Schneider, Shanklin Biological Laboratory, Wesleyan University, for reading the entire manuscript ; Dr. Francis R. Hunter, Rhode Island State College, for reading the entire proof ; Dr. Aurel O. Foster, Gorgas Memorial Laboratory, Panama, for reading section XII ; Dr. Hurbert B. Goodrich, Shanklin Biological Laboratory, Wesleyan University, for reading sections XIX-XXIII ; Dr. Frederick L. Hisaw, Biological Laboratories, Harvard University, for reading section XVIII ; Dr. John A. AIcGeoch, Psychological Laboratory, Wesleyan University, and Dr. Bernard C. Ewer, Depart- ment of Psychology, Pomona College, for reading section XVH ; Dr. Philip A. Munz, Department of Biology, Pomona College, for reading the botanical portions of the book ; Messrs. Emil Kotcher and Wilson C. Grant for aid in preparing the index. Acknowledgment is also made to organizations and individuals without whose co-operation it would have been impossible to secure many of the instructive and attractive illustrations. vn CONTENTS PAGE 1 NATURAL HISTORY CHAPTER I. The Stage Settin"g (Ecology) II. The Biological Conquest of the Would . . . 20 III. The Interdependence of Living Things — The Web of Life 44 IV. Roll Call (33 FUNDAMENTALS OF STRUCTURE AND FUNCTION V. Life and Protoplasm VL Cells and Tissues 125 138 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES VII. Beginnings: The Large Group of the Smallest Organlsms VHI. The Development of Sexuality in Plants IX. Division of Labor in Coelenterates X. Being a Worm XI. The Popular Insect Plan . NIL The Art of Parasitism XIII. Advantages of Being a Vertebrate 151 lf)8 17!i 187 199 .) 233 THE MAINTENANCE OF THE INDIVIDU.VL XIV. The Role of Green Plants XV. The Metabolic Machinery of Animals XVI. Support, Motion, and Sensation XVII. The Display of Energy XVIII. Chemical Regulators 237 274 32() 3C.4 390 THE MAINTENANCE OF SPECIES XIX. Reproduction and Life Cycles XX. The Great Relay Race IX 405 434 IS X CONTENTS THE CHANGING WORLD CHAPTER PAGL XXI. Time Spent (Palaeontology) 473 XXII. The Epic of Evolution 483 XXIII. That Animal, Man (Anthropology) .... 530 MAN AS A CONQUEROR XXIV. Man's Conquest of Nature ...... 567 XXV. Conservation and Its Meaning 589 XXVI. Man's Fight for Survival 608 XXVII. The Next Million Years 637 Index 645 NATURM. IIISToin T THE STAGE SETTI\(; (ECOLOGY) Preview. Ecology of a typical region • How to study ocolog>' • IMaiit and animal associations • Basic environments : water as a factor ; tempera- ture ; light as a factor ; chemical factors ; gravity as a factor ; substratum ; molar agencies ; biotic factors • Life in the water • Life in the air • Life on land • Suggested readings. PREVIEW "My heart is fixed firm and stable in the belief that ultimately the sun- shine and the summer, the flowers and the azure sky, shall Ijecome, as it were, interwoven into man's existence. He shall take from all their beauty and enjoy their glory." — Richard Jefferies : The Life of the Fields. There is a lure in knowing something intimate about i)lant and animal neighbors, their habits and the places where thoy live. A trout fisherman finds almost as keen enjoyment in watching a king- fisher make its catch as in having a trout take his own fly. Flic banks, meadow\s, and woods along a trout stream are aliv'e with interesting plants and animals. Even a slight acquaintance with what may be expected along the path makes a hike througii the forest and field immensely more worth while. An early morning walk, if one knows a few permanent bird residents and can recognize a migrant here and there, takes on an absorbing interest for tlie observer. Such trips in the open are eventful experiences, the joy of which is not easily forgotten. One may see the beauty of living things, and enjoy the songs of birds and the gay colors of insects, or get a thrill out of the sight of the first violet or bluebird, as he drinks in the sweet odors of the flowery meadow. From the standpoint of the more observant, another side than passive enjoyment of nature is to be found. It is discovered in asking and trying to answer the how and why of life aroimd us. Charles Elton has called the science of ecology "scientific natural history." This deals with the occurrences and behavior of organ- isms in a given habitat or home. Anyone who feels a genuine response to the call from the natural environment surrounding him cammf 1 2 NATURAL HISTORY fail to find an interest in this approach to natural history. Why, for example, do eertaiii kinds of animals live in the swift water of trout streams, while different ones are associated with plants in a quiet pond? Why are the types of life found along the seashore so unlike those around the edge of an inland lake? Why do forest trees grow tall in the dense woodland, more spreading in the open, and stunted near the tops of mountains? These and hundreds of like questions can be answered truthfully with the background afforded by the science of ecology. Ecology of a Typical Region ^ New England scenery is characterized by rounded granite hills, often heavily wooded with second or even third growth. In the hollows surrounded by these hills nestle little lakes, bodies of water varying in area from a few hundred square feet of surface to many scores or even square miles in extent. A survey of the inhabitants of one of these smaller lakes, chosen as a typical example, reveals relatively few fish and fewer plants in the open water. Nearer shore are found unmistakable zoning of plants and animals, depending on whether the shore is rocky, sandy, or muddy. In sheltered bays having a bottom of soft mud are found numbers of pond lilies and other aquatic plants, which give shelter to pickerel, bass, and smaller fish, as well as a vast array of small crustaceans, insect larvae, and microscopic plants and animals. Part of the lake shore is a sandy beach, at one end of which a slug- gish stream, after meandering through a meadow, empties into the lake. This constitutes quite a typical environment and will yield abundant material if searched carefully. The edge of the lake bordering on the beach contains relatively few plants and animals. It is exposed to the wind and consequently to wavelets which cause more or less movement of the loose sand, thus giving slight protection to living things. We find here almost no ' BOOKS USEFUL FOR FIELD WORK Downing, Our Living World, Longmans, Green, 1924. Johnson and Snook, Seashore Animals of the Pacific Coast, Maemillan, 1927. Lntz, Field Book of Insects, Putnam, 1921. Mann and Hastings, Out of Doors, Holt, 19.32. Morgan, Field Book of Ponds and Streams, Putnam, 19.30. Needhani and Needham, Guide to the Study of Fresh Water Biology, 3rd ed., Comstock Publ. Co., 193.-). Weaver and Clement, Plant Ecology, McGraw-Hill, 1929. THE STAGE SETTING ./. N lOinkin. Jr. A slow-ilowing stream presents a habitat for characteristic plants ami animals adapted to this type of environment. Head i)afjes ."5-1. plants and only occasional bass, pickerel, or minnows. A few dragonfly nymphs live under the small stones in shallow water, while ninnerous snails (Campeloma) are foimd buried in the sand or crawl- ing along the bottom.^ It is possible to collect a few specimens of plankton, which consists of minute free-swimming or floating organ- isms, but, on the whole, it is a relatively inhospitable environment inhabited by comparatively few organisms. Within a few yards of this beach the stream flows gently over a shallow sandbar, flanked by cattails and rushes. Here are nimierous representatives of several groups of plants : in the water a \'ariety of algae, Spirogyra (pond scum), streaming filaments of Ocdogonium, Oscillatoria, and Cladophora, and iimiunerable unicellular organisms, such as desmids and diatoms. Water cress, water plantain, water smart-weed, and burr-weed grow along the banks, while in sheltered bays the surface of the water may be covered with duckwec'd or per- haps yellow and white water lilies. H(>re and there in boggy i)I;ic(>s are dense masses of cattails, yellow flowering rushes, and numerons sedges, while on the banks are fotmd grasses of se\'eral species. 'It is expected that the student will make free use of IV, ' Roll Call." for cencral idcntiBcation and of the books of reference noted for more intimate and exact classification. 4 NATURAL IIISTOHY buttercups, Jack-in-the-pulpit, bog arrow-grass, and a few shrubs such as button-bush and willow. The vegetation shows a zonal arrangement of, first, submerged or floating water plants, then emer- gent forms, growing in the water and along the banks, while other plants such as grasses and shrubs are found at a little distance from the water. This zonal distribution is characteristic of shore associa- tions of plants and animals. In the slow-flowing stream live two species of sunfish, two or three species of pickerel, bass, three species of frogs, bullfrogs, green frogs, and pickerel frogs with their tadpoles, also an occasional painted turtle and water snake. Of birds, the redwing blackbirds are numer- ous, with occasional kingfishers, and more rarely a great blue heron. Although no mammals are in sight, a telltale mound of sticks shows that muskrats live there. Of the smaller organisms, the nymphs and larvae of the dragonfly and Mayfly are the most abundant. The water swarms with two species of water bugs and diving beetles, while beetle larvae and the larvae of mosquitoes are numerous. Many crustaceans, tiny amphipods and isopods, may be seen swimming or feeding on the aquatic plants. The snails, Physa and Lymnaea, are very abundant, while a few aquatic worms, Tuhifex, may be found in the mud. Colonies of bryozoans may also be found, incrusting the stems of water plants, as well as an occasional mass of fresh-water sponge. These two regions, the lake shore and the stream, although only a few yards apart, present tremendous differences in populations. Why these differences? At first sight, one might say it was due entirely to abundance of food, but this is only begging the question. Evidently many factors are at work. The fauna and flora of other localities visited would show even greater changes. Across the meadow and up into the nearby woods each locality would be found to be inhabited by groups of living plants and animals differing in many respects from those in neighboring localities. In each of these localities there would be certain dominant organisms better fitted than any others to live there. These become permanent species in that locality. How to Study Ecology To understand much al)out ecology, one must be able to do much more than simply study a book. The place to study the stage setting is the stage. The place to learn about the relation of living things THE STM^E SKTTINd to their cnviroiuncnt is the luihitat. Kltoii ' in his iiiteivstiiijr intro- duction to ecology cU^scribes the attack on a ceilain ccolofrical prob- lem in these words : "Suppose one is studying the factors limiting the distribution of animals living in an estuary. One would need to know amongst other things what the tides were (but not the theories as to how and why they occur in a par- ticular way) ; the chemical composition of the water and how to estimate the chloride content (but not tlie reasons why silver nitrate precipitates sodium chloride) ; how the rainfall at different times of the year affected the muddiness of the water; something about the physiology of sulphur bacteria which prevent animals from living in certain parts of the estuary ; the names of common plants growing in salt-marshes ; sometliing about the periodicity of droughts (but not the reasons for their occurrence). One would also have to learn how to talk politely to a fisherman or to the man who catches prawns, how to stalk a bird witli field-glasses, and possibly how to drive a car or sail a boat. Knowing all these things, and a great deal more, the main part of one's work would still be the observation and coUeo tion of animals with a view to finding out their distribution and habits." This gives us our approach. Our own interests, our reading, and the time involved must largely determine the extent to whicii we solve the ecological problems of our own environment. Plant and Animal Associations In making an ecological study of living communities we notice that one kind of plant or one kind of animal is never found li^-ing entirely alone. Plants, for example, are associated together by lack or abundance of water ; those living under abundant water conditions being called hydrophytes ; those associated in a condition of moderate Water lilies, catta (•haracteristi( hulriislu's pliyle."*. 1 From Elton, Charles. Animal Ecology, p. 35. By permission of The Macmillan Company. publishers. NATllRAr. HISTORY Typical xerophytic plants of the desert areas. Hau'oTtIt water supply, mesophytes ; and those which associate in desert condi- tions, xewphytes. Animals which live in the water are said to be aquatic, those on land terrestrial, while those that live both on land and in water are called amphibious. Animals and plants associated in still water are quite different from those in running water, while different types of plants and animals are found close to shore, in deep water, in rapid water, on rocky shores or on sandy shores, in salt or in fresh water, and in tidal pools or on the sand. Everywhere we find dif- ferent associations of plants and animals. Many explanations are given, but no one explains everything. One investigator, Merriman, emphasizes temperature as an all-important factor ; Walker gives atmospheric pressure ; Heilprin, food ; and Shelford, in recent experi- ments, indicates that the conditions under which an animal breeds may greatly influence its distribution. He experimented with tiger beetles, using different soils such as clay, clay and humus, humus, humus and sand, and pure sand. The beetles lay their eggs only in moist soil, therefore this factor was constant with all the soils. In this experiment the soils were also placed at a level and on slants. THE STAGE SETTING Eighty per cent of all the eggs were laid in steep elay, and <)S per cent in sloping soil. Thus ho concludes that the egg-laying hahits of these beetles determine tiieir habi- tat, for if they could not get the kind of soil and the slope needed, they would not breed. In this case the fluctuation and distribution of a spe- cies would be dependent upon a single factor. This may be true in the dis- tribution of a great many plants and animals. Basic Environments There are three states of matter, gas, liquid, and solid. These are evi- dent in the land, the water, and the air in which living things are found. Life is only found in conditions where it is at least partially fitted or adapted to live. These conditions, called factors of the en\iroinnent. are air or its contained gases; water or moisture; temperature; light; chemical constituents in soil, water, or foods; gravity; the presence of a substratum on which the organism rests, such as soil, moving objects in the water, or the sea bottom ; molar agencies, such as wind, water currents, or any moving force in the environment ; and finally, biotic factors which come through the interaction of other organisms in the same environment. A birch forest is composed of typical me.sophyte.s. Water as a Factor Water is absolutely essential to life, from 40 to 95 per cent of all living things being formed of this substance. It is generally true that no growth or life process of either plants or animals can take place without water. An example of this relationship of moisture to life is shown in the story of the British Mu.seum snail related by Mr. Baird.' " On the 25th of March 1846 two specimens of Helix desertorum, colloc-ted by Charles Lamb, Esq., in Egypt some time previously, were fixed ui>on tablets and placed in the collection among the other ^h)llusca of the .Musmnn. There they remained fast gummed to the tal)let. About the loth of .Marcli 1850, having occasion to examine some shells in the same ca.se, Mr. Il'iird 1 Ann. Mag. Nat. Hisl. (2) vi. (1850). p. 68. H. W. H, — 2 NATURAL HISTORY w|i»4«jiK. '-t**-». WiiylU J'itrcc These photographs were taken from the same spot on the Mohave desert floor. The upper was made at the end of the rainy season, the lower about two months later. What one factor causes this difference.!^ noticed a recently formed epiphragm over the mouth of one of these snails. On removing the snails from the tablet and placing them in tepid water, one of them came out of its shell, and the next day ate some cabbage leaf. A month or two afterwards it began repairing the lip of its shell, which was broken when it was first affixed to the tablet." THE STAGE SETTING ., The uses to which water is put by an organism are nianit'old. It is necessary as a solvent for foods within the body. In HvIiik tissues it becomes a medium of exchange between different parts of tlio body, while in higher animals it carries off body heat, thus helping in tiic regulation of their temperature. In air it causes humidity. In soil it carries the raw food materials of green plants. In many alkali lakes, such as Great Salt Lake, fish life is practically absent and the numbers of insects and crustaceans inhabiting such water are greatly reduced because of the high mineral content of the water. On the other hand certain crustaceans, such as the brine shrimps, are only found in water containing a high concentration of salts. Acid lakes and streams contain only certain types of fish, and according to in- vestigation by Jewell ^ are lacking in snails, possibly because of the absence of lime from which snails build their shells. Temperature Differences in climate (which after all are largely differences in temperature and water supply) are accompanied by changes in the appearance and kinds, of plants and animals. The life processes of organisms proceed between certain maximum and minimum limits of temperature. Somewhere between these is an optimum temi^era- ture at which the life processes function most normally. In i)lants optimum temperatures vary greatly for different species, and are largely instrumental in determining what plants will grow in a gi\-cn locality. For example, apple-raising regions must have a mean summer temperature of not more than 70° F. The optimum of most tropical plants ranges over 90° F., while alpine species require a temperature slightly above freezing. The temperature of plants changes rapidly, depending on the amount of external heat they re- ceive. This has an important bearing on horticulture. Lemons on the trees, for example, freeze at a temperature of 28° F., and oranges at 26° F. They are often kept from freezing by means of heaters. Plant injuries caused by freezing are due to the rapid withdrawal of water from the soft parts, therefore plants with a high water con- tent are more easily injured. This accounts for the freezing of the young tips of trees. Seeds which have a small water conteut are capable of withstanding very low temi)eratures. In animals, as in plants, the lif(> processes proceed best at oi)timuiii temperatures which differ with the species. Mast p rotozo a divide 1 Jewell, •• The Fishes of an Acid Lake." Tran.. Amer. M \ol. XLIII, 1924. pp. 77-84. 10 NATURAL HISTORY ^g» 4 *"■ ■ ■>^---''^'>j*S3B||)iiN^:'- Hk nil wl^^^^^^^H y^^: (,?) ir. L. Macchtlin During the freezing weather in January, 1937, in California, citrus groves which were adequately protected by heaters lost relatively little fruit, while many unprotected groves suffered a complete loss of fruit as well as some trees. much more rapidly at warm than at cold temperatures, and this is true of the reproduction of many animals. Many tropical animals may withstand cold temperatures, but will not propagate at those tem- peratures. H. B. Ward ^ has made observations on the sockeye salmon which indicate that these fish in swimming up rivers to spawn always take the river of slightly cooler temperature, a difference of 1° F. being sufficient to divert the fish. Seasonal cycles of activity are largely influenced by temperature, this being particularly true of reproductive activity, which plays a part in the migrations of birds, the rapid multiplication of plankton and other forms. Some animals respond to a cold temperature by going into a resting state or hiberna- tion, while others go into a dormant condition because of unfavorable conditions of heat and dryness. This latter state, aestivation, is often seen in regions having marked periods of alternating rain and drought. I Ward, H. B. " Some Responses of Sockeye Salmon to Environmental Influences during Fresh- water Migration." Ann. and Mag. of Nat. Hist., Vol. VI, pp. 18-36. THE STAGE SETTING II Animals are said to be warm-blooded or cold-blooded. The foriiK r term means that they have a constant body temi)erature {honwio- thermal), while the latter means that the body temperature varies with the external temperature {poikilothcnnal) . Frogs can often be frozen stiff and, when thawed out gradually, will live. This is true of many animals and is an undoubted adaptation which enables them to withstand great cold. Homoiothermal animals, however, are more or less independent of the external temperature because their internal body heat remains at a constant temperature regard- less of outside fluctuations. Animals are divided into two groups depending on whether they can easily stand changes in external temperature, some being restricted to a relatively narrow range of temperature changes {steno- thermaV), while others have not only the ability to withstand a large range of temperature, but also may become acclimated to new tem- perature ranges if they are changed gradually from one environment to another {eur y thermal) . A classic series of experiments by Dallinger with protozoans showed that he could change their li\'ing conditions from 15.6° to 70° C. without having the animals die. It is this ability that gives us the plant and animal populations in some hot springs. Light as a Factor Light is a form of radiant energy. Passed througli a prism it is broken up into the primary colors of the spectrum, each of which has Left : A nasturtium plant exposed to ordinary Kreeniu...se lif^hl sin.e ^.vnuuMum. Right: Same plant exposed to onr-sideddlunnnal.on for .SIX hours. I^ettuce. a long-day plant. Salvia, a short-day plant. These series of plants were grown experimentally at the Boyce Thompson Institute for Plant Research, Yonkers, N. Y. 12 THE STAGE SETTING i:i a different wave length. In addition there is the non-visible radiant energy of the ultra-spectrum. These different wave lengths ha\c various effects on plants and animals. Chlorophyll, the gnnMi color- ing matter of plants, which depends on the presence of light, absorbs light waves only from the red and blue bands of the spectrum. Whereas most of the radiant energy absorbed by a plant changes to heat, a very small part of it, estimated at not more than 0.5 per cent to 3 per cent, is used by the chlorophyll in the process of starch making. As in the case of temperature, optimum light is necessary for the best work of plants, some preferring shade and others living at their best in bright sunlight. Light causes movements in leaves and stems as well as changes in the size and shape of these organs. Plants respond to light, tlie leaves being placed so as to get the most light possible. The amount of light largely determines the shape of the entire plant, trees in a thick forest having a very different shape from similar trees in the open. The length of daylight has an effect on plants. Some plants, like the radish, spinach, and clover, require a long day to produce flowers and fruit, while fall flowers, such as cosmos, dahlia, and ragweed, require a short day in order to form flowers and fruit. It has been shown experimentally that for each species there appears to be a most favorable length of day for flowering, fruiting, tuber formation, and other food-storing activities. This discovery is of great value to agriculturists. 60 50 40 30 V 20 10 Legend Diaptomus Lake Eaton Holopfdium 8 II riiilil I'll rcc Can you explain THE STAGE SETTING Fishing boats at the mouth of the Klamath River in northern Cahfornia. Salmon run in on the outgoing tide apparently in response to the fresh water coming out through the narrow mouth of the river. Chemical Factors Under this heading are inckided all of the chemical factors in the environment of living things. Such are soil, rocks, and the various salts and chemical substances found in food and water. Experi- mental evidence shows that certain mineral substances are needed for plant growth, and that these minerals are found in the composi- tion of living matter. Alkali soils form a great problem of agriculture. In sixteen west- ern states this is the greatest problem outside of the water supply. In thirteen irrigated states there is enough alkali present to be harm- ful to crops. Alkalies are chiefly harmful because their presence causes the soil water to become permeated with these salts, thus hindering absorption of water by the plant. Acidity of the soil is another problem for the agriculturist. It is produced by a number of factors, such as the removal of calciinn from the soil, or the production of acids by certain bacteria or from decom- position. Acid affects the plant growth by checking the multii)lica- tion of useful bacteria and keeps earthworms and other useful animalN out of the soil. However, some species of plants demand aciti soils. Mountain laurel, rhododendron, blueberries, and cranberries are examples, as are sphagnum mosses found in certain bogs. 16 NATURAL HISTORY The distribution of fishes and other organisms in water depends largely on whether these waters are neutral, acid, or alkaline. Brook trout, for example, are usually found in acid and neutral waters, while sunfish, bass, perch, and certain other fish are typically asso- ciated with alkaline waters. Carbon dioxide in the atmosphere is another factor which deter- mines plant distribution, three parts to 10,000 being necessary if plants are to make starch. Oxygen is essential for living things. Certain so-called anaerobic bacteria and a few animals appear to be able to live without oxygen. Some insect larvae, worms, and molluscs live a part of the year in deep lakes where little or no free oxygen is present, due to decomposition of the algae. Certainly one factor in the distribution of aquatic animals appears to be the oxygen content of the water. Gravity as a Factor The pull w^e call gravity brings about differences in pressure both of air and of water. Plants and animals must adjust themselves to this factor. In a general way gravity determines the size of organisms. Insects and birds which move about swiftly in the air must be small, otherwise gravity would bring them down. Gravity is important in the growth and orienta- tion of plants. It is a stimulus for the direc- tion taken by the plant body, apparently caus- ing the root to grow downward and the stem to grow upward, while horizontal branches are neutral to the pull of gravity. This same force acts upon sessile or rooted animals, such as hydroids and sponges. Adaptations to offset the force of gravity are seen in the air spaces of floating plants, oil drops in eggs, spines and long hairs on the surfaces of aquatic plants and animals, and the air spaces in bones and other tissues of birds, and in the construction of feathers. Successive positions, from photographs, showing effect of gravity on a green plant {Impatiens glandii- ligera). — After Pfeffer. THE STAGE SETT1\(; ' 2^ '• r^' ij^ f^ 1 .l/M/irs Cypress trees have become adapted to live in swampy lands by developing buttressed bases of the trunks and erect growths (knees) from the roots. Tliese enable the tree to get sufficient air. Substratum Anything in which a plant grows or on which an animal comes to rest is known as substratum. Types of soil differ from cold, dense, clayey soils, which though they hold water do not readily give it up to humus that is well aerated, has a high nitrogen content, hokls water, and gives it up readily. The distribution of plants depends to a considerable extent on the kind of soil found in a given locality. For example, mosses and ferns grow in moist soil, while cacti are found in sandy desert soils. Varying soil temperatures are brought about by the kind of soil, whether coarse or fine ; by the pre.'^ence of a blanket of living things over it ; by its color (dark soils absorb heat more readily than light-colored soils) ; and by the water it will hold (wet soils are cooler than dry). Great variations occur in the air content of soils and this again determines the plants and animals found in a given area. Water-soaked soil, for examj^le, contains practically no air and does not ordinarily have a large jjiant or animal population. In some cases a plant adapts itself to water- soaked soil, as seen in th(> bald cypress. 18 NATURAL HISTORY Animals also differ with different types of soil. This is particularly true of the bottoms of lakes or streams. A different fauna is found on the rocky stream bed from the soft mud of the pool below. Mud contains more food, but it is also more difficult for organisms living in it to carry on respiration. Soil is also the home of such burrowing animals as nematode worms, earthworms, ants, beetles, digger wasps, and the larvae of various insects. Molar Agencies Such are any moving agencies. Running water and winds erode ; ice moves soil and rocks. Tides cause great differences in aggrega- tions of plant and animal life, animals living between tides having different problems to face from those below the tidal flow. Moving air has a definite effect on vegetation, as is often seen in the wind- blown trees on mountainsides or plains. Moving air acts upon seeds, tumbleweeds, spores, and fruits, thus spreading plants over vast American Museum uf Xatural History Tidal shores, along the New England coast, show wide variations in habitat. The flora and fauna of the intertidal zone differs greatly from that of the regions above and below the tidal flow. TUE STAGE SETTING 1«» \y ritjlil /'it re, The ell'ect of differences in environiiient upon the same s[)e(ies of tree (Piiins ponderosa). Here molar agencies are largely responsible for Ihi; changed ai)pear- ance of the tree. areas, but it also fells much timber, breaks off branches, and destroys crops. Winds may either help or hinder in the migration of insects. The cotton boll weevil travels north more rapidly in the years when more wind is recorded. Winds blow birds and insects out to sea, thus destroying them, or they may land them in a new location where they may multiply rapidly. Currents of air as well as water currents distribute plants and animals. Many animal forms react to wind and water currents. Fish head upstream, an adaptation favorable to food-getting. The swiftness of the current not only determines tiic distribution of fishes, but also of other forms, such as caddis fly larvae and "water pennies." Biotic Factors These are factors arising from the presence of other li^•ing organ- isms. One is concerned when studying ecology not only with the environment of living things but also with how li\ing things react on others in their immediate environment. There is competition not only between plants and animals, but al.so between plants of the 20 NATURAL HISTORY same and of different species for a place under tlie sun ; literallj'- under the sun, for competition is caused by the limited amount of light that will fall in a given area. Young plants often die because of the shad- ing by the parent plant, and larger plants preempt areas of soil which give little or no space for young growth. Feeding by animals, such Effect of sheep grazing upon trees. Thousands of young trees are destroyed every year in this way. as rabbits or sheep, may change the entire flora of a region, while parasitic organisms injure a vast number of the hosts on which they live. There are also marked cases of partnership between organisms, bacteria in the soil giving and taking from both plants and animals, and helping to create a cycle of food substances which pass through the bodies of both animals and plants. The feeding of animals is their biggest business in life, and the presence of a food supply determines very largely the presence of animals in a given locality. It is said that the oak tree serves as food for over 500 species of insects, the apple for 400, clover and corn, over 200 each. Thus man with his tilling of the soil, destruction of the forest, and domestication of plants and animals has changed the fauna and flora of the land. Having discussed the effects of the factors of the environment on living (jrganisms, let us now see how the interaction of these factors affects life in the situations that living things are forced to meet. Animals and plants must be adapted to live either in the water, in air, or on land. The pages that follow show some of these adaptations. THE STAGE SETTING 21 Life in the Water Plants are adapted for lite in water hy a mucli reduced root system, by leaves which either float, are ribbonlike, or are finely divided with air passages and air spaces. The latter spaces help buoy up the plant and also allow for an accumulation of oxyu-en and carbon (hoxidc Green coloring matter is abundant, such plants being better fitted for vegetative propagation than reproduction by flowers and fruits, as is shown by their numerous horizontal and thickened stems. In general, aquatic plants are restricted to relatively shallow water, many species being found floating near the surface. Animals, usually locomotor and having definite adaptations for movement in the water, have a much wider ^•ertical range. The bodies of most fishes are more or less streamlined, and protected by mucus which covers the backward-pointing scales, their fins being placed where they offer the least possible resistance to the medium. In some animals, the limbs are transformed into flii)pers, while in lower types, such as protozoa, threads of living matter, cilia, are used as whiplike organs of locomotion. Since the oxygen content of water is only about 1 per cent as against over 20 per cent in air, we find special adaptations for taking in oxygen. These structures are usually in the form of gills, delicate struc- tures which will be discussed more fully later. The water forms an ideal medium for vast numbers of small, free-swimming, or float- ing organisms, the plankton. Oceans and lakes swarm with them. Every small pool has its plankton, and even rapidly flowing waters will disclose some of these tiny organisms. In certain tested regions in the Atlantic, plants form about 56 per cent and animals 44 jx-r cent of the total plankton. The flora consists mostly of diatoms, bac- teria, and many forms of algae, while the fauna includes numerous dinoflagellates and other one-celled animals, eggs of fish, molluscs, Diatoms have various forms aFid may !><• colonial as well as unicellular. riicrc arc probably I. "),()()(» species known. 22 NATURAL HISTORY numerous crustaceans mostly copepods, jellyfish, and the larvae of many crustaceans, molluscs, and fish. Some of the plankton, such as small crustaceans, tunicates, medusae, small fishes, and larger algae, may be visible to the naked eye, but most of it is microscopic. LAKE PLACID UPPER SARANAC MIDDLE SARANAC LOWER SARANAC deptm\ g o CD < m t— Q- LJ >- CO C3 < .1- t— a. > -> a < a. Ui 5 < CO Q- BLUE GREEN ALGAE 1 M i_ _ ■ ■ BOTTOM ^_ GREEN ALGAE : 1 M „ ■ l_ BOT 1 OM ^^ ^ DIATOMS 1 M ■ ■ ^ ^ ■ ^ BOTTOM J_ Mi iV. y. Siaie Conseruation Dept. Comparison of the distribution of nannoplankton (minute forms that will pass through the meshes of a plankton net) at the surface and bottom of four Adirondack lakes. The larger pelagic organisms mostly found in the ocean, such as fish, squid, whales, turtles, and seals, are called collectively nekton. Currents, wind action, the shapes of bays and coasts, migrations of various animals, all cause differences in the horizontal distribution of plankton. Sometimes given forms, as Cladocera, will multiply very rapidly, even coloring the water in a large area. The vertical dis- tribution is much more regular with reference to plants, since algae and other green plants depend upon sunlight. Plants get very little light at a depth of 100 meters. At 75 meters' depth, only half as many plants are found as at 50 meters, and careful investigation in various areas shows that most of the plant plankton lives within a few feet of the surface. On the other hand, animals exist at great depths. Beebe reports jellyfish, shrimps, and other plankton at a depth of over 1000 feet and the tunicate, Salpa, as well as fishes, at his greatest depth of 3028 feet. Dredgings from the "Challenger" and other expeditions reveal many living organisms, particularly protozoans, in the abysmal depths. Till': sr\(;|.: si:rnN(; o., Towins with a phnikton i.cl (a llun-mcshcd uv\ cf l,oltii,K ,-l„tl,) near the surlacc of tlu> ocean on an early summer day would yield a very different distribution of organisms from those collected "on ;, fall or winter day. There is a seasonal variation in distrihutioii. The eggs and larvae of animals ar(> abundant in the spring and early summer, while great numbers of algae appear then which are Hot found later. This rhythm of plant life is believed to be correlated with a turn-over of the available phosphates and nitrates in the water. In the winter, the (^ooler top layer of water sinks and pushes up the water rich in the salts necessary for plant growth from underneath, so that with the coming of warmer weather tlu^ life cycle goes on and a seasonal rhythm of algae appears. This turnover of plant and animal life is very great. The fishing industry on the Grand Banks and in the North Sea is largely due to the occurrence of this great seasonal rhythm of plankton. There is also a considerable variation in the numbers of plankton near the surface of the water during the day and night. Many crus- taceans, for example, come to the surface at night and go down in the daytime, while green algae are usually nearer the surface during the day. In oceans and lakes, there is a more or less distinct zoning of living forms, depending on the depth of water, the type of shore, or the kind of bottom. A very different fauna and flora exist on a rocky coast from that along a sandy beach. The forms of botii plants and ani- mals are different in salt and fresh water areas. Life in the Air Here life is more circumscribed. There are no true air plants unless they be the so-called epiphytes of the tropical rain forest, some algae, such as the Pleurococcus found on the bark of trees, or the lichens, which encrust rocks and tree trunks. The reproducti\'e bodies of plants, such as spores, seeds, and fruits, are furnished with adai)ta- tions which enable them to pass long distances through the air, thus allowing new areas to be populated. In animals where locomotion is possible various special adaptations exist. Flying animals hiivv their wings placed wdiere they will not onlv cause the liody to mo\e forward, but also assist in balancing it. Instead of one ])ropeller placed astern, as in fish, flying animals have two paired i)roi)ellers placed forward at a greater breadth of beam. The body is not onl.\- streamlined, but in higher forms special adaptations exist for protec- H. w. H. — 3 21. NATURAL HISTORY Epiphytes in a semitropical forest. Note the aerial roots for securiiif? moisture from the air. tion against low temperatures and moisture. Oiled skin and feathers of birds are examples. Bones are hollow and large air spaces are found between muscles. In insects a special aerating system exists, since in these heavier-than-air machines a very rapid oxidation of fuel material must take place if the organism is to be efficient in the medium. Life on the Land Adaptations in plants for life on the land are seen in the widely branching root systems, the woody stiffened stems, the leaves placed in positions where light may reach them, and in the various adaptive movements which enable green plants to get a share of the much needed light. In tropical rain forests, this relation to light is seen in a vertical zoning where sun plants form long twining stems, making their way up the tall trimks of trees to an upper zone where light is available, while in the lower areas are found shade-loving plants which prefer less sunlight. In animals, where movement is much more evident, there are special adaptations in the form of legs, which support the body off the ground and allow of various types of loco- motion such as climbing, crawling, walking, running, and leaping. THE STAGE SETTLNG Various other types of movement are found as, for example, tlir waves of muscular contraction in the foot of the slug ; the crawling of earthworms where tiny setae are used as levers; the erawlinfr of the snake with its definite use of scales as "ground grippers" ; the adaptations for leaping in the grasshopper and the frog; adaptations for climbing, such as the sucking disks on tiie toes of tree frogs {Ilyln) and of some lizards, or the arrangement of the toes in climbing birds. These and scores of other adaptations for obtaining food, for brcatii- ing, and for protection may be recalled. SUGGESTIONS FOR FURTHER READLXG Borradaile, L. A., The Animal and Its Environment, Oxford University Press, London, 1923. A general book on the natural history of animals. Elton, C., Animal Ecology, The IVIacmillan Co., 1927. Chs. I, II, III, I\', \'. A fascinating book, written in a charming style. Accurate and authentic. Jordan, D. S., and Kellogg, V. L., Animal Life, D. Appleton tV: Co., 1900. Contains some valuable chapters fundamental to an understanding of ecology. Needham, J. C, and Lloyd, J. T., The Life of Inland Waters, Charles C. Thomas, 1930. Chs. Ill and V. Interesting aquatic natural history. Pearse, A. S., Animal Ecology, McGraw-Hill Book Co., 192G. Chs. II and III. Rather technical. Shelford, V. E., Animcil Communities in Temperate America, University of Chicago Press, 1913. A pioneer work, but still reliable and usable. Shelford, V. E., Laboratory and Field Ecology, The Williams & Wilkins Co., 1929. Very usable for field work. Weaver, J. E., and Clements, F. E., Plant Ecology, McGraw-Hill Book Co., 1929. Chs. IX, X, XI, XII, XIII, XV. Authentic and well written. It should be of great value in the field. II THE BIOLOGICAL CONQUEST OF THE WORLD Preview. A comparison of two forests • The why of distribution ; barriers ; successions and their causes ; overpopulation and its results • The shifting world of organisms • Ways of locomotion • Adaptability to new conditions • Human interference • Life zones • Life Realms • Sug- gested readings. PREVIEW The science of Ecology, or the distribution of animals and plants in a given habitat, was considered in the preceding section. Chorology attempts to determine the laws governing the distribution of animals and plants over the surface of the earth. So long as man accepted the naive assum]3tion that the earth was originally populated by means of isolated creative acts, there was no point in attempting to explain the distribution of living things. They had all been put arbitrarily in the places where they occurred, and that was all there was to it. With the rise of the belief which culminated in Darwin's famous theory, that dissimilar species have arisen by modification from other species, and that all organisms are related, more or less distantly, to one another, the interpretation of plant and animal distribution became a very interesting and challeng- ing field for study. How about the varied populations of living things in arctic, tem- perate, and torrid climates ; the absence of animals and plants from areas quite suited to their existence? Why is it that tapirs are found only in South America and the East Indies, while certain fishes, such as the pickerel, occur only in North America and north- em Europe ? Equally difficult aspects of distribution cropped out, notably in the Australian fauna and flora, which differ so greatly from that of the rest of the world, while most perplexing of all, probably, the habit of migration that makes certain animals, such as birds, seals, salmon, and eels, change residence regularly from one region to another. A gradual suspici(jn that two environments quite similar in general appearance might nevertheless be populated by species of plants and animals different from each other gave the clue 26 TfTE BIOLOGICAL CONQUEST OF Till: Would 27 to a scientific differoiitiution of specific distribution, AVWor///, Irom .aionoral distribution, Choroloc/!/. A Comparison of Two Forests Two writers, Victor E. Shclford, the well-known ecologist. and William Beebe, ornithologist and naturalist, have given two widely different pictures, one, an accurate description of a hard-wood forest in Illinois, and the other, a survey of life in a British Guiana jungle forest. A typical beech-maple forest, such as Dr. Shelford describes, can be found anywhere in the vicinity of Chicago. A.ssociated with the two dominant trees are ash, elm, walnut, linden, and a wealth of smaller trees and shrubs forming a lower layer under the higher trees. Wild cherry, sassafras, and dogwood are abundant, and in some of the more northern forests, azalea and rhododendron form an inter- mediate growth. The floor of the forest is covered with herbs and flowering plants, large and small, which change with the season. In spring, trilliums, violets, wild geraniums, anemones, phlox, and scores of other plants are in bloom, succeeded in the fall by asters and other composites, in areas having ample light. A relatively large number of plants having spiny or hooked fruits occur, which aid in their accidental distribution by wandering animals. A few large mammals, deer, fox, and hares, are found occa.sionally, though are rarely seen. The woodchuck is perhaps the mo.st numerous of the mammals, and the red, gray, and fox squirrels are not uncommon. Of birds the crested flycatcher, wood pewee, blue jay, scarlet tanager, wood thrush, and red-eyed vireo nest in the lower trees, while the oven-bird conceals its curious architecture on the ground. The wood frog, red-backed salamander, and Pickering's tree frog are found, although not always in evidence, and insects abound, especially those that live on trees, such as borers of various sorts, beetles, millipeds, spiders, and in.sect larvae. Inhabiting the lower layer of the forest are snails, centipedes, sowbugs, and earthworms. This represents, \\\\]\ \aria- tions, a typical association of life in a northern deciduous forest. At first sight the jungle forest does not appear to be very difTerent from the northern forest. Both contain large and small trees, the larger ones in the jungle, such as mora and greatheart. towering to a height of two hundred feet or more, but here the likeness stops. There is an almost complete absence of large horizontal branches in the tropical forest, the trunks of trees shooting straight up for si.xty ■m NATURAL HISTORY }\'iUiam Beebe American Museum of Xatural History CouTtestj U. S. Forest Serrice A comparison of two widely separated forests. The right-hand photograph is a typical northern mesophyte beech-maple association, the left-hand photograph a tropical rain forest of British Guiana. Note the superficial likenesses and dif- ferences. or seventy feet without a branch, festooned with long cHmbing hanas, which in this way work from the forest floor into the upper zones. Four general horizontal regions, or zones of life, are distinguishable, namely, the forest floor, the lower jungle up to about twenty feet, the mid-jungle up to seventy feet, and the tree-tops, towering a hundred and fifty or two himdred feet high. Life at first seems almost absent in the jungle to the casual observer, but if one stops, and simply looks, the jungle wakes up and life appears everyu^here. The forest floor is covered with the accumulated debris of ages, fallen trees in different stages of decay, fungi, mosses, and lichens, with a generous covering of brown leaves, for here the leaves fall all the year around, instead of only in the autumn season as in northern regions. The ground area is occupied by occasional deer, paca, and tapirs, with agoutis and armadillos found more frequently. Partridge and the strange tropical tinamou are seen here and there, as well as jungle mice and rats, salamanders, frogs, a few snakes, innumerable scorpions, beetles, grubs, worms, and rarely, the unique and interesting Peripatus. In the low jungle are found manikins of several species, ant-birds, with trumpeters and jungle-wrens, while at night opossums climb THE BiULUCilCAL CONQUEST OF Till-: Would u) about through the underbrush. During the daytime tiie wonderful morphous butterflies, brilhant spots of blue, add a touch of col.,!- to the picture. The mid-jungle contains the most life. Here iimumeraljlc birds, curassows, guans, pigeons, barbets, jacamars, trogons, and smaller feathered species abound, in company witli ant-eaters, sloths, squir- rels, bats, coatis, and small monkeys such as marmosets. The upper jungle of the tree-tops is the mo.st difficult region to know. Red howlers and be.som monkeys move about in the tree- tops, and occasional glimpses may be had of toucans, macaws, and great flocks of parakeets and parrots that live ther(\ Fierce ants prevent tree-climbing, and the relatively great height and mass of foliage make living things not easily acce.ssible to observers in this upper layer of the tropical rain forest. These two forests, the northern maple-birch and the jungle, by their entirely dissimilar populations illustrate contrasts that might be found in many parts of the world. Sometimes conditions in widely separated areas may be almost similar, with diverse populations inhabiting them, and again, localities close at hand may show remark- able diversities in their living inhabitants. When regions far apart have similar populations, which does not commonly happen, the biologist is faced by a puzzling problem. The Why of Distribution Jordan and Kellogg ^ give three laws to account for the distril)u- tion of organisms which they state as follows : E\ery species is found everywhere that conditions are suitable for it unless (I) it was unable to reach there in the first place, or (2) having reached there it was unable to stay because it could not adapt itself to the new condi- tions, or (3) having entered the new^ environment it became modified into another species. It is not only the normal habitat that deter- mines the presence of a given plant or animal, but its accessibility from the place of origin. Although every species originated historicaUy from some i)receding species at some definite place, its present distribution results from the working of two opposing factors, expansion and repression. The factors of expansion will be mentioned later, lliose of n^pression are, first, inadequate means of dispersal because slow-moving animals 1. Jordan, D. S.. and KellofiK. V. L., Animal Life. Appletou, 190(). 30 NATURAL HISTORY are necessarily limited in their distribution. A second means of repression lies in the poor adaptability of organisms to new localities which they have invaded. A round peg will not fit in a square hole, nor a square peg in a round hole, but if the peg consists of a plastic material it will adapt itself. The normal habitat for a species is the place where the organism is most nearly in physiological equilibrium, the geographic range being determined by the fluctuation of a factor, or factors, which are necessary for the life of a species. Barriers Each species widens its range of distribution as far as possible and tries to overcome obstacles which nature has put in its way. These obstacles may be chemico-physical, geographical, or biological bar- riers. In general chemico-physical barriers are climatic in nature, such as unfavorable conditions of moisture, soil, or temperature. Soil deficiencies, salinity, the presence or absence of light, or character of the surrounding medium might also be mentioned. These climatic Friislur Why might such a mountain barrier restrict the distribution of certain plants and animals .3 THE BIOLOGICAL CONQUEST OK TIIL WolU.O :U barriers may be in vertical zones, extending from tlie ocean level to mountain tops, as well as horizontal, spreading out north and south from the equator in zones of latitude. Map showing ancient and modern ranges of the elephants and their ancestors. The shaded area shows the former habitat of the maniniolh and mastodon, ant^estor of the modern elephant. A land connection probal)Iy existed I)etween Asia and North America. Note the restricted range of the present-day elephants indicated by heavy shading. How can this be accounted for.^ Sometimes natural barriers occur, such as high mountain ranges with eternal snow, deserts with unfavorable conditions of moisture, or in the case of water-distributed animals such as fishes, high water- falls may prevent them from moving up a stream beyond a cciiain point. The barrier for one organism, however, might l)e a highway for another. A desert would be an impassable barrier- to a squirn^l but not to a camel. Geographical barriers have not always been fixed. Geological history reveals the fact that some land surfaces were once occujiied by water and what is now water may have been land. The presence of fo.ssil sea.shells in the Panama Canal area indicates that the Isthmus was formerly submerged, and there is evidence that as late as Eocene times there was a land connection acro.ss Bering Straits. As bar- riers have changed so has the resulting distribution of organisms. Distribution often indicates the geography of the i)ast. .Mnnbers of the same genus may differ widely in certain isolated localities, as, for example, the tapirs found in tropical America and the Malay 32 NATURAL HISTORY Peninsula with its adjacent islands. In early geological times mem- bers of this genus were widespread and abundant, whereas now, due to the disappearance of former land connections, there are but two widely isolated species in existence. The distribution of animals is bound up in their food supply. Hence carnivorous animals are restricted to areas wiiere the animals on which they prey live. Often a biological barrier is created by the presence of animals which are parasitic on a given form. The tsetse fly, Glossina, which frequents the river bottoms and shores of lakes in certain parts of Africa, prevents the ranging of other than native cattle in these areas because of the fact that they transmit a blood parasite fatal to such animals. Man himself is most active in both creating and breaking down barriers. He introduces new animals and plants either purposely or by chance into areas where they thrive and replace other species, or by building dams, irrigating, deforesta- tion, or accidentally burning over areas, he destroys one kind of life perhaps never to replace it with another. Successions and Their Causes Succession means that in a given area organisms succeed one an- other because of changes in the environment, migration taking place so that they may reach conditions favorable to their development. An example of plant succession may be seen in almost any pond that is gradually drying up. In deep water there are a few submerged aquatic plants ; in water from 6 to 8 feet deep floating plants such as pond lilies are found ; in shallow water from 1 to 4 feet deep, cat- tails and reeds are abundant ; while at the edge we find a meadow of sedges and some bushy plants. As the pond becomes drier, these plants slowly push outward until eventually it may be completely filled with plants which build up soil, making first a swamp and eventually a meadow, while around the edge of the former pond will now be a forest of trees and bushes. In the tropical oceans different corals succeed each other, growing on the skeletons of other species, thus building their way into shallow and warmer water, or along the ocean shore colonial diatoms may occur, to be followed by hydroids and seaweeds, the latter becoming a dominant climax formation, a group of species that are better fitted to survive in that habitat than any others. Erosion, which carries away the original inhabitants, or a deposit of new soil by running water, wind, or other agencies, gives oppor- tunity for the establishment of new life in a region thus devastated. THE BI()LO(;iC,\L CONQUEST ()\- nil] \\(,|u,|) M\ The question of how long seeds will survive, uiidci- whal condilions they will germinate, and how fast they will grow is of g,vat inipor- tance in the repopulation of areas after soil erosion oi- fire. Beale reports an experiment where ten out of twenty-two species of seeds sprouted after hav- ing been buried in open bot- tles in moist sand at a depth of three feet for over forty years. After a coniferous for- est has been devastated by fire, an entirely new series of plants spring up in the area ; first herbs, such as fireweed or wild mustard ; then trees or bushes, the seeds of which may be brought by birds, as raspberry, blackberry, or wild cherry ; later a stage of trees having wind-blown or bird- carried seeds, such as aspen, cottonwoods, or birches. Still later the forest may become repeopled by its original in- habitants, which becomes the climax. Conditions of wind, mois- ture, sunlight, and weather, the sum total of which constitutes climate, play a most important part in succession. If drought destroys life in a given region, an entirely new group of plants may come to occujiy that area, bringing with them a new group of animals. Migrations of animals may be brought about by changing seasons. The biotic conditions governing successions are many. Man, through clearing forests, throwing wastes into ri\ers, or introducing new plants or animals which may compete with existing species, often completely upsets the balance of life and causes succe.'^sioiis. Indus- trial pollution may completely depopulate streams of fish life, bac- terial growth replacing the original plants and animals. Sometimes new organisms add so many competing mouths to feed in a gix'cn terri- tory that it becomes necessary for some to break away if any are to li\('. Wriijlu I'itrct A lypiciil undt'CKruwtti succession after a I'orcsl (ire. 34 NATURAL HISTORY Overpopulation and Its Results One of the I'aclors in dcteniiiiiiug tlie .spread and distribution of organisms is overpopulation. An annual plant, for example, pro- ducing only two seeds a year, which is far below the actual number, and always developing these into mature plants, in only twenty-one years would have 1,048,576 descendants. A pair of common house- flies which usually produces eggs six times a year, each batch con- taining 150 to 200 eggs, with the young flies beginning in turn to lay eggs in about fourteen days after hatching and repeating the life cycle, might, it is calculated, beginning to breed in April, if all the eggs were hatched and no individuals died, give rise to 191,010,000,- 000,000,000,000 descendants by the end of August. However, each species, year in and year out, tends to remain about stationary in number. Indeed, many species are actually disappearing. The reasons for this check of potential populations are found in lack of adequate food supply, lack of favorable breeding conditions, and in the fact that many animals and plants become food for others. The Shifting World of Organisms There is no doubt that desire for food furnishes the greatest urge to locomotion and exploration in animals. Dr. Crothers once said in one of his essays that the "haps and mis-haps of the hungry make up natural history." Indirectly there is the same necessity for food on the part of plants, but here the urge is expressed not so much in locomotion as in a struggle for position with reference to light, which is essential to every green plant in the manufacture of its own food. Changing environmental conditions may force the movements of organisms and produce faunal and floral repopulations. For example, it is known that drifting coconuts frequently float long distances and grow into trees upon some distant shore. A recent cataclysm of nature has given us an opportunity to see the repopulation of a devastated area taking place. In 1883, the volcanic island of Kra- katao was literally blown to pieces by a series of terrific explosions that destroyed every living thing on the island. Less than three years after the volcano became quiescent, a Dutch botanist visiting the island found the ash which covered its surface completely car- peted with a layer of bacteria, diatoms, and primitive blue-green algae. Here and there ferns were found, along with several kinds of mosses. There were even a few flowering plants, but no trees or shrubs. In THE BIOLOGICAL CONQUEST OF THE Would r, that short time, the naked land had been partially ropoijulatcd with these several low forms of life, by spores or seeds blown througli tho air, or floated in water from the nearest islands, wliich wore aboiil fifteen miles away. Twenty-three years after the explosion, I'rofessor Ernst visited Krakatao and reijorted a forest of cocoinit pahns and figs growing near the shore line, a luxuriant jungle in the interior, and considerable animal life, represented by species that could either fly or drift to the island on floating wood. Ernst estimated that within another fifty years this island would differ in no respect from its neighbors, a prediction, however, wliich seems doomed to failure of confirmation because the volcano has again gone on a rampage. Variations in temperature, brought about by the changing seasons, are a factor in the movements of animals. This is particularly true in the case of the annual migrations of such animals as crabs, lobsters, and squid, which go into deep water in winter, returning to shallow shore-water in spring. Movements apparently dependent to some extent upon temperature occur in the case of many marine fishes, and birds, certain butterflies, and bats, that go north and south according to the season. Many animals mo\T up and down mountain slojjes probably for the same reason. Sometimes other factors than scarcity of food, em'ironmental changes, or seasonal differences cause migration. I^emmings. for instance, small rodents living in the mountainous districts of Scan- dinavia, at intervals of from five to twenty years suddenly mo\'e forth in vast numbers, with no apparent Pied Pi])er of Hamelin to lead them, but always in the same general direction, swimming rivers and lakes, overcoming all sorts of obstacles, and eventually ending the mysterious trek in the ocean. Although they feed on the way and consume enormous amounts of food material, the search for food is not sufficient to explain their fatal pilgrimages. The relation of different degrees of salinity to th(> breeding habits of food-fishes jjrobably influences their distribution also, by det(>r- mining the character of organisms in their feeding grounds. Tetters- son found that herring only enter the Baltic when the .salinity g<'ts to a certain degree, whereas Galtsoff found that in America tli(> mi- gration of mackerel is due not so much to salinity as to temperature. Thus, different factors appear to influence different species in deter- mining their movements. Birds, becau.se of their ability to fly, are better aide to seek out :i favorable place for abode than most animals. Many tlifferent reasons 36 i; c ti ^ -=; s c "S ^ ^ 2;° y^ f o •^ "^i X ^ ^ O 3 A C TT ^ i; s *-. c« • ^ - J •w - ^ t£_ ^ •- X •*^ — > a ^ 3i «r .^^^ w ** > aj a __^' — _w A ^ =t: ^• u .^ • o = % s ,^ -£ 55 1^ ^ — s ^ 'lI — 7 * vT ^ r '/: ^ — 2 ^' ^ -^ — — -T 1^ — ;. c u ^ ■^ E, t£ 37 38 NATURAL HISTORY have been given to account for the long-distance migrations of ducks, geese, the Arctic tern, golden plover, and other remarkable feathered travelers. Food cannot be the deciding factor, jjecause many birds leave for the south while food is still abundant. Neither can tempera- ture be the only cause, because a majority of migrating birds go south when the weather is still warm, while robins and other lairds often stay behind and win- ter successfully in cold climates. Humidity, atmospheric pressure, winds, have all been considered as playing a part in migration, but it is more likely that some- thing within the bird rather than any external environmental factor is the impelling cause for this impressive phenomenon. For in- stance, among the hormones pro- duced by the ductless glands, are sex hormones which may stimu- late the bird to the extraordinary activity that results in long mi- gratory flights. How to account for the direction and exactness of these migratory flights is another matter, even more difficult to explain. Changing climatic conditions probably influence plants more directly than animals, because the latter are more capable of move- ment, and, consequently, better able to escape from unfavorable surroundings. Nevertheless, living things make up a world of shifting organisms, always on the move. Ways of Locomotion Much of the delight that the naturalist experiences comes from observing and interpreting the ways and devices by which the move- ments of organisms are brought about. WINTER HOME The annual migration routes of the Arctic tern. It covers about 22,000 miles in its yearly round trip from its winter range in the Antarctic to the summer breeding range in the Arctic. Note the different routes taken going and coming. THE BTOLOGICAI. CONQUEST OF 'll||.; Woiuh .'.'» II riijhl I'll rci The Russian thistle {Salsolu) introduced into this country in 1!571. Today it covers the entire country. What adaptations have enabled this pesi to do tliis? In the world of attached animals, like sea-anemones and corals, that apparently are doomed to remain in one place, the free-swimming larvae seize the opportunity to break away from the maternal apron- strings before settling down for life, just as stationary plants by means of spores, seeds, and chmbing or trailing vegetative parts are enabled to shift about and occupy new territory. Seeds of orchids and certain spores of fungi, mosses, and ferns, for example, are light as dust and may be wafted hundreds of miles in the air before settling down to germinate on some distant soil. Seeds of dandelions and other plants, such as milkweed, willow, and cottonwood, have feathery paracluite- like structures, which support them in the air for some time, e\-en in a wind blowing only two miles an hour. Insects, ballooning spiders, and birds make use of air currents, sometimes being carried long distances, particularly by heavy winds. Whole plants, like the Russian lliistl(\ and the "resurrection plants" of desert regions, may dry uj) and break loose from their anchoring roots, and roll along the ground or ride the breeze scattering their seeds, thus taking root in newly invatknl regions. H. w. H. — 4 40 NATURAL HISTORY Estimate ok Seeds Produced by a Single LARtiB WE>;n Dandelion . Cockle-bur . Oxeye daisy Prickly lettuce Beggar's ticks Ragweed 1,700 9,700 9,750 10.000 10,500 23,000 Crabgrass . Russian thistle Pigweed Purslane (large) Tumble mustard Lamb's-quarters 89,600 150,000 305,000 1,250,000 1,500,000 1,600,000 Some fruits, like those of violets and the witch-hazel, explode, send- ing their seeds to a distance. Even gravity may sometimes be re- sponsible for spreading plants by means of soil-slides, while animals in such accidentally disturbed soil may be carried considerable dis- tances to a new situation. Birds inadvertently scatter fruits and seeds by first swallowing and then depositing them elsewhere with their droppings. As a result cherry bushes and poison-ivy vines may often be seen growing along fences where birds have roosted. Adaptability to New Conditions The fact that some organisms do not invariably adapt themselves to new localities which they have invaded is a great deterrent to their permanent spread. Successful invaders that gain a new foothold as pioneers, and retain it as settlers, are conspicuous enough to be discovered and remembered, but unsuccessful ones, reaching the Promised Land but unable to establish themselves there, escape atten- tion. Indian corn, for example, seems unable to reproduce and main- tain itself if allowed to run wild. The yellow-fever mosquito has a certain dead-line, north of which it cannot successfully continue to live. Just as in economic life, so in communities of plants and animals, undesirable individuals frequently appear, bumming their way into places where they are not wanted. Weecis are notorious plant- hoboes that are pre-eminently successful on their own part, but are unwanted by man, and reckoned as outlaws with a bad reputation, because they rob other plants which man favors, of food, moisture, and sunlight. Having great natural vitality, they are successful because they usually grow even in unfavorable conditions which would kill competing plants, and produce enormous numbers of seed. Their persistence and varied means of seed dispersal are easily realized by anyone who has tried to pick "beggar's ticks," and "sticktights," and burrs from his clothes after a ramble in the autumn woods. THE BlULOCilCAL CONQUEST OF THE WOULD n Human Interference Man is often the unwitting cau.se of sliifts, .sometimes with serious results, of animal and ])lant ixjpulations. Tlie Russian thistle, already mentioned, was introdueed into South Dakota in 1S74 with flax-seed from Europe. By 1888, it was reported as a troublesome weed in both the Dakotas. By 1898, it had covered all the area east of the Rocky Mountains from the Gulf of Saskatchewan, and today ranges over the whole country. There are many curious cases of the accidental transport by human agency of animals and plants to regions far from their point of origin. Recently a tropical boa landed in Middletown. Connecticut, with a bunch of bananas. Tropical tarantulas, too, are known to be carried over long distances in the shipment of this fruit. Such instances as these, however, usually have no lasting effect on the general spread of organisms, yet they emphasize the fact that unanticipated develop- ments in distribution are quite jiossible from very insignificant and unsuspected beginnings. Man's interferences with the distribution of organisms have by no means always been unfortunate or disastrous. In many instances his rearrangements of plant and animal popula- tions have been eminently successful. The planting of various species of trout in new streams has proved to be a wise move, \\hile the introduction of reindeer into Alaska and Labrador is of incal- culable benefit to both man and beast. The list of cases where man has lifted the lid of Pandora's box and set free plants and animals for weal or woe into new localities could be extended indefinitely. Life Zones Reference has already been made to a zonal distribution of i)hints and animals in a pond. A similar condition is easily seen in climbing any high mountain. Life zones are often rather sharply marked, but usually show transitional areas between them. A region which has been carefully studied and which shows this zonal distribution in a marked way is the San Francisco mountain region in north Arizona. Here, a mountain nearly 13,000 feet in height rises out of a desert plain. This mountain shows successively two tyj^es of desert zone, a lower and upper, each with its own desert fauna and flora, cacti, sagebrush, a few birds, mice, lizards, and snakes. Then a r(>gion at between 6000 and 7000 feet of pinon pines and red cedars, inhabited by more birds and a small number of mammals. Between 7000 and 42 NATURAL HISTORY Zonal distribution of flora on a moun- tain peak rising from a desert area. How would you account for these differ- ent life zones ? 8200 feet there are forests of Douglas and balsam fir, with such mam- mals as meadow mice, chipmunks, deer, lynx, and puma. Higher still between 8200 and 9500 feet, is a typical Canadian vegetation, timber pine, Douglas and balsam fir, and aspens, while the wood- chuck, porcupine, rabbit, mar- ten, fox, wolf, and other northern forms are found. From 9500 to 11,500 feet we find a fauna and flora almost like that of northern Canada and called Hudsonian. Stunted spruce and pine exist up to the timber line with a few typical mountain mammals such as the marmot, and pika or mountain hare. Above this area lies the rocky Alpine zone, snow-clad for half the year even in this warm, sunny climate. Lichens on the rocks and a few stunted herbs are the only plant life visible, while a limited number of insects and an occasional mammal from the Hudsonian zone are the only signs of animal life. The facts that the chorologist has discovered concerning life zones have been put to practical use by the Biological Survey of the United States Department of Agriculture. A life zone map has been pre- pared so that the settler going into a new region will know at once the kind of plants and animals best adapted to live there. In addi- tion, information is available about the character of the soil, the rainfall, temperature range, and the particular cereals, fruits, and vegetables that can be grown in the region. Life Realms Different parts of the world, each with its several life zones, have been separated into life regions, or realms. If we plot the distribu- tion of a given family of animals or plants, we often find that species within the group have a wide distribution, in some instances covering more than a single continent. Australia has long been set aside as a distinct realm because its peculiar fauna and flora differ from those in other parts of the earth and so is called the Australian Realm. THE BIOLOGICAL CONQUEST OF Till: WOULD \:\ "L,_ Hblarctic p,. 3 / ^^^^^4i/^• Ethiopian Auslralia-ri ^:;» "Rsalin Map showing life realms. Similarly there are the South American, or Xcotroi)i('al, Etliioi)iaii, Oriental, and Holarctic realms, the latter comprising most of the land surface of the Tropic of Cancer. Each of these regions has animals and plants peculiar to itself, although resemblances are often found between inhabitants in different realms. SUGGESTED READINGS Beebe, C. W., Hartley, G., Howes, P. G., Tropical Wild Life in Britif^h Guiana, New York Zool. See, 1917. Ch. VI. Contains an interesting description of a tropical rain-forest. Borradaile, L. A., The Animal and Its Environment, O.xford University Press. London, 1923. Chs. VII, VIII, X, XI, XIII. Excellent for general reading. Elton, C, Animal Ecology, The Macmillan Co., 1927. Chs. Ill, V, X. Fascinating reading. Jordan, D. S., Kellogg, V. L., and Heath, H., Animals, D. Applcton ct Co., 1909. Chs. VII, XVI. Old but reliable. Pearse, A. S., A7iimal Ecology, McGraw-Hill Rook Co., 192G. Ch. IV. Rather a book of reference than a reading book. Roule, L., Fishes, Their Journeys and Migrations, W. ^^■. Norton & Co., 1933. All of this book makes interesting reading. Walter, H. E., Biology of the Vertebrates, The Macmillan Co., 192S. Cli. III. Interesting and reliable. Weaver, J. E., and Clements, F. E., Plant Ecology, McGraw-Hill Book Co., 1929. Chs. IV, V, VII, XVIII. Very' scientific and yet interesting. II J THE INTERDEPENDENCE OF LIVING THINGS — THE WEB OF LIFE Preview. Relations between members of the same species; care of eggs by parents; care of young • Relations of mutual aid • Animal can- nibalism • Relations of competition • Relation of members of different species ■ Adaptations for food-getting in animals • Scavengers • Food- getting in plants ; carnivorous plants • Symbiosis • Commensalism • Par- asitism • The chemical relationship of plants and animals • Life habits of bacteria • Relation of bacteria to free nitrogen • Rotation of crops • The relations between insects and flowers • Suggested readings. PREVIEW Those who have been fortunate enough to be in California or Flor- ida when the oranges are in bloom will never forget their odor ; nor will they, when examining the grove, fail to notice the large number of bees vi-siting the flowers. The bees are after nectar and pollen, yet without these winged agents, the crop of oranges for the follow- ing year would probably be small. This interrelationship between insects and flowers was noticed by Charles Darwin, who pointed out that the size of the clover crop in England depended upon the num- ber of cats in a given region. His friend Huxley, who knew better than Darwin how to popularize science, immediately went him one better and added that the size of the clover crop depended upon the number of old maids. When asked to explain, he gave this logical se- quence of events. Old maids keep cats ; cats prey upon field mice ; mice provide nesting places for bumblebees ; bumblebees pollinate clover, upon which pollination the next year's crop depends. So he had a perfectly logical chain of events. Throughout nature there is this give and take between different organisms which we call the web of life. When man interrupts or displaces a link in the chain of interre- lationships, the web is broken and the whole fauna or flora of a region may be changed, as in the case of the Englishman who took a bit of water cress to Australia, planting some in a nearby stream to remind him of home. This foreign plant, having no enemies and finding conditions favorable for its growth, literally overran the waterways until today the rivers of Australia are choked with water cress. Look- 11 THE INTERDEPENDENCE OF LIVING THINGS i:, ing over the world of plants and animals an unescajDabie dcixMidenro of one form of life upon another is found in the food relationship by which green plants supply animals with food and in the shelter relationship, by which animals find safety in the protection given by plants. Reducing this search for food and shelter to its ultimate, we find that all animals are dependent upon green plants. But does the green plant get anything from the animal ? At first sight it would seem as though it were all give and no take. As we study the situation more closely, however, we find that food-making is dependent upon certain raw materials, some of which, such as nitrogenous wastes, can only be supplied from the dead bodies of organisms or their excreta. Moreover, another important raw material, carbon dioxide, used by green plants in starch-making, is given off as a respiratory by-product by animals, and in this same process oxygen is released. All of these facts suggest certain problems. Why, for example, when some animals produce enormous numbers of eggs and others only a few, do not the former outnumber the latter? Of what significance is the mutual aid so frequently observed in nature? What is symbiosis and why is it significant? What is the \'alue of pollination by insects as compared with pollination by other means? What part do bacteria play in the fives of plants and animals? What is the reason for parasitism ? Can the oft-repeated statement that green plants make food for the world be proved ? A start on the answers to some of these questions will be made in the pages that follow. Relations between Members of the Same Species Many examples of helpful relationships can be .seen between ani- mals of the same species, especially in the care of young. Although in low forms, such as sponges, coelenterat(>s. echinoderms, and a good many fishes, large numbers of eggs are laid and given little or no parental care, the production by the male of immense numbers of sperm cells in the vicinity of the eggs insures chance fertilization and continuity of the species. For example, Norman ' reports that a cod w^hich weighed 21^ pounds produced over 6,650,000 eggs. At tiie time of egg laying each male of the above .species throws billions of sperm cells into the water near the eggs. Higher in the animal scale we find greater provision for care of the young correlatcnl with a re- duction in the number of eggs laid. Many insects lay their eggs on 1 Norman, J. R., A History of Fishes. Stokes, 1931. 46 NATURAL HISTORY Bruinu II Ichneumon fly {Ophion macnirum) laying eggs in the cocoon of a Cecropia moth. plants which will become food for the larvae or caterpillars. Others lay their eggs either in the ground where they are protected, or in dead bodies of animals on which the larvae may feed, as in the case of certain beetles, or in a ball of dung, as in the case of the dung beetle. Certain ichneumon flies bore deep into tree trunks in order to lay their eggs in the larvae of wood-boring insects. Some w^asps paralyze caterpillars or spiders, laying eggs in the still living victim so that when the eggs hatch the young larvae will have food. In many animals, food is provided in the yolk of the egg, the eggs of fish and birds being examples. Spiders and earthworms form cocoons, which in the case of the earthworm are usually filled with a nutritive fluid on which the young feed after they are hatched, while in the cocoon of the spider the young feed upon each other, the strongest of the group surviving. Care of Eggs by Parents Some of us as youngsters have angled for sunfish and will always remember the thrill that came when a brightly colored male dashed at the bait dangled over the hollowed nest containing eggs which he was guarding. From the simple nest of sunfish and salmon through the more complicated nests of the stickleback or lake catfish we come to the more elaborate nesting habits of birds. Some birds, as terns, sandpipers, or gulls, simply make shallow holes in the sand, as does the sand ostrich. Grebes and rails make nests of floating decaying vegetation. Nuthatches and woodpeckers make nests in holes in trees where the young are protected. At the top of the ladder are more elaborate nests such as those of the oriole and oven-bird of our latitude or the tailor bird and weaver bird of the tropics. Care of Young Sir Arthur Newsholme has said that the most dangerous work in the world is that of being a baby. If the young of plants and ani- THE INTKHDEI'llNDENCE OF MV|\(; ril|\(;s M A. 1'. .sV((/i CtinserrniUin iJcpl Stickleback and nest. Of what advantage would this be to the species? mals survive this dangerous stage, their chances of growing to adults are very considerable. Although parental care is not associated with plants, nevertheless in low forms of plant lif(> locomotor stages occur, called zoospores or swarm spores, by means of which the plants gain footholds in new areas. Many devices have already been men- tioned by means of which seeds are scattered far from the parent plant. In higher plants, hard shells, spiny coverings, or inedible pulp protect seeds within the mature fruit, thus giving greater ojjpor- tunity for the scattering and germination of seeds. Adaptations for the protection of young are more evident among animals. In crustaceans, the larvae of which form the chief food for great numbers of fish, there are not a few protective adaptations. In some instances crustaceans have brood pouches in which the young are kept, or, as in the case of crayfish and lobster, the developing eggs are cemented to the abdominal appendages of the mother and carried around by her. The male bullhead .swims arountl with and broods over his young, while the male sea horse has a brood pouch in which the young are held. In some worms and crustaceans, the eggs may be retained in the burrow of the parent, or they may be held in the mantle cavity or a space similar to it, as in the fresh-water mussels, barnacles, and tunicates. Some spiders, notably the wolf spiders, carry the egg cocoon about with them and when the yoimg are hatched, they are carried on the backs and legs of the female 48 NATURAL HISTORY Huijh Spiricer A spider with its egg cocoon. until large enough to care for themselves. The male of the so-called midwife toad (Alytes) carries the eggs entangled around the legs. The male Surinam toad places the eggs on the back of the female, where each sinks into a tiny pouch as it develops. Animals that lay eggs which hatch outside of the mother's body are said to be oviparous. A modified form of this procedure is seen in some nematodes, arthropods, fish, amphibia, and reptiles. Here the eggs remain in the oviduct or uterus of the mother until they are almost ready to hatch, the body of the mother acting as an incubator. Such forms are said to be ovoviparous. Most of the mammals which retain the eggs in the body until the young are born are said to be viviparous. Here the young are held as embryos within the body of the mother and nourished by means of an organ called the placenta. The young of mammals are suckled at the breasts of the mother until they are able to eat solid food. Relations of Mutual Aid A certain amount of protection is afforded plants from their habit of living in communities. Examples are the aggregations of cacti in our western deserts or the acacia and "thorn bush" communities of Australia. The animal world, too, shows many examples of protec- tion among gregarious forms. The schooling of fishes not only is a defense for the group from larger fish, but it also enables small fish, working concertedly, to prey on organisms much larger than them- selves. The driver ants in Africa, traveling in great swarms, often overcome and devour animals hundreds of times larger than them- selves. Wolves hunt in packs, several of them rushing together to bring down their larger prey. Deer and other herbivorous animals move in herds for mutual protection. Another relation of mutual aid results from the development of division of labor among certain animals. Although social division of labor is well seen in the human species, there are many examples in the insect world, particularly among the social bees and wasps, THE INTERDEPENDENCE OF LIV1\(, Tl||\f;s 49 such as tho division of the colony into castes thai include nuih-s (drones), fertile females ((lueens), and infertile females (workers). Castes are even more mmierous among ants, there being winged and wingless females, intermediates between females and workers, soldiers, several groups of workers, and winged and wingless males. Not all of these forms, however, are found in any one species. By means of such division of labor, life in the colony goes on at a very efficient level. Animal Cannibalism Most of us have had the experience of having some pet destroy her young when they were in danger, or of having laboratory-bred rats or mice eat their newborn young. This is probably a perverted instinct, but nevertheless animal cannibalism is .seen rather fre- quently. The destruction of a wounded member of a pack of wolves when hunting is usual. The female spider usually kills the male after fertilization of the eggs, this habit being common to some other forms. Similarly the eggs may be destroyed by the male, as in the case of the mole cricket and centipede, w^hich eat the eggs shortly after they are laid, the mothers resorting to numerous pro- tective devices in order to thwart the cannibalistic fathers. Many fish eat the eggs of their own species. Even the domestic hen at times will eat her own eggs. Relations of Competition Evidences of competition in the plant world are numerous. Be- cause of their sessile habit, older plants may overshadow and crowd out the young ones, or one group of plants may prevent the growth of other plants in the vicinity. Weeds and plants in general pro- duce enormous quantities of seed, which are kept from germinat- ing by the rapid growth of the older plants. Many grasses and some shrubs grow rapidly by means of underground shoots, in this way securing territory which might be used by other plants. Thus plants with favorable adaptations may completely pre-empt new territory for themselves at the expense of others le.ss able to use the environment. In animals, competition between individuals of a species is almost universal. Males fight each other for the possession of females, or sometimes just for the sake of fighting. There is a contituial struggle for food, for water, and for a place to live. I>arger animals, as we have seen, prey on smaller ones and in general those best fitted to compete in the battle of \Uo, survive. 50 NATURAL HISTORY Wright Pierce This desert weed, rabbit brush (Chrysothamnus nauseosus) has pre-empted newly cleared areas along the border of the Mohave Desert. How would you account for its rapid spread ? Relation of Members of Dififerent Species No one who has carefully watched the life that goes on in a grove or forest can escape seeing there the enactment of a drama that repre- sents the larger picture of relationships between living things the world over. Insects are flying through the air, crawling along the ground, or burrowing into decaying logs and the ground. Spiders and ground beetles may occasionally be observed making off with a victim, while here and there birds such as woodpeckers, flycatchers, and warblers may be seen feeding on adult insects or their larvae, while a hawk may be watching to pounce upon some one of the insect-eating birds. If we were able to make a prolonged study of the area we would find that squirrels, rabbits, and wood mice are food for larger flesh-eating animals or carnivores, such as foxes. In such an area we might also find a series of herbivorous animals ranging from plant lice (aphids) living on the leaves of trees to occasional deer which browse on the leaves of the same plants. zSn plants rrxike the fooct fbr- tha- worloC THE INTERDEPENDENCE OF L1VT.N(; TIIIN(JS :,i It will be noted in the illustrations given that animals almost mvanably feed upon others smaller than themselves. The same relationship is seen in lakes or oceans where microscopic plants and animals (plankton) form the food of other larger organisms, especially fish. These living things form definite "food chains" in which larger animals feed on smaller and smaller ones until ultimately the lowest forms subsist on tiny green plants or bacteria. For example, in a small pond we may find billions of diatoms, unicellular algae, and protozoa and feed- ing on them millions of small crustaceans. With them are thousands of insect larvae, hundreds of small fish, and a few large fish, such as bass, pickerel, or perch, which are dependent upon all the other forms of life. In this case a few large animals are depend- ent for food upon the development of myriads of smaller organisms, the basis of this food being very simple plants. Take away any link in the food chain and life in the pond becomes disorganized, with the ensuing death of many of the inhabitants. Since smaller animals reproduce more rapidly than larger ones, the food supply for those "on the top of the heap" remains fairly constant. It should be borne in mind, however, that the larger animals require a range of sufficient size to support them. Adaptations for Food-getting in Animals Protozoans, if ameboid, engulf their food, but in other members of this group, food passes into the cell through a definite opening or through the plasma membrane. Sponges and many molluscs pick up microscopic food as it comes to them in water currents. Some molluscs bore holes through the hard shells of bivalves, in that way securing the soft parts of the animal for food. Insects have biting, chewing, or sucking mouthparts, each type being fitted to utilize a drassViopptrs- ° Gat gi-ass \\ hy will a break in the food chain often cause disorganization of life in that locality ? NATURAL HISTORY Wright Pierce Adaptations of beaks of birds for food -getting. different kind of food. Carnivorous mammals have sharp teeth fitted for tearing and holding prey ; herbivorous mammals have flat, corrugated teeth ; rodents, gnawing or chisel-like teeth ; while snakes, which swallow their prey whole, have pointed, needlelike teeth to hold their food securely. More striking adaptations for food-getting are found in birds whose beaks and feet both give clues to their food habits. The flesh-eating birds have hooked beaks and curved claws ; aquatic birds have feet shaped like paddles and scooplike bills for straining out small organisms from the water ; wad- ing birds display a remarkable variety of highly specialized beaks and feet ; and the smaller land birds show equally interesting adaptations for se- curing food. Bizarre adaptations for procuring food characterize the giraffe, with its long neck that enables it to reach up to feed on branches of trees fifteen feet from the ground, the ant- eater, with its sticky tongue, and the walrus, which digs bivalves with its tusks. Scavengers Some forms of life are not only om- nivorous in their diet, but are actually scavengers, living on dead organic ma- terials. The bacteria,^ smallest of all plants, feed upon or destroy millions of tons of organic wastes which other- wise would make life on earth impossi- ble. Think of a world without decay. Land and water would soon become ' See pages 165-166. THi: INTEHDKPKNDKNCE OF I.IVINC T|||\,;s ;,.•, roverod witli the dead bodi(>s of plants and animals. TUr i)acteriu of decay are very numerous in rich, damp soils containing large amounts of organic material. They decompose organic materials, changing them to compounds that can be absorbed by plants to be used ii, building protoplasm. Without decay life would be impossible. f„r green plants would otherwise be unable to get the raw food materials to make food and living matter. In general all plants, both colorless and green, may be said to play a part in ridding the earth of organic wastes. The fungi, or colorless plants, get their nourishment from the dead bodies of plants and animals, while the green plants take organic wastes from the soil to be used in the manufacture of foods. Many animals also take part in scavenging. Some of the food of the protozoa is made up of decaying unicellular material and the bacteria which cause its decay. Certain forms, especially insects, feed upon and lay their eggs in decaying flesh, while myriads of insects and their larvae help to break down decaying wood in a forest. These are only a few instances of this important function. Food-getting in Plants Although green plants make foods and use raw food materials ' from their environment to do this, there are some that destroy foods. Fungi, such as bacteria, molds, smuts, and rusts, ruin billions of dollars' worth of food plants and plant jiroducts each year. This is seen in damage to crops, fruits, stored foods, and animals used as food by man. Carnivorous Plants A curious exception to ordinary green plant nutrition exists in carnivorous plants, which also illustrates a different interrelationshij) between plants and animals. Carnivorous plants add to their nitro- gen requirement in several ways. The fresh-water aquatic plants known as bladderworts {Utricidaria) catch water fleius and other small crustaceans in hving bladderlike traps. Just what lure urges the crustaceans to destruction is hard to say, but the fact that they are caught in numbers is verified by their decomposed remains found in the bladders. Other animal-eating forms are the various pitch(>r plants (Sarracema sp.), some of which are found in our northern swamps. Insects are apparently lured to the urn-shaped leave^' > See pages 253-262. 54 NATURAL HISTORY by a trail of sweet nectar secreted just outside the mouth of the pitcher. Once inside, a shppery surface and incurving hairs prevent egress, and the insect is soon digested by the enzymes in the fluid contained inside the pitcher. Still another leaf modification with a similar function is seen in the sundew (Drosera sp.). Here the leaves are covered on one surface by sticky glandular hairs, which close The leaf of a bladder wort {Uiricu- laria vulgaris). Many of its numerous divisions bear bladders (6), especially near the place of attachment to the main leaf axis (a). Note the aper- tures of the bladders (p) into which small aquatic animals may crawl or swim. The modified leaf of a sundew {Drosera rotund if olia) showing the conspicuous glandular hairs (g) covering the upper surface, the hairs at the right having caught an insect. Note that the hairs are tipped by a drop of secreted liquid {d), which attracts insects to the leaf and also entangles them. — After Kerner. over the insect, hold it fast, and ultimately digest it and absorb its juices. In the Venus's-flytrap (Dionaea sp.), another carnivorous plant found in some parts of this country, the leaves have two sensi- tive lobes provided with marginal hairs. If an insect lights on a leaf, the two lobes close over it and the insect is trapped. After its prey is digested, the lobes of the leaf open up and the plant is ready for action again. Symbiosis The process of living together for mutual advantage is called symbiosis. Plants may join forces as may animals, or in some instances, plants with animals. Lichens, for example, illustrate this THE INTERDEPENDENCE OF LIVI\(. TIIIN(;S W I III III I'll rci An encnistiiif^ licht-n. Why docs lliis pl;inl suc- ceed in such an unfavorable cm ironiucul i' mutual partnership in an interesting way. A lichen is composed of two kinds of plants, a green alga and a fungus, one of which at least may live alone. The two plants form a part- nership for life, the alga making the food and nourishing the fungus, while the latter gives the alga raw food materials, protects it, and keeps it from dying when the humidity of the air is low. Other examples are bac- teria and the mycelial filaments of fungi {my- corhiza) which live sym- biotically on the roots of certain plants, taking food from the plants, but giving them nitrogen in a usable form in return. A common example of symbiosis between plants and animals is the green Hydra {Chlorohydra viri- dissima), which holds in its body wall a unicellular alga known as Zoochlorella. These plants contain chlorophyll, using the sun to make food. In this partnership, the algae get carbon dioxide and ni- trogenous wastes from the animal, to which, in turn, they give food and the oxygen set free in the process of starch-making. There are numerous examples of this kind of symbiosis in tlio animal world, as is seen in many of the protozoa, sjionges, A root' tip of the coelenterates, flatworms, molluscs, and sea urchins. European beech xhe symbiotic relationship of animals to each {Fagus sylvalica), ^^^ -^ ^j^^^^.^^ y ^j^^ ^j, protozoans iixing in showing ectotrophic '^ „ . i •. * mycorhiza, the fun- the digestive tracts of termites or white ants. gal hyphae forming These Httlc animals act as digestive cells for the :nS:the?tLtLt termites, making it possible for them to use -After Frank. wood fibers on which they live. In return th(> H. W. H. — 5 56 NATURAL HISTORY a shark- s-cccker- protozoans receive food and are protected by their hosts. A some- what similar situation prevails in the large intestine of man, where certain types of useful bacteria are found. These forms help keep down putrefying bacteria, receiving in return a home, food, and a favorable temperature in which to live. Certain species of ants protect and feed aphids, in turn feeding upon the sweet fluid secreted by the aphid. Commensalism Some associations are not obviously to the advantage of either organism, the two feeding together as messmates. Animals like the small crabs that live in the water canals of certain sponges, or the tiny fishes that live in the lower part of the body of a "trepang," a sea cucumber, are ex- amples. The young of some species of rudder- fish (Stromateidae) ac- The shark sucker (Remora brachypiera, Lowe) company large jellyfish, showing sucking disk and its method of attach- geekina; shelter under ment to the shark. The Remora gets free trans- ,, ■ +• • j. 4. i portation and makes sudden forays after food as their stmgmg tentacles well as sharing the "left-overs" of the shark's food, when chased by larger But it seems doubtful if the shark gains anything ggj^ while another fish from the association. . ^ . {Nomeus) lives in con- stant association with the beautiful coelenterate known as the Portuguese man-of-war. Parasitism Not all life is give and take. Some plants and animals live at the expense of others, giving nothing and taking all. These are known as parasites, the organism which entertains them being called the host. From the lowest to the highest forms in the plant and animal kingdom there are few which are not attacked by parasites at some stage of their existence. Parasitism implies plenty of food, shelter, and a relatively protected life for the parasite, but it also usually spells degradation in structure and loss of activity. It may mean only inconvenience, but more Ukely a shorter and disturbed life for the host, especially if the parasite THE INTERDEPENDENCE OF LIVING THINGS 57 causes disease. In some instances, the complicated life history is so bound up with more than one host that if one of the hosts is absent a hnk in the chain of life is broken, the life cycle cannot be completed' and the parasite dies. The black-stem grain rust, which ref,uires ^^ recC spore blov/n to anotber^ stem recC or Sxtmreierc rixst on •wheat stem barlserrv rtcst spore in?ectintf ths cells of -^heat stem ira spring- "bocrbei^ry leaves mfscts stem through breo-^hing" ?«""«. red rust syjrecuil^ , from stem to stem/ cCixriijg' Sixmme'P blaclc or ^vinter rust lives on straw thrcuflh , winter- * " infection form', barberry rust onborberrjlea.^ a cluster cup' The life history of black stem grain rust. controlled. r, ■!, black spora inject ing^ bocfics, sporicCa a spoT~id.ium infects the Cells of a barberry leaf Explain how this rust may he both the barberry plant and the wheat to complete its life history ; the pine tree blister, which lives on the currant or gooseberry at one stage of its life history, and on the pine at another ; and the parasite causing malaria, which requires both the anopheline mosquito and the blood of man to complete its cycle, are examples. The Chemical Relationship of Plants and Animals The study of plant and animal ecology may be said to be analogous to the study of human economics. Social conditions among men, animals, and plants are all determined by the environmental factors present, but chiefly by the availability and abundance of food. The world's food supply in the long run depends upon the chemical ele- ments making up the environment and energy derived from the sun. 58 NATURAL HISTORY Plants and animals are made out of the same chemical elements. Burn some beans or a piece of beefsteak, a piece of wood or a bit of living bone, an entire green plant or a dead mouse, and the chemist would tell us that the same chemical elements are present in animals and plants ; that certain of these elements passed off in the smoke, others into the air as colorless gases, leaving still others as a whitish ash. All living things are composed mainly of carbon, oxygen, hydrogen, nitrogen, with about twelve other chemical ele- ments found in very minute quantities. These elements are all present in the immediate environment of plants and animals, air, water, and soil. How they get from the basic environment into living things can be briefly stated. Carbon, which is contained in all organic foods and in this condition is taken into the animal body, can only be absorbed in the form of carbon dioxide by food-making green plants. This gas, which is present in the atmosphere to the average amount of about 0.03 per cent, gets there as a result of oxidative processes taking place in plants and animals, as well as by the com- bustion of organic substances. Factories and volcanoes alike form their quota of carbon dioxide to diffuse out into the atmosphere. The cycle of the passage of carbon from plants to animals and from animals back to plants is shown in the accompanying figure. Hydrogen, another component part of living things, cannot be used in its pure state by either plants or animals. In water (H2O) , it becomes an important part of the food of animals, and as water vapor it is used in starch-making by green plants. Oxygen is freely available to both plants and animals. As a gas, making up over 20 per cent of the air, capable of being dissolved in water for aquatic plants and animals, it is used by all living things in respiration. Green plants add this gas to the air during the process of starch-making. Nitrogen is one of the most important elements found in living things. Making up 79 per cent of the air, it is not usable in the form of a gas except by the nitrogen-fixing bacteria. The carbon and oxygen cycles in a balanced aquarium. Trace the pas- sage of an atom of carbon from a green plant back to the plant. THE INTERDEPENDENCE OK LIVING THINGS :><> The other mmeral components of living matter, of wliich sulpliur phosphorus, calcium, potassium, and iron are among tlie most impor- tant, are all found either in water, soil, or both. How the plant makes use of them and turns them over for the use of animals is an interesting story to be told later. But enough has been said to show that foods made by the green plants form the supply on which all animals live. carbohydrates^,, .^^^^ carbon iiox.ae^^ /'0>^yg«?'v proteins^ X -^-ureot /^ >\ n'til«ts slltsZI^ AnimaU (Gr^enPlantC r^f"' >.^ter- \^ y-sctlts V y^-^^'■-1^<.^ The food relationships between green plants and animals. Life Habits of Bacteria In this web we call life, bacteria play a most important part. Since bacteria contain no chlorophyll, they are unable to make carbo- hydrate food, and must obtain their foods from decaying organic matter. In order to absorb such food it must be made soluble so that it will pass into their bodies. This they do by digesting food substances by means of enzymes ^ which they secrete. Bacteria that grow or thrive in the presence of oxygen are called aerobic, while those which live without free oxygen are called anaerobic. The latter need oxygen, like other living things, obtaining it by breaking down the foods on which they live, and utilizing oxygen freed in this process. Relation of Bacteria to Free Nitrogen It has been known since the time of the Romans that the growth of clover, peas, beans, and other legumes causes soil to become more favorable for the growth of other plants, but the reason for this was not discovered until modern times. On the roots of the plants mentioned are found little nodules, or tubercles, in each of which are millions of nitrogen-fixing bacteria {Rhizobium leguminosarum) , that take nitrogen gas from the air between the soil particles and build it into nitrites which arc tlu>n converted by otluM- bacteria (Nitrobacter) into nitrates. In this form it can be used by plants. Nitrogen-fixing bacteria live in a symbiotic relationship with the plants on which they form tubercles, their hosts pro\iding them with organic food. J See pages 127-128. 60 NATURAL HISTORY fjring' JoactcHee Bacteria also act upon ammonia formed from plant and animal wastes, one kind (Nitrosomonas) producing nitrites, or nitrate salts, and others (Nitrobader) converting the nitrites into the more stable nitrates. Thus all of the compounds of nitrogen are used over and over, first by plants, then as food by animals, eventually returning to the soil, or in part being released as free nitrogen. This process is called the nitrogen cycle. Although free nitrogen is fixed for use by means of electrical discharges dur- ing thunderstorms, by man-made machines, by ultraviolet light (which is estimated to return 100,000,000 tons a year to the earth's surface), and from other sources, yet these means give an almost negligible amount of usable nitrogen to the soil, compared with what is used in crop produc- tion, especially since so much nitrogen is lost from the soil in various ways. The nitrogen-fixing bacteria supply the deficiency, thus form- ing one of the most important inter-relationships between plants and animals because of their direct relationship to the production of the food of the world. Rotation of Crops Plants that are hosts for the nitrogen-fixing bacteria are raised early in the season, then plowed under and a second crop of a differ- ent kind is planted. The latter grows quickly and luxuriantly be- cause of the nitrates left in the soil by the bacteria which lived with the first crop. For this reason, clover is often grown on land used later for corn, or cowpeas will be followed by a crop of potatoes. On well-managed farms, different crops are planted in succession in a given field in different years so that one crop may replace some of the elements taken from the soil by the previous crop. This is known as rotation of crops. ^ ' Crop rotation is not only a process to conserve the fertility of the soil, but also a sanitary meas- ure to prevent infection of the soil. The nitrogen cycle. What additions could made to this diagram? be THE INTERDEPENDENCE OF LIVINt; Tll|\(;s <)l Wriilhl I'Urcc Tiger Swallow-tail (Papilio turnns) on rose. The Relations between Insects and Flowers One of the most interesting symbiotic relationships is that which exists between msects and flowers. Flowering plants produce .seed.s and fruits, and from these come new gen- erations of plants, but if it were not for the visits of insects, many plants would not produce seeds. Insects visit flowers in order to obtain nectar, a sugary sub- stance formed by the nectar glands, and pollen. The glands which produce the nectar are usually so placed that an insect has to push its way past the stamens and pistil of the flower in order to reach the desired food. In doing this, pollen grains may adhere to the hairy covering of the insect and be transferred to the sticky surface of the upper end of the pistil (stigma). Inside the pollen grains are the male re- productive cells (sperms), while in the ovary of the pistil are held the female reproductive cells (eggs). In order to ha\e develop- ment of a new plant, it is essen- tial for a sperm cell to unite with an egg cell. Pollen grains on the stigma are stimulated to send out hairlike tubes, wiiich j)enetrate the stalk (style) of the j)istil and eventually reach the ovary. The pollen tube carries one or more •derminatino: anther* •filameriti ovulell'' . .-finicropyle-' A longitudinal section of the repro- ductive organs of a flower showing the penetration of a pollen tube through the opening in the pistil called the micropyle, and the growth of the pollen tube to the ovule. 62 NATURAL HISTORY sperm cells, which are thus enabled to unite, each with a single egg cell, in the ovule of the pistil. The union of the sperm nucleus with its egg nucleus is called fertilization. As a result of this process the fertilized egg develops into an embryo or young plant which is held in the seed. When favorable conditions arise, this embryo m.ay develop into a plant. Bees are the chief pollinizing agents, although butterflies, moths, flies, and a few other insects perform this service as well. Hum- mingbirds often pollinate tubular flowers, while other small birds, snails, and even bats are agents in the pollination of certain forms. Man and animals may accidentally pollinate flowers in brushing past them through the fields. The value of cross-pollination is obvious and is an example of the close weaving of life in which man, animals, and plants are all inescapably entangled. SUGGESTED READINGS Borradaile, L. A., The Animal and Its Environment, Oxford University Press, London, 1923. Chs. IV, V, XIV. Excellent for reference. Elton, C, Animal Ecology, The Macmillan Co., 1935. Chs. V, VI, VIII. Particularly valuable on the animal community and the relationship of animals to a food supply. Needham, J. C, and Lloyd, J. T., Life of Inland Waters, 2nd ed., Charles C. Thomas, 1930. Ch. V. Interrelationships among fresh-water organisms. Pearse, A. S., Animal Ecologij, McGraw-Hill Book Co., 1926. Chs. VIII, X. A wealth of material on interrelationships. Rau, Phil., Jungle Bees and Was-ps of Barro Colorado Island, privately printed, Kirkwood, St. Louis, 1933. An ecological study of a tropical environment. Wallace, A. R., The Geographical Distribution of Animals, 1876. Books I and II. Ch. IV, especially. This book forms the basis for most of the modern work in distribution. All of Part III, Books I and II, is extremely interesting. Weaver, J. E., and Clements, F. E., Plant Ecology, McGraw-Hill Book Co., 1935. Ch. XVI. An interesting chapter on relations between plants and animals, with especial emphasis on insect pollination. Wells, H. G., Huxley, J. S., and Wells, C. P., The Science of Life, Doubleday, Doran & Co., 1931. Book 6, Chs. IV and V. A fascinating book for general reading. IV ROLL CALL Preview. Earh^ contributions to classification • Binomial nomencla- ture ■ Law of priority • What is a species? • A classification of plants and animals • Classification of the plant kingdom • Classification of the animal kingdom ■ Glossary of terms occurring in the Roll Call. PREVIEW It is hoped that this section will be freely used by the student. It is not expected that the classification of plants and animals will be learned by rote, but rather used for reference from time to time as new forms are seen. By this means the diagnostic characteristics of different phyla and classes will gradually be learned as needed, and the relationship of one group to another become more apparent. In order to enjoy hikes or longer trips, the student should be able to recognize the larger groups of the plant and animal kingdoms. Fortunately there are museums, botanical gardens, and zoological parks to which one may refer, all the more intelligently of course if he has himself first discovered living animals and plants. Identifying plants and animals correctly becomes more of a plea.s- ure than a task, if the principles of scientific, as well as common, nomenclature are understood. Both scientific and common names will be encountered. The former are written in the dead, unchanging Latin language, and are of more universal usefulness, since the latter are frequently misleading and confu.sing, as more than one common name may be applied in different countries, or in different parts of the same country, to a single plant or animal. For example, the common "chain pickerel" is listed under the scientific name of Esox, indicating the larger or generic group to which the fish belongs, and niger, which is its specific name, but it has at least twenty-two connnon names in different parts of this country. Here are a few of them: black pickerel, pike, common eastern pickerel, duck-bill pickerel, green p'lkv, little pickerel, and lake pickerel. The terms pike, pickerel, aii.l lake pickerel are also quite commonly used in some parts of the country to designate another fish, the great northern pike, Esox lucius. In still other localities "pike" refers to an entirely different group, the pike-perches, belonging to the genus Stizostedion. This examph^ will 63 64 NATURAL HISTORY serve to indicate the necessity for the use of Latin scientific names in classification. There may be other members of the genus Esox, but there is only one niger, although varieties of the same are possible in different environments. The terms of genus and species were intro- duced to the scientific world in the middle of the 18th century by Carl von Linn^ (1707-1777), of Sweden. The study of classification is called Taxonomy and is subdivided into zoological taxonomy, or Systematic Zoology, and botanical taxon- omy, or Systematic Botany. Early Contributions to Classification In order to secure an idea of the development of taxonomy it is necessary to go back several hundred years to some of the earlier biologists and glance at a few of the contributions of these students. Obviously such an excursion can hope to touch upon only a few of the more important workers. Logically, one should go all the way back to Aristotle's time, but lack of space forbids such an interesting excursion. Consequently we must confine ourselves to the immediate forerunners of Linne, or Linnaeus as he came to be called, who intro- duced the concept of binomial nomenclature and with it a more ade- quate idea of genus and species. In 1576, Matthias de TObel published an important work on plants. This was an attempt to arrange plants according to their structure. He took the shape of the leaf as the basis for this classification, and it led him to put such things as ferns in the same group with trees because the fronds of the fern bore a superficial resemblance to the needles of the hemlock. Another botanist was the Swiss, Kasper Bauhin (1560-1624), who described in order 6000 species of plants, beginning with the ones he considered most primitive. He approached the concept of genus and species, because he grouped together plants which resembled one another externally. John Ray (1627-1705) deserves recognition along with Linnaeus as the founder of the science of systematic biology. This enthusiast published a catalogue of British plants in 1670 and later works (1703) in which he introduced and explained the groups of Monocotyledons and Dicotyledons. He also made less extensive contributions to the classification of animals. Some of these he published with his good friend Willughby (1635-1672). Ray gave evidence in his work that he realized the fundamental differences between genus and species ; furthermore, he had the keenness to group together both related plants ROLL CALL 63 and animals. Ray also advanced the idea that fossils are extinct species. Linnaeus was born in 1707, the son of a Swedish clergyman. Ho would have been destined to become a cobbler had it not been for the influence of a physician who recognizcnl the lad's abilities. To make a long story short, he finally secured his medical degree, aided in no small amount by the contributions of his fiancee, and eventually became a professor of natural history at Upsala. It seems that Lin- naeus had a passion for natural history and for classifying everything which came to hand. He initiated several changes in the study of systematic biology, many of which are still in use today. Binomial Nomenclature The most important contributions of Linnaeus center about (1) brief, clear, and concise diagnoses ; (2) sharper divisions between groups ; and (3) a definite, clear-cut system of scientific terminology, known as hinomial nomenclature. These innovations appeared in the 1753 edition of Species Plantarum and the 1758, or tenth, edition of his great work, the Systema Naturae. The tenth edition of this latter work is taken as the starting point of zoological nomenclature. Linnaeus divided the plant and animal kingdoms into Classes, Orders, Genera, and Species. This was a great step over the use of popular common descriptive terms, as you can now appreciate if you refer back to the example of the pickerel. However, a big mistake made by Linnaeus was his concept of fixity of species. In 1898 the International Congress of Zoology appointed an inter- national commission which drew up a set of rules ajjplying to the divisions of the animal kingdom. Thus classification today is really an expansion of the Linnaean system which now includes in the case of the animal kingdom, for example, the following : Animal Kingdom — is made up of Phyla — each of which is composed of Classes — in turn made up of Orders — then Families — and finally Genera — and Species. In the plant kingdom a comparable arrangement is utilized, beginning with Divisions (= phyla). 66 NATURAL HISTORY Law of Priority In describing species it sometimes happened that more than one person described the same form, giving it different names. In such cases the name assigned by the one who first described it is used, the second being considered a synonym. This is the reason for writing the describer's name and date of pubHcation after the specific name. Ordinarily the date and frequently the describer's name is omitted. Thus the true daisy is properly Bellis perennis, Linn. 1758, or the English sparrow, Passer domesticus, Linn. What Is a Species? We have taken a glimpse at the contributions of some of the con- temporaries and near contemporaries of Linnaeus and have gained a sHght concept of the problems these early workers faced in defining and describing a species. Biological scientists of today are still working on this problem. The principle involved is readily under- stood when we look at a sheep, a cat, and a dog. One can easily sepa- rate them from each other, various cats being put in one group and diverse dogs in another. All domestic cats, whether they be the alley variety or pet Persians, and all dogs, whether they be a "dog in the manger" or "man's best friend," fall into well-marked and easily separable groups, known as species. To continue further, one finds in looking over representative mammals that many other species such as the jaguars, ocelots, jaguarundis, and cougars, all have certain char- acteristics in common with our domestic cats. These characteristics are size, build, shape of head, nature of claws, teeth, and fur. The zoological systematist, therefore, places them in one larger group or genus which is called Felis, a relationship expressed below. Kingdom. Animal . Plarzb 'Phylum - Onovdcdxx . Arthropod a. 'Koiltx£r. Gla55 - "rlocmiTacxIioD , Pisces .Pept/ilia, Aves.cstc-. Order"- Carpi vonx . 'R'ocCe-aticc.Chiroptera,ete- Kimil/- PeUcLcce^ . coa5Cir^ Class "C. Ascomycetes gac ■{ ^UT7^i wheat 2 smuts ^"puff balls SUBDIVISION B FU-NGI 3ctsicCiomycetes smutts ondrtxstls ROLL CALL 71 Chakacteristics : Chlorophyll associated with carotin and xantlKjphyll; marine or fresh water organisms, or inhabitants of moist hind; nucleus and one or more chloroplasts present; starch synthesizeil in pi/rentmls; plant composed of single cells, colony, filament, or plate of cells; most species produce motile i-eproductive cells (zoospores) ; botli equal {iso-) and different sized (hetero-) gametes present. Class III — Charophyceae — Stonewarts {Chara and Nitella). Characteristics: Vegetative body consisting of long, jointed stems with whorls of short branches arising at joints {nodes) ; asexual spores absent ; more complicated antheridia and oogonia than found in Thallophytes borne along branches. Class IV — Phaeophyceae — Brown algae, kelps, rockweeds, sargassum {Lami- naria, Fucus, Ulopteryx). Characteristics: Multicellulate; exclusively marine ; brown color (due to one or more brown pigments associated with chlorophyll) ; normally found in intertidal zone. Class V — Diatomaceae — Diatoms (Meridion, Diatoma, Denticula Fragillaria). Characteristics : Large group of unicellular algae ; related in color to brown algae ; common as plankton organisms in both fresh and salt water ; siliceous walls. Class VI — Rhodophyceae — Red algae {Nemalion, Polysiphonia, Phyllophora, Corallopsis) . Characteristics : Mostly marine ; characteristically reddish in color ; branched, vegetative body filamentous and delicate; grow entirely sub- mersed; cell wall often thick, gelatinous; color due to pigment, phyco- erythrin ; no motile cells ; sexual reproduction highly specialized. Subdivision B — Fungi — Fungi, bacteria, and molds. Characteristics : Chlorophyll lacking; exist as parasites or saprophj-tes. Class I — Schizomycetes — Bacteria (Diplococcus, Staphylococcus, Streptococ- cus, Bacillus, Bacterium, Spirillum). Characteristics : Unicellular plants, usually without pigment, dividing in one, two, or three planes; apparently structureless, but probably con- taining a diffuse nucleus. Class II — Saccharomycetes — Yeasts (Saccharoimjces). Characteristics: Sometimes regarded as reduced Ascomycetes; single cells with definite nucleus ; cytoplasm and sap cavity ; buds a.sexua!ly ; under unfavorable conditions forms four spores, in a modified ascus. Class III — Myxomycetes — Slime fungi, slime molds (Hemitrichia, Coma- tricha, Trichamphora) . Characteristics: Border-Hne plants; spores borne by fruiting bodies, germinating into small, naked mass of protoplasm without a wall ; indi- vidual cells fuse, forming a Plasmodium. Class IV — Phycomycetes — Algalike fungi, molds, and bliglits {Saprolegnia, Mucor). H. w. h. — 6 72 NATURAL HISTORY Hcpa'ticoce 1 L vei^-NVortS BRYOPHyfA , iverwor ts , mosses r^^^-Sporxs capsule CD 6'phcc^nixxn peoct moss •e^C Ccctbcarinia acomrrion moss arche^nium Qnt"hei4diuTn of corartiorL moss Olo-SS IC ROLL CALL Characteristics : Resemble algae ; plant body consists of filaments {hyphae) which are not divided into cells by cross walls ; multinucleate. Class V — Ascomycetes — Sac fungi {Morchella, Exoascus, Microsphaera). Characteristics: Includes over 20,000 species, mostly saprophytes or parasites; body consists of branching mycdium throughout substratum and a definite fruiting body at surface ; produce spore sacs {asci) contain- ing eight spores (ascospores) ; group of asci embedded in sterile hyphae may or may not be surrounded by protective envelope. Class VI — Basidiomycetes — Basidia fungi, smuts and rusts, wheat rust (Puccinia), puff balls. Characteristics: Large and varied group; specialized reproductive struc- ture (basidium) is swollen terminal cell of hypha, in mushrooms the ija- sidium usually bears four basidios pores, each carried on a delicate stalk {sterigma) ; sexual reproduction rare; lichens — composite plants in which algal cells are entangled in mycelium. Usually regarded as a parasitism of algal member rather than an example of symbiosis. DIVISION II — BRYOPHYTA — Liverworts and Mosses. Characteristics : Alternation of generations in which sexual (gametophyfic) stage dominates; asexual (sporophyiic) stage typically parasitic upon the gametophyte ; archegonium and nmlticcUulate antheridium pre.sent ; gametophyte contains x number of chromosomes while the 2 x number occurs in the sporophyte; careful study of archegonium reveals typical flask shape, with sterile cells (neck and venter) surrounding the egg and associated cells ; antheridium more or less stalked and consisting of layer of jacket cells surrounding cuboidal sperm mother cells. Class I — Hepaticae — Liverworts (Marchantia, Riccia). Characteristics: Intermediate between green algae and higher plants; thaUus flattened and attached to soil by rhizoids; growth by repeated division of single large apical cell. Class II — Musci — Mosses (Sphagnum, Polytrichum, Catherinia). Characteristics: In every habitat except salt water; very common in alpine and arctic regions; gametophyte erect, consisting of stalk with spirally arranged leaves ; attachment by rhizoids. 74 NATURAL HISTORY Subdivision A pi"! mi live, vasculctr plants WTJVjynia Devonian plant ^ RsilophyCalcs ^-"^ -.i <'..^ .(2)p5notum (3)Tmesipteris $UBDIVI5I0M B Lycopsicta club mosses sporopVr/te y,- Lycopodium. :5UBD I VISION C ophejiopsido. / TI?ACH£OPHYTA vascular' plants Subdivision!) Pberopsida ferns . seed plants, polkw >^ cell i eicxssi Filicineae arc Gymr205perma<2 dicotyledon ii20i20Ccit/kfGn AndioSpermae oclVc, iTioLple . elm , ccc . ROLL CALL <■> DIVISION III - TRACHEOPHYTA - Vascular Plants. Chakacteristics : Fibro- vascular system for transportati..,i „f raw mate- rials up and food down; separation of specialized cl.l..njphyll-lK.ari..K tissue ; adaptation to absorption of water from soil. Subdivision A — Primitive Vascular Plants — {Psilotum and Tmesipteris, Rhynia). Characteristics : Fossil primitive vascular plants giving rise in tliree lines to Lycopsida, Sphenopsida, and Pteropsida. Subdivision B — Lycopsida — Club mosses, ground pines (Lycopodium, Selagi- nella). Characteristics: Stem clothed with small, numerous, spirally arranged leaves; sporangia borne on upper surface of spowphyll; latter usually grouped into terminal cones. Subdivision C — Sphenopsida — Horsetails (Equisetum). Characteristics : Hollow, typically jointed stems, bearing small leaves at joints (nodes); stems ribbed; diaphragms often across stem at nodes; sporangia borne in groups on stalked shield-shaped structures forming terminal cones ; ribs opposite fibro-vascular bundles which are associated with small air-filled canal; abundant in Paleozoic age; now only about 35 species. Subdivision D — Pteropsida — Ferns and seed-bearing plants. Characteristics: Typically large leaves; sporophytic generation domi- nates ; sporangia relatively large. Class I — Filicineae — Ferns. Characteristics : Small, herbaceous plants with typical pinnately com- pound leaves (fronds) ; stem relatively weak and inconspicuous ; roots numerous but do not form an extensive system; small sporangia borne on lower surface of leaf in groups usually protected by membrane (iiulu- simu) ; spore germinates, forming small, thin gametophyte (prothallus), which in turn bears antheridial and archegonial structures. About 15,000 species, some of which reach a height of 30 feet. From forms like the ferns evolved the higher vascular plants whic-h dominate the earth's surface today. Class II — Gymnosperil\e — Evergreens, pines, hemlocks, spruces, junipers. Characteristics: Seeds freely exposed to air; usually nondeciduous types; megaspore retained within megasporangium where it germinates producing female gametophyte; integument, a new structure, enclo.ses a sporangium and embryo sac; reduced male gametophyte transferred directly to vicinity of female ; male obtains access to female gametophyte by new structure (pollen tube); young sporophyte develops in contact with and at expense of parental sporophyte; gametophyte with haploid (x) number of chromosomes entirely parasitic upon sporophyte. Mem- bers of this group are phylogenetically ancient; only about 450 living species. Class III — Angiospermae — Deciduous trees and plants. Dicotyledons, oak, maple, beech ; Monocotyledons, corn. 76 NATURAL HISTORY Class I SarcocCina (2) ATCella C3) "'-J "Radiolarla Clctss IE Kastigbphorec PROTOZOA one cellecC animals c'C®) \^ m^ •«oY^ ^^-^ ctsexuctl \^^9© Cycle in. / 1 mosquito Bisali- PlasmocCium. Class JSL 5porq3oa Vorticella (2) ^^ Stentor^ (3) 5tyionych i cc Class IST ly^ftcsoricc ROLL CALL 77 Characteristics: Seeds enclosed by a case (ovary), so that pollen Rrain does not reach the ovule but rests on surface of carpel ; closure to form case probably arose by folding together of edges of megasporophyll {carpd) ; pollen received on special organ (stigma) at tip of ovary. Members of this group probably were derived from gymnosperm stock ; now number 135,000 species and are subdivided into dicotyledons and monocotyledons which may be separated by the following ciiaracteristics : Dicotyledons Monocotyledons Number of cotyledons of embryo . . . two one Vascular bundles . . arrange to form vas-cylinder enclosing pith scattered Leaves open venation, veinlets end- ing freely in margin, which is often toothed or lobed closed venation (i.e. parallel) margin therefore entire Flowers in sets of four or five in sets of three CLASSIFICATION OF THE ANIMAL KINGDOM (mainly after Hegner) All members of the animal kingdom are characteristically free-moving organ- isms; generally capable of assimilating organic foods; rarely possessing chloro- phyll ; cell membranes composed of protoplasm or proteins. PHYLUM I — PROTOZOA — One-celled animals. Characteristics : Single cells or colonies of loosely aggregated unspecial- ized cells ; rarely differentiated into germ cells ; 8500 species. Class I — Sarcodixa — Naked protozoa (Ameba,^ Arcella, Radiolaria). Characteristics : Locomotion by means of pseudopodia. Class II — Mastigophora — Flagellate protozoa (Euglena, Trypanosoma). Characteristics : Locomotion by means of flagella. Class III — Sporozoa — Parasitic protozoa (Lankesieria, Myxosporidia, Plas- modium). Characteristics : Xo organs of locomotion in adults ; endo-parasites repnn ducing by schizogony and spore formation. Class IV — Infusoria — Ciliate protozoa (Vorticella, Stentor, Stylorjychia, Paramecium). Characteristics : Locomotion by means of cilia. 1 See footnote at beginning of classification of Plant Kingiioni. 78 NATURAL HISTORY Class T Calcarea KexactiY^ell ioCa Grantia (1^ Euplectella PORIFERA sponges (D @ Spongilla fresh- wcxter-Spon^e ELc5pong"ia ROLL GALL 79 PHYLUM II — PORIFERA — Sponges. Characteristics : Usually considered as diploblastic animals ; body con- sists of a perforated (inhalent pores) cylinder, leading to central canal opening to outside through exhalent pore; [peculiar flagellate, collared cells (choanocyfes) typically present; body structure frequently compli- cated by budding ; 2500 species. Class I — Calcarea — (Grantia). Characteristics : Small marine sponges possessing one-, two-, or four-rayed calcareous spicules. Class II — Hexactinellida — Deep-sea sponges (Euplectella). Characteristics : Sponges with six-rayed siliceous spicules. Class III — Desmospongia — Finger sponge, bath sponge (Chalina, Spongilla, Euspongia) . Characteristics: Diverse groups of sponges possessing spicules of silicon, not six-rayed, with spongin, or a combination of spicules and spongin. 80 NATURAL HISTORY Olass I H/cCro3oa; (1^ Otoe-lia C3) PhyBcclicc Portuguese mar?- of -^/ar COELENTERATA (IV Aurelia Class IE 5c/pbo3)Oa Secc anerrzor^e Astra n^ioc Class HE Antbo^oa ROLL CALL ni PHYLUM III — COELENTERATA — Jellyfishes and corals. Characteristics: Mostly marine; radially syinniotrioal ; diploblastic- ani- mals with a noncellular layer of niesoglea lying between; po.ssL's.sing tentacles, armed with nematocysts; body composed of a single gastro- vascular cavity ; 4500 species. Class I — Hydrozoa — Fresh-water polyps, jellyfishes, and a few stony corals {Hydra, Obelia, Physalia). Characteristics : Mostly marine ; usually hydroid and jellyfish forms occur in the same life cycle; the jellyfish (medusae) po.ssess a shelflike velum extending inward from the margin toward the mouth (manubrium) ; a few species like Hydra possess no medusoid stage; the stony coral, Millepora, represents a colony with a coral-like skeleton of calcium car- bonate. Cl.\ss II — Scyphozoa — (Amelia). Characteristics: Entirely marine, with the medusoid stage dominating; produced from subordinate polyp by terminal budding (strobilalion) ; velum usually absent; lobate, typically eight-notched. Class III — Anthozoa — Sea-anemones, sea-pens, and stony corals (Metridium, Pennatula, Astrangia, Sagartia). Characteristics : Entirely marine with medusoid stage suppres.sed ; organ- isms characterized by an introverted ectodermal mouth (sto7nodaeum) anti vertical radiating mesenteries extending inward from the body wall; one, two, or more rarely three cihated gullet grooves (siphonoglijphs) carry a stream of oxygenated water to interior. Corals produce islands and reefs; in addition they sometimes protect a shore from wave action. 82 NATUIIAL HISTORY a) CTENOPHORA ytonna iphorroc Comb i<2-^Vy "'^i^^'S^^w^'i^pM C2) Venas* gxindLie ROLL C^LL 8:{ PHYLUM IV — CTEXOPHORA — Sea-walnuts {Cestus, Hormiphora, Mnemi- opsis) . Characteristics : Eight radially arranged rows of comhjlike plates typi- cally present; fundamentally bilaterally syninictrical; with a distinct mesodermal layer (therefore triploblastic); no nematocysts : 100 species. 84 NATURAL HISTORY Class I Tarbellaria #'G>' ■■^ m 1. 'Planoria ^' Microsbmum Clccss IT TrematooCa twoflUKZf Yrom, Turtles mouth 3 'PnGUtTiono®2ia]f7r?elicCa Polygordlius cMass "K Cl2aetopocCa Nsreis Chaetoptert£5 c\ccro..^»/'or-m. tube vorm. (^3) ANNELIDA segmentecC "^orms KirucCo •■medicinal leech Class HL Hirudinaa Lambricud Garthvorm 1 fhaecolosoTQa arrow vorm Class sr Csrephyrea Cla&S"y Chaetognatha ROLL CALL y , PHYLUM I X — ANNELIDA — Segmented worms. Characteristics : Segmented animals bearing distinct head, digestive tube, coelom, and sometimes nonjointed appendages; frequently supplied with chitinous bristles (setae) ; 6500 species. Class I — Archiannelida — (Polygordius). Characteristics : Marine worms lacking setae or parapodia ; trochophore larvae present. Class II — Chaetopoda — Clam worms, tube worms, earthworms (Nereis, Glycera, Chaetopierus, Lumbricus). Characteristics : Members of this class marine, terrestrial, or fresh water ; paired setae characteristically arranged in integumentary pits or upon parapodia ; further subdivision based upon number of setae present : Oligochaeta, a few ; Polychaeta, many. Class III — Hirudinea — Leeches (Hirudo, Glossiphonia). Characteristics : Hermaphroditic, dorso-ventrally flattened annelids with 32 body segments, two suckers, one surrounding mouth, the other the posterior end ; setae and parapodia absent ; growth of mesenchyniatous cells reduces coelom. Class IV — Gephyrea — Sipunculid worms (Phascolosoma). Characteristics : Non-segmented when adult, without setae or parapodia ; characterized by a large coelom and trochophore larvae. Class V — Chaetognatha — Arrow worms (Sagitta). Characteristics: Small, transparent, marine invertebrates with well- developed body cavity, alimentary canal, nervous system, two eyes; lobes on sides of mouth armed with bristles which aid in capturing food. 94 NATURAL HISTORY Class I Asteroidea Clccss X Ophijiroidea OphioglypVja brittle -star- ECHINODERMATA starfishes, etc m'w ■'-i'lyfi"'-:.- K'#^- Her) Arbcxoicx sect urcHin i\» Thj/one. sea: - cucuiTzber EcVjinarachniuS sccncC dCollocr Class is: Holothuroidea Class IE Echirzoidea l^er^tacrmus Class ^ Crinoidea ROLL CALL 95 PHYLUM X — ECHINODERMATA — Starfishes, sea-urchins, sea-curumbors. Characteristics : Adults radially symmetrical (pentamerous) ; marine ; tube-feet, water vascular system, distinct alimentary canal, large body cavity usually present ; frequently a spiny skeleton of calcareous plates ; larvae bilaterally symmetrical ; 4800 species. Class I — Asteroidea — Starfishes (Asterias, Mediaster). Characteristics : Typically five rays or arms not marked off from central disk ; each ray possessing ventral ambulacral groove through which numer- ous tube-feet extend ; gastric pouches and hepatic caeca extend into rays ; blunt spines and pedicellariae present; respiration by dermal branchiae. Class II — Ophiuroidea — Brittle-stars (Ophiopholis, Ophiothrix, Ophioglypha, Ophioderma). Characteristics: Typically pentamerous with arms sharply marked off from disk ; no ambulacral groove ; hepatic caeca and anal opening lacking. Class III — Echinoidea — Sea-urchins, sand-dollars, spatangoids (Arbacia, Strong ylocentrotus, Echinarachnius, Spatangus, Moira). Characteristics : Typically pentamerous without arms or free rays ; test of calcareous plates bears movable spines; i)ediceilariae usually three- jawed ; mouth with five conspicuous teeth constituting part of Aristotle's lantern. Class IV — Holothuroidea — Sea-cucumbers {Holothuria. Thyone, Leptosy- napta). Characteristics : Long, ovoid, soft-bodied cchinoderms ; tentacles about mouth; body wall muscular ; skeleton greatly reduced. Class V — Crinoidea — Sea-lilies or feather-stars {Antcdon, Halhromelra, Co- rnadinia, Pentacrinus). Characteristics : Usually five branched arms, possessing featherlike divi- sions (pinnules) ; aboral pole sometimes possessing cirri but more gener- ally a stalk for temporary or permanent attachment ; a few modern types, most forms known as fossils. 96 NATURAL HISTORY Class I AiTiphineura Class I GastropocCa Class HE ScaphopocCa 1 5cb r2och itoio. chiton Helix Iccnd. snccil marine Snail C5). Limccx tootlri snocil MOLLUSC A clams , Snccils, etc soctllop Class 3Sr PslecypocCa ra^or-shell Cicom (2) OCtopLCS Class ^ Cephalopoda ROLL CXLL 97 PHYLUM XI — MOLLUSCA — Snails, clams, and oysters. Characteristics : Unsegmented, bilatorally synunotiical, triijloblastic ani- mals bearing a shell, muscular foot, and mantle; four main pairs of nerv- ous ganglia ; 70,000 species. Class I — Amphineura — Chitons (Chaetopleura, Ischnochiton) . Characteristics: Bilaterally symmetrical; shell typically composed of eight transverse calcareous plates with many pairs of gill filaments. Class II — Gastropoda — Snails, slugs, whelks {Umax, Physa, Helix, Lymnaea, Campelotna, Busy con). Characteristics : Asymmetrical animals with well-developed head ; spi- rally-coiled shell. Class III — Scaphopoda — Elephant's-tusk shells (Dentalium, Siphonodenta- lium). Characteristics: Both shell and mantle tubular; protrusible foot ; rudi- mentary head. Class IV — Pelecypoda — Clams, mussels, oysters, and scallops {Ensis, Ano- donta, Venus, Teredo, Ostrea, Pecten). Characteristics: Usually bivalved shells with two-lobed mantle; no head ; body laterally compressed ; bilaterally symmetrical. Class V — Cephalopoda — Squids, cuttlefishes, octopus, nautilus (Loligo, Polypus, Dosidieus). Characteristics: Bilaterally symmetrical; with foot divided into siphon and arms provided with suckers; well-developed nervous system con- centrated in head; mouth possesses strong jaws. /.<^ V 98 NATURAL HISTORY ClctSS I Cmstacea Olci-ss -jn Oiiychopbortt 4 PecLiculus /^TXi-jh Class IS" liasecta "Po-pilio . ciccss"sr Aractiiaoidea ROLL CALL ^,j PHYLUM XII - ARTHROPODA - Lobsters, crabs, spider., millir>odes insects. ' * ' Characteristics: External evidence of segmentation, body at least beine divisible into a well-defined head, thorax, and abdomen; jointed append- ages ; chitinous exo-skeleton ; nervous system of ladder f vpo witl, tondcnry toward concentration in head region; main longitudinal blood vessel with heart dorsal to alimentary canal; coelom reduced; body cavity filled with blood (hemocele) ; 640,000 species. Class I — Crustacea — Crayfish, crabs, water fleas, barnacles, sowbugs (Cam- barus, Callinectes, Gammarus, Asellus, Trior thrus). Characteristics: Mostly aquatic; usually bearing gills; with two pairs of antennae (feelers) ; chitinous exo-skeleton ; body divided into head, thorax, and abdomen ; head and thorax sometimes fused {cephalolhorax) ; further subdivision depending largely upon characteristics of carapace. Class II — Onychophora — Annelidlike arthropods (Peripatus). Characteristics: Tropical, primitive, wormlike tyi)os j)resumably inter- mediate between the segmented worms and the arthropods; excretory system of annelid type (nephridial) ; respiratory organ resembles tracheae of insect group ; external appendages ringed, suggesting segmentation of arthropods. Class III — Myriapoda — Centipedes and millipedes {Scolopendra, Spirobolus). Characteristics : Body relatively long and definitely nietamcric ; one pair of antennae ; appendages segmented ; legs similar ; respiratory sys- tem of tracheal type ; in millipedes there are two pairs of legs per somit«, in centipedes one. Class IV — Insecta — Insects, as butterflies, grasshoppers, beetles, bees. Characteristics: L^sually possess wings; one pair of antennae; tracheal respiratory system ; segmented legs. Order 1 — Thysanura — Bristletails, Silverfish (Lepistna, Campodea, Thermobia). Characteristics : Wingless arthropods ; primitive ; probably derived from wingless ancestors ; 11 abdominal segments; chewing mouth parts; usu- ally two or three long, threadlike, segmented caudal appendages; less than 20 species in the United States ; no metamorphosis. Order 2 — Collembola — Springtails (Archorules). Characteristics : Primitive wingless insects with chewing or sucking mouth parts; four segmented antennae; usually no tracheae; six abdominal segments; a springing organ (furcida) present on ventral side of fourth abdominal segment in most species ; no metamorphosis. Order 3 — Orthoptera — Grasshoppers, cockroaches, walking sticks {Melanoplus, Periplaneta, Diapheromera). Characteristics: Members of this order are characterized by two pairs of wings (sometimes greatly reduced) ; the fore wings usually thickened. sometimes leathery ; hind wings folded fanlike beneath fore wings ; biting mouth parts ; gradual or simple metamorphosis. 100 NATURAL HISTORY Order 4 — Isoptera — Termites or white ants {Reticulitermes) . Characteristics : Four similar wings lying flat on back when at rest ; workers are wingless; chewing mouth parts; abdomen joined directly to thorax ; gradual or simple metamorphosis. Orders — Neuroptera — Dobson flies, alder flies, lacewings, ant-lions {Corydalis, Chrysopa, Myrmeleon). Characteristics : Four membranous wings with many veins ; chewing mouth parts ; larvae carnivorous ; tracheal gills usually present on aquatic larvae; the larvae of the horned Corydalis known as hellgrammites are used by fishermen as bait ; complete metamorphosis. Order 6 — Ephemerida — Mayflies (Ephemera). Characteristics : Mouth parts of adult vestigial ; two pairs of membra- nous, more or less triangular, wings ; fore wings larger than hind wings ; caudal filaments and cerci very long; aquatic larvae breathe by tracheal gills, usually located on either side of abdomen ; adult's span of life short ; mouth parts poorly developed, probably making organism incapable of taking food; nymph remains one to three years in water; adults moult within 24 hours after acquiring wings, therefore called sub-imagos ; gradual or simple metamorphosis. Order 7 — Odonata — Dragonflies and damsel flies (Macromia, Agrion). Characteristics : Chewing mouth parts ; two pairs of membranous veined wings; characteristic joint (nodus) on anterior margin of each wing; eyes large, compound ; nymphs are aquatic ; gradual or simple metamorphosis. When at rest dragonflies hold their wings horizontally and at right angles to body, while damsel flies maintain theirs vei-tically. Order 8 — Plecoptera — Stone flies (Allocapnia, Taeniopteryx). Characteristics : Chewing mouth parts often poorly developed in adults ; two pairs of wings; hind wings usually larger and folded beneath fore wings ; nymphs aquatic, bearing filamentous tracheal gills ; usually be- neath stones in flowing water; gradual or simple metamorphosis. The salmon fly, Taeniopteryx pacifica, is a dangerous pest in the State of Washington because it destroys buds. Order 9 — Corrodentia — Book- and bark-lice (Trodes). Characteristics : Either wingless, or two pairs of membranous wings char- acterized by a few prominent veins; fore wings larger than hind wings; when at rest held over body like sides of a roof; chewing mouth parts; gradual metamorphosis. Book-lice often eat paper and bindings of old books. Order 10 — Mallophaga — Chewing lice or bird-lice (Menopon, Trichodectes). Characteristics : Chewing mouth parts ; wings absent ; eyes degenerate ; metamorphosis gradual or wanting. Members of this group are ecto- parasitic upon hair and scales of birds and mammals. Order 11 — Embiidina — Emhiids {Emhia). Characteristics : Chewing mouth parts ; wingless or possessing two pairs of delicate membranous wings with few veins ; cerci present on two seg- ments ; males usually winged, females wingless ; gradual metamorphosis. These organisms live under stones, etc., in tunnels formed of silk produced in tarsal glands. ROLL CALL 101 Order 12— Thysanoptera — Thrips (Thnps, Franklinella, Crypiolhnps). Characteristics: Piercing mouth parts; either wingless or with two pairn of long, narrow membranous wings, practically veinless; large, free pro- thorax; feet clawless but possessing small protrusible membranous sacs for clinging; manj^ parthenogenotic ; gradual metamorphosis. Order 13 — Anoplura — Sucking lice {Pediculus, I'hthirins). Characteristics: Wingless ectoparasitic lice with piercing and sucking mouth parts; eyes poorly developed or absent; parasitic on bodies of mammals ; gradual metamorphosis. At least two species, the head louse and crab louse, occur on man. Order 14 — Hemiptera — True bugs {Artocorixa, Lethocercus). Characteristics : Either wingless, or with two })airs of wings ; in such cases fore wings are thickened at base ; mouth parts adapted for piercing and sucking; gradual or simple metamorphosis. Members of this group con- tain many interesting and sometimes economically important forms. The water-boatmen (Corixidae) have long, flat, fringed metathoracic legs which are adapted for swimming. These peculiar forms carry a film of air about body when under w^ater. The leaf bugs (Xeridae) are frequently numer- ous and injurious to plants. Bedbugs (Cimicidae) have been accused of transmitting various diseases. The cabbage bug does damage to garden vegetables. Order 15 — Homoptera — Cicadas, aphids, leaf-hoppers, and scales {Euscclis, Empoasca, Rhopalosiphum) . Characteristics : Mouth parts adapted for piercing and sucking ; two pairs of wings of uniform thickness held over back like sides of a roof. The cica- das (Cicadidae) are better known as the "seventeen-year locust." Plant- lice (Aphididae) are mostly small green insects that suck juices from plants and have a gradual metamorphosis. Order 16 — Dermaptera — Earwigs (Anisolabis, Labia). Characteristics : Either wingless, or possessing one or two pairs of wings ; in such cases fore wings are small and leathery, meeting in straight line along back; chewing mouth parts ; gradual metamorphosis. Earwigs are nocturnal and feed principally upon vegetation. Order 17 — Coleoptera — Beetles and weevils {Hydrous, Dytiscits, Photinus, Anthonomus). Characteristics : Either wingless or with two pairs of wings, fore wings being hard and sheathlike {elytra); hind wings membranous and are folded two ways under elytra; large movable prothorax; chewing mouth parts; complete metamorphosis. Many forms are found in this group. as the tiger beetles, fireflies, click beetles, whirligig, ladybird, and leaf beetles. Order 18 — Strepsiptera — Stylopeds {Xenos). Characteristics: Mouth parts reduced or wanting; nutrition by absorp- tion; males possessing club-shaped fore wings and large membranous hind wings ; females wingless and legless ; life cycle complex ; para.sitic on bees, wasps, and homopterous bugs. 102 NATURAL HISTORY Order 19 — Mecoptera — Scorpion-flies {Panorpa, Bittacus). Characteristics : Members of this group are wingless or characterized by two pairs of long membranous wings containing many veins; head pro- longed into beak; antennae long and slender; mouth parts adapted for chewing ; males with olasping-organ on caudal extremity resembling sting of a scorpion ; metamorphosis complete. Order 20 — Trichoptera — Caddis flies {Phryganea, Molanna). Characteristics : Adults with vestigial mouth parts ; two pairs of mem- branous wings obscurely colored by long silky hairs and narrow scales; antennae long and slender; metamorphosis complete; larvae and pupae aquatic, constructing portable cases of sand grains or vegetable debris fastened together with silk from modified salivary glands. Order 21 — Lepidoptera — Butterflies and moths {Tinea, Alsophila, Papilio). Characteristics : Wingless, or with two pairs of membranous wings cov- ered with overlapping scales; sucking mouth parts coiled beneath head consist of two maxillae fastened to form a tube; metamorphosis com- plete ; larvae known as caterpillars ; many species known. Order 22 — Diptera — Flies and mosquitoes (Tipula, Culex, Prosimulium, Musca, Drosophila). Characteristics : One pair of membranous fore wings on mesothorax, or wingless ; knobbed threads (halteres) on metathorax represents hind wings ; mouth adapted for piercing and sucking, forming proboscis ; larvae known as maggots ; complete metamorphosis. Order 23 — Siphonaptera — Fleas (Ctenocephalus, Pulex). Characteristics : Wingless insects with laterally compressed body ; head small ; no compound eyes ; mouth adapted for piercing and sucking, legs for leaping; metamorphosis complete; ectoparasites of mammals and more rarely birds. Order 24 — Hymenoptera — Saw flies, ichneumon flies, ants, wasps, and bees (Cladius, Ophion, Formica, Vespa, Apis). Characteristics : Wingless or with two pairs of membranous wings ; fore wings usually larger ; venation reduced ; wings held together on each side by hooks (hamuli); mouth parts adapted for chewing or sucking; first abdominal segment fused with thorax ; complete metamorphosis. Class V — Arachnoidea — Spiders, scorpions, ticks, mites, and king crabs {Caddo, Lycosa, Phalangium, Buthus, Argas, Sarcoptes, Limulus). Characteristics : No antennae nor true jaws ; two of six pairs of jointed appendages modified for mouth parts; respiration by lung-books or tracheae; first pair of appendages usually contain poison glands, second pair used as jaws ; terminal portions as sensory organs ; body usually divided into anterior cephalothorax and posterior abdomen ; former bears four pairs of legs for locomotion. THE ANIMAL KINGDOM > KM Phvllm Chordata Arthropoda Mollusca Echinodermata Annelida (Annulata) Molluscoidea Platyhelminthes Nemathelminthes Troehelmi n thes Coelenterata Porifera Protozoa Claj; Mammalia Aves Reptilia Amphibia Pisces Miiior Cl asses Onychophora Crustacea Myriapoda Insecta Arachnoidea Examples Kmtimatek iNl-MMEU (IK I.IVIM. .Si'EciEM DkhciiiiiEU VERlEliUATES Man, cat, horse, bat, whale liirds, fowls Turtles, snakes, lizards, alli- gators I'rofis, toads, salamanders I'ishos Tunicates, Balanoglossus, etc. Total Chordata INVERTEBRATES Crayfish, crabs, water fleas, barnacles, sowliugs Centipedes, millipedes, etc. All true insects Spiders, scorpions, ticks, mites, and king crabs Total .\rthropoda Snails, slugs, clams, oysters Starfish, sand dollar, sea- urchin Earthworm, leeches Bryozoa, Ijrachiopods P'latworms, flukes, tapeworms Roundworms, Trichinclla, Filaria Rotifers, wheel animalcules Jelly-fishes, coral animals. Hydra Sponges Ameba, Paramecium, Euglena, malarial organ- isms, trypanosomes Grand total :{.7.')() l.l.."j(t() •4.000 1.7.">0 lIl.jOO 1,500 70 20,000 2,430 625,000 27.500 38.000 fi75,000 S0.(JO0 5,000 5,000 2,5(X) G.500 .3.500 1.500 5.000 3.000 15,000 S40,000 ' Modified from Metf-alf and Flint, Destmrtirp and Useful Insects. By pprmis.xjon r)f the McOrnw- Hill Book Company, publishers. The discrepancies between this table and the tt'Xt illusirato the pragmatic nature of taxonomy. H. W. H. — 8 104 NATURAL HISTORY Sub -phylum I HEKIC«ORT>ATA half - CL ' cViorcC Sub • pliylum H Urockorbata chor*vit/Vi "toccc-Vctoones "Mammalia ROLL CALL jqs PHYLUM XIII — CHORD ATA — Animals with notochord. Characteristics: All possess a dorsal supporting rod or notochord and pharyngeal gill clefts at some stage in life cycle; tubular nerve cord dorsal to digestive tract ; 36,000 species. Sub-Phylum I — Hemichordata — (Balanoglossus). Characteristics : Wormlike marine oiganisms of doubtful relationship that burrow in sand and resemble the larval echinoderms in development ; head- end with proboscis and collar; with or without a notochord. Sub-Phylum II — Urochordata — Tunicates and ascidians. Characteristics : Marine organisms with saclike covering {tunic) ; larvae resemble tadpoles, possessing notochord in tail; gill slits and endostyle present in pharynx. Sub- Phylum III — Cephalochordata — Lancelots (Amphioxus). Characteristics : Segmented primitive chordates, burrowing in sand ; lat- erally compressed ; notochord extending from anterior tip to tail. Sub-Phylum IV — Vertebrata (or Craniata) — Vertebrates. Characteristics : Animals with definite head, sense organs, closed circula- tory system, and axial notochord at some period in life cycle; skull and vertebral column present either in cartilaginous or bony stage. Super-Class A — Agnatha — Fossil, armored Ostracoderms, lampreys and hag- fishes (Cyclostomata). Primitive fishlike forms (Pterichthys, Petromy- zon). Characteristics: Animals w'ithout jaws; sucking mouth and primitive brain present. Super-Class B — Pisces — True fishes. Characteristics : Organisms with true jaws ; typically scaled ; charac- teristically aquatic; appendages developed into fins; two-chambered heart. 106 NATURAL HISTORY Clocss I Ela5mobranchn Class IE Holocepholi (1^ Chimaera spook fislT. (B) Superclass PISCES FISHES PROPER. 1£> Protopterus lungfish Class 12: 9^ 'Pe^rccc class "JT Tsleostei l\OLL CALL jy^ Class I — Elasmobranchii — Gristle-fishes {Sgualus, Raia). Characteristics: Cold-blooded fishlike vertebrates witli jaws; charac- terized by a cartilaginous skeleton, i)(>rsistent notochord and placoid scales; upper jaw suspended to ci-aniuin indirectly by means of ligaments and cartilages (hyostylic). Class II — Holocephali -^ Elephant-fishes (Chimaera). Characteristics : Immovable upper jaw fused with cranium (autostyiic) resembling higher forms; gill slits covered by flap (operculum); tail heterocercal. Class III — Ganoidei — Enamel-scaled fishes (Acipenser, Lepisosteus, Polyp- terus). Characteristics : More or less armored fish ; remnants of group dominant in Devonian seas; degenerating spiral valve in intestine associated with presence of pyloric caeca; scales usually rhomboidal, fitting together rather than overlapping; dorsal fin usually close to caudal fin. Class IV — Dipnoi — Lung-fishes (Neoceratodus, Lepidosiren, Protopterus). Characteristics: Semitropical fishes, passing dry season by aestivating in slimy cocoon ; during period of active life use gills, and while aestivating breathe air, the modified swim bladder acting as a lung; cycloid scales; auricle of heart partially divided. Class V — Teleostei — Bony fishes (Ctenolabriis, Perca, Gadus, Microptcrtis). Characteristics : Bony fishes, breathing primarily by gills ; well-develoiM-d operculate bones, cycloid or ctenoid scales ; tail homocercal. These fishes constitute about 90 per cent of all known varieties. 108 NATURAL HISTORY E OrcCe^r- (i) OrdiQr (2") jg^css amphibia Rftstorations Steg'ooepViali (1"^ Cccecilicc "blincC" ^vo^mUke amphibian. (C)5icperclass TE CLASS I AMPH 'RAPODA IBIA (1^ Triturus spottccC incvt Necturzxs Order (3) UrodLela . . . ., gtnpbibitt wttn taits OncCer- (4-) Anurcc taillC96 amphibia ROLL CALL ,„y Super-Class C — Tetrapoda — Four-footed vertebrates. Characteristics : Well-defined limbs witli hands and feet typically con- structed on plan of five digits; stapes or coluniolla present in ear; Rirdles adapted to bear weight on land; body divisible into neck and trunk, tail present. Class I — Amphibia — Frogs and salamander.'?. Characteristics: Cold-blooded, naked vertebrates undergoing a meta- morphosis ; usually with five-fingered limbs (pentadactylous) ; young u.sually aquatic, breathing by gills; adults using lungs and skin, u-suallj' air breathers. Order 1 — Stegoccphalia — Extinct fossil amphibians (Erynps, Loxomma). Ch.^racteristics : Fossil forms resembling amphibia, flourishing in car- boniferous age ; probably earliest four-footed air breathers. Order 2 — Apoda — Legless amphibia (Herpeles, Siphonops, Caecilia). Characteristics : Small, tropical, wormlike, often blind amphibia, burrow- ing in ground. Order 3 — Urodela — Salamanders {Desmognathus, Necturus, Cryptobranchus, Triturus). Characteristics: Tadpole-like tail retained throughout life; some never emerge from water; a few retain external gills in adult stage. Order 4 — Anura — Frogs and toads {Rana, Bufo, Ilyla). Characteristics : Tailless upon completing their metamorphosis ; capable of singing ; characterized by the possession of movable eyelids. no NATURAL HISTORY OrcCeJr, 1 , ^. RViy n cho ctepnou loc 'tJhe. old, t-imer-^'' Spherzodon OrcL©3~ 2 • Cro: turtle^ OrcCer -3 Chelonia turtles, tortoises ^ 'osoxers fish -like reptile^ sub ordter Soci^ria lixccr-cCs Plesiosaurs Pberodoc'tyls ./(2) -^^.^^.ffla^ (4) '' sub order Serper^teS Hiriosaurs SriccKe.S ^lant reptiles Ordei^ 4r 5c|uamata snokss , li3ards Orders 5-8 fossil rejjtiles Ichthyosouria ,Plesio5auria Ptcrc?ctactylia,T)inoscturia UOLL CALL ijj Class II — Reptilia — Turtles, snakes, alligators, and lizards. Characteristics: Cold-blooded; usually covered with scales and fre- quently bony plates ; air breathers. Order 1 — Rhynchocephalia — "The old-timers," Sphenodon. Characteristics : Biconcave vertebrae often containing remnants of noto- chord; quadrate bone immovable; parietal eye present. This group i.« represented by one genus of lizards, Sphenodon, found only in New Zea- land. Order 2 — Crocodilia — Crocodiles and alligators {Crocodiliis, Alligator). Characteristics : Anterior appendages bearing five digits, jiosterior four with trace of fifth ; longitudinal slit constitutes cloacal opening; vertebrae procoelous. Order 3 — Chelonia — Turtles and tortoises (Amyda, Eretmochelye, Terrapene, Testudo, Chelonia). Characteristics : Body surrounded by bony case forming a carapace and plastron; toothless jaws ; immovable quadrate bone; appendages typi- cally with five digits. Order 4 — Squamata — Snakes and lizards (Phrynosoma, Heloderma, Tham- nophis). Characteristics: Usually with horny epidermal scales or plates; movable quadrate bone; vertebrae usually procoelous; ril)s with single heads. This order is usually subdivided into two sub-orders: lizards (Sauria) ; and snakes (Serpentes). Orders 5-8 — /)z/;o.s<7;/m — Fossil reptiles (Ichthyosaurs, Plesiosaurs, Pterodac- tyls, Dinosaurs). In these groups belong such forms as the fishlike reptiles (Ichthyosaurs) ; the long-necked reptiles (Plesiosaurs) ; the flying reptiles (Pterodactyls) ; and the giant reptiles (Dinosaurs). 112 NATURAL HISTORY Subcbss A - Arcbaeomithes r| fossil reptile-like "bircCs I SUPERCLASS TETRAPO"DA ClassI AVE5 BIRDS Kestserornithifbrmes ^, ^5sil toothe^blrasl Aptery^iforme? « Ca5i:arii|brTOes Kivi^^ ^Caseovarie^ ^ Ciconiifomes C)Icbtbxor^^i7orm<25 ^ /« stork -like bircfs Grui formes rails at2ct coots (10) cViarBucCrji^nTMs glover, 5nipi« ,^115 11^ ^ . ^ m.^s??^ Cuculi formes stratHorjiformes |f^^ M-^WxV Talcomfl^nes Afncan oitrich jV ]j col^'mbilbrmcs falcon-like-binis (-r) rmes ^"^^W ^^^'^S^l ^^^^ -"^ssi^*^ Moots ^-r-'^'^/ j'^^'^,.// Coraciiformas extinct :^^/ <^^f< — ^'^ (8) (^^^ ^ Gcclhformss =^ elep hant birds aibatrossa.s .petrels RheifoT^m©© . American ostrioh Subclass D -Neorrzitbes (2/) fixsseriformes percViing- birds HOI.L CALL ,lj Class III — Aves — Birds. Characteristics: Typically featliered and toothless; \varm-l)Ioo(lo(l. Subclass A — Archaeornithes — Fossil birds (Archaeopteryx). Characteristics: Ancient re[)tilelike fossil birds; only three specimens of a single genus (Archaeopteryx) are known. Subclass B — Neornithes — Recent birds. Characteristics: Mostly composed of birds which are represented by living forms; 21 orders. Order 1 — Hesperornithiformes — (Hesperornis). Characteristics : Fossil, toothed birds from America ; teetli set in a groove. Order 2 — Ichthyornithiformes — {I chthyornis) . Characteristics : Fossil, toothed birds from America, whose teeth are set in sockets. Order 3 — Struthioniformes — Ostriches (Stridhio). Characteristics : Naked head, neck, and legs ; flightless, terrestrial forms ; feet with two toes; no keel on breastbone {sternum). Order 4 — Rheiformes — Rheas (Rhea). Characteristics : Distinguished from preceding order by a partially feath- ered head and neck; flightless terrestrial birds, with three-toed feet; feathers without aftershaft. Order 5 — Casuariiformes — Cassowaries and emus (Dromalus). Characteristics: Terrestrial, flightless birds, possessing small wings; feathers with large aftershaft. Order 6 — Crypturiformes — Tinamous (Rhynchotus) . Characteristics : Flying, terrestrial birds, with a short tail ; no pygostyle. Order 7 — Dinornithiformes — Moas (Palapteryx). Characteristics : Recently extinct, flightless, terrestrial birds, with large hind limbs ; wing bones absent. Order 8 — Aepyornithiformes — Elephant birds (Aepyornis). Characteristics: Extinct terrestrial flightless birds with large hind limbs; small sternum and wings ; large eggs. Order 9 — Apterygiformes — Kiwis (Apteryx). Characteristics: Small flightless terrestrial birds; hairlike feathers with- out aftershaft. Order 10 — Sphenisciform.es — Penguins (Eudyptes). Characteristics: Marine antarctic birds, incapable of flight, with small scalelike feathers; wings modified as paddles for swimming. Order 11 — Colymbiformes — Loons and grebes (Gavia, Podiceps). Characteristics : Aquatic birds with feet far back with webbed or lobed toes. Order 12 — Procellariiformes — Albatrosses and petrels (Diomedea, Hydrobates). Characteristics : Marine birds with great powers of flight ; webbed toes ; bill sheath of several pieces. Order 13 — Ciconiiformes — Storks, birds, pelicans, cormorants, snake-birds, herons, ibises, and flamingos (Phalacrocorax, Ardea, Phoetncoptcru.f). Characteristics: Long-legged aquatic marsh birds with feet adapted for wading. 114 NATUllAL lilSTORY Order 14 — Anseriformes — Swans, geese, and ducks (Mergus, Anas, Cygnus). Characteristics : Aquatic birds whose beak is covered by soft sensitive membrane edged with horny lamellae. Order 15 — Falconiformes — Falcons, vultures, eagles, hawks, and secretary-birds {Cathartes, Gymnogyps, Sagittarius, Falco). Characteristics : Carnivorous birds with curved, hooked beak ; feet adapted for perching and provided with sharp, strong claws. Order 16 — GalUformes — Tui'keys, fowls, quails, and pheasants ; also the hoactzin (Meleagris, Colinus, Bonasa). Characteristics : Arboreal or terrestrial birds ; feet adapted for perching. Order 17 — Gruiform.es — Rails and cranes {Rallus, Gallinula, Fulica). Characteristics : Mostly marsh birds. Order 18 — Charadriiform.es — Plovers, snipes, gulls, terns, auks, and pigeons {Jacana, Larus, Rhynchops). Characteristics : Marine, arboreal, or terrestrial forms. Order 19 — Cuculiformes — Cuckoos and parrots {Conuropsis, Coccyzus). Characteristics : Arboreal birds, first and fourth toes directed backwards ; the latter may be reversible. Order 20 — Coraciiformes — Kingfishers, owls, hummingbirds, swifts, and wood- peckers (Streptoceryle, Antrostomus) . Characteristics : Tree-inhabiting forms with short legs. Order 21 — Passeriformes — Perching birds (Passer, Sayornis, Tyrannus). Characteristics : More than half of all known birds belong in this order. In America representatives of 25 families are found. A few of these are the flycatchers, larks, thrushes, thrashers, wrens, warblers, swallows, shrikes, nuthatches, crows, orioles, finches, and creepers. ROLL CALL ,,- Class IV — Mammalia — Mammals. Characteristics: Members of this class are readily distiiiKuisl)0(l by a covering of hair at some time in their existence; the females pos.seKs mammary glands which secret(> milk for nourishment of young. Subclass A — Prototheria — Monotremes {Echidna and Ornithorhynchus). Characteristics: Egg-laying mammals; in case of Ecliidim the egg is placed in a temporary pouch and incubated until hatched. Subclass B — Metatheria — Marsupials {Didelphys, Petrogale, Macropus). Characteristics : Carry young in marsupium or pouch ; allantoic placenta typically absent. Subclass C — Eutheria — Viviparous mammals. Characteristics : Bring forth their young alive ; young never carried in pouch ; nourished before birth by placenta. section a — unguiculata — Clawed mammals. Order 1 — Insectivora — Insect-eaters, moles, and European hedgehogs. Characteristics : Small terrestrial clawed mammals with typically planti- grade feet ; molar teeth enameled, rooted, and tuberculate. Order 2 — Dermaptera — Flying lemurs. Characteristics: Members of this group resemble the insectivores in the structure of the skull and the canine teeth ; only two genera are known, which inhabit the forests of Malaysia and the Philippines. Order S — Chiroptera — Insectivorous bats, fruit-bats, and blood-sucking vam- pires. Characteristics : Mammals with claws whose fore limbs are modified for flight. Order 4 — Carnivora — Flesh-eating mammals, hyenas, raccoons, dogs, cats, weasels, bears, sea-lions, seals, and walruses. « Characteristics : Carnivorous mammals with claws and large projecting canine teeth; incisors small; premolars adapted for flesh-cutting. Order 5 — Rodentia — Gnawing animals, hares, rats, mice, squirrels, beavers, porcupines, guinea pigs. Characteristics : Members of this group are usually separated into two suborders depending upon the possession of one or two pairs of incisors in upper jaw. Order 6 — Edentata — So-called toothless mammals, three-toed sloth, armadillo, and pangolin. Characteristics: Clawed mammals; teeth entirely absent or mi.-^sing from anterior part of jaw ; teeth usually without enamel ; tongue often long and protractile. section b — primates. Order 7 — Primates — M&mmaAs with nails; tarsiers, lemurs, monkeys, apes, man. Characteristics: Toe or thumb usually is opposable to other digits; dentition rather primitive ; eye orbits directed forward ; posture usujilly semierect. 116 NATURAL HISTORY Subclass A PROTOTHERIA MONOTREMES ®gig-layin^ mamnrzals OrnitlQorbyr2c"bu4 cUxckbill SubcktssB NETATHEQIA HAR5(JP)AL5 mammals >vitl3 torOQgt pOLCCVx Macropas l^ OrcCai- v5 RocCent-icc gnaviog mammals Dermaptera flying ]emtxi~5 OroCer 6 Edentata OrcCejT 8 Artiodoctyla even- toed. \^ Section C/ UNGULATA hoof«gcC mammali M^ OrcLai- 9 ' PerissododMct octd-toedL "^ OroCer lO Probosrcixfea trttnk ondC tusl fnndamentally similar. Prac-tic-iUy every cell that is microscopically visible possesses several difTerent kmds of structures located within its borders. Some of these struc- tures are alive, some lifeless. In the first group may be placed the -plastiih of plant cells, the mitochondria or chrondrio somes, some of which probably give rise to plastids, fibers of various kinds, the Golgi bodies and the centrosomes, the latter of importance in animal cell division. In the second group may be placed such inclusions as yolk, or other food substances, fatty droplets, granules of pigment or of secretions (as in gland cells), and crystals of various kinds, such as calcium oxalate in plant cells. To this list may also be added vacuoles, which in plant cells often occupy the major space within the cell membrane. All of these structures are confined to the cyto- plastn or part of the protoplasm outside the nucleus. In Elodea, the cells present a green t^. appearance, due to the presence of many tmy pi.yii cell of a K-af; ovoid bodies, the chloroplasts, which are plastids <"• ihl()ro[)last ; n, nu- containing chlorophyll. Careful obser^•ation of a single cell shows that the chloroplasts move slowly down one side of the cell, across one end, and uj) the other side, keeping rather close to the outer edge of the cell during the process. This is due to the movement of the cytoplasm. In the cells of the hairlike stamen of Tradescantia, the movement of the cytopla.'^m is also evident. Here it can be seen actively streaming in currents within the cell, carrying along within it tiny crystals of inorganic origin, as well as colorless plastids and granules. The latter term is usually applied to inert materials, such as granules of stored food in the form of starch grains (in plants), fat or yolk granules, or pigment granules which frequently occur scattered througiiout |)r()toi)lasm. Between the strands of (ytoplasm are spaces or vaeuolrs filled with a watery fluid, called cell sap. In young jilant cells, the vacuoles are small and the cytoplasm occupies the greater part of the cell, but in mature plant cells the cytoplasm is found clo.se {'k'us ; r u\ cell wal \ ay found free in the foods or waste products, but rather as various kinds of chemical compounds which 132 FUNDAMENTALS OF STRUCTURE AND FUNCTION may be further subdivided into inorganic and organic compounds. The former comprise most of the non-Hving compounds such as soil and rocks and their decomposition products. However, in proto- plasm, inorganic compounds are usually present as water, salts, or gases. Water is important not only because it comprises 70-98 per cent of protoplasm by weight, but also because it dissolves so many different substances. Furthermore, water is an important factor in promoting the dissociation of many salts into their constituent ions. The inorganic salts which occur in marine organi.sms, for example, are usually those commonly found in sea water. Some, such as ni- trates and nitrites, occur chiefly in plants, while compounds con- taining sodium and chlorides are characteristic of animal tissues. Only three gases are found in varying amounts in the living cell, — free oxygen, carbon dioxide, and ammonia. Protoplasm a Colloidal Mixture Matter exists in three states, gaseous, liquid, and solid. Frequently it passes from one state to another, as when ice melts under the in- fluence of heat, turn- ing to steam as the water boils away. That protoplasm at different times and under different con- ditions varies in ap- pearance is probably due to the fact that A B c it is a colloid and as Colloidal constitutions. The continuous phase in a such can change from being fluid; in a jell (C) solid; while in the "sol " or liouid tO sol (A intermediate phase (B) the solid forms a net through which the fluid is continuous. a "gel," or solid state and then, under cer- tain conditions, back again. The scientist examines protoplasm under the ultramicroscope and finds tiny dancing particles which are invisible under the ordinary illumination of the microscopic field (Brownian movement). This condition is known as a dispersion, the dispersed particles being carried in the dispersion medium, in this case water. A fog composed of tiny droplets of water is an example of dispersion in nature. If the particles in a disper- sion are small, the substance is called a crystalloid, when large it LIFE AND PROTOPLASM i:{3 is called a colloid. Xow these terms are not api)lie(l t(j fixed sub- stances but to states of matt(M-. Gelatin passes from a li{|uid to ii solid state on being heated or cooled. A study of the diap;ram shows how this might be possible. In the left-hand diagram the solid i)ar- ticles are floating freely in the fluid of the medium ; in the middle diagram the solid portion is becoming a loose mesh; while in the right-hand diagram the mesh has become a solid mass, including the liquid within it . The protoplasm within the cells of plants and animals probably behaves in a similar manner, under some conditions assum- ing the "sol," and at others the "gel" state. Remembering that protoplasm is not a single protein substance, but rather a mi.xture of proteins, fats, carbohydrates, and sometimes even other sub- stances, it is clear why there are many slightly different protoplasms depending on the part of the animal or plant examined. This fact may help us to see why the living matter of a muscle, the blood, or the brain differs visibly in structure. For one thing, the water con- tent differs greatly. Living bone is said to be 25 per cent water, muscles about 75 per cent, the jellyfish almost 99 per cent, and some fruits as high as 98 per cent water. Diffusion We have spoken of the work of the enzymes in making food sub- stances soluble. Let us now see why .'^olul)ility is necessary for the life processes of cells. The physical phe- nomenon of diffusion is easily demonstrated by the slow spread of red ink when a droj) is put into a glass of water. Brownian movement of dancing particles visible under the high power of the microscope is a mani- festation of molecular kinetic energy caused by the water molecules bombarding these particles. It is a similar movement of molecules that occurs when diffusion takes place. Molecules of any substance are always in motion. If this substance is soluble (the solute) in another substance (the solvent), there is always a tendency for these molecules to move from the place of their greatest concentration to i^laces where they are not so highly concentrated, until an equilibrium is reached and there Lonfjit udiiial sect ion through a tumbler of watt-r containing soluble crystal. sliowiiif; by arrows tlic direc- tion of (lillusion. ami by (lotted circles the lines of equal concentration. 134 FUNDAMENTALS OF STRUCTURE AND FUNCTION are just as many molecules of the solute in one part of the solvent as in another. In the case of the diffusion of red ink in water, the eosin (which is the coloring material used) was more concen- trated in the drop than in the water, so the molecules of eosin began moving away from this place of high concentration until they were equally dispersed throughout the water. As a general rule we may say that, if other conditions are equal, the diffusion rate between two points is proportional to the differences in concentration of the substances at these two points. One thing which affects the diffusion rate is the nature of the medium, w^hether it be a gel, emulsion, or some sort of semisolid (porous). Gelatin, for example, which is a gel, offers no effective resistance to the diffusion of molecules of a crystalloid nature through its meshes, but, upon the other hand, this network may serve to block effectively the passage of colloidal substances. Suppose a membrane were stretched crosswise in a jar where diffusion was taking place. Could the molecules of the diffusing substance pass through the membrane? This depends on whether the membrane is permeable to the diffusing substance. In some membranes the ultramicroscopic "pores" are believed to be quite large, thus letting through molecules of larger sizes, while in other membranes the "pores" through which substances can diffuse are very small. Other substances penetrate in proportion to their lipoid solubility. Thus some membranes allow certain sub- stances to pass through, while they keep out others. Such mem- branes are said to be selectively 'permeable. An ordinary parch- ment membrane will allow the '^l ff^ " . • - •'•. ^«.-- ■',■ ■' i'ly. l,v-i .'•:~-r '■f.^:.:^-. ■■"iPV^i^- a W e p "'"-"-^f V Diagram of an imaginary section . i •. t-> - -u through the cell wall and protoplast to eosm to pass through it. But the show a, outer water ; iv, cell wall ; c, ecto- cell membrane does not act in the plast or cytoplasmic membrane next ^^^^ manner, as it is a vldsma to the cell wall; p, general cytoplasm; i i j.- i /. tonoplast or inner cytoplasmic mem- membrane, and selectively per- brane next to the water, thus forming meable. a continuous pathway which carries ^j^^ plasma membranes sur- solutes irom (a) to (?)) ; i\ vacuole. ,. ,. . , ,. , rounding living cells are believed to be colloidal in nature, made of a combination of fatty and protein substances. Careful experiments have demonstrated them to be LIFE AND PROTOPLASM i.r, selectively permeable. Most living cells allow oxygen and carhon dioxide to pass freely through their mcmijranes, while diss(jlved sugars and digested proteins in the form of amino acids dilTu.sc through more slowly. Water of course passes through, acting as a vehicle for other substances. Such membranes are impernicablc to certain salts and not to others. The permeability of living cells to dissolved substances differs with the cell, and naturally with tlu; organism. Salt- and fresh-water fishes are examples of types, the cells of whose gills exhibit different permeabilities. Dead cell mem- branes are usually permeable to crystalloid solutes, while living cell membranes permit but few salts to enter. In general, cells are not permeable to colloids, because of the large size of the particles constituting the colloid. Osmosis and Its Significance to Living Cells We have already seen that if a membrane is sc^lectiveiy permeable, then some substances, such as water or certain solutes, will pass through readily, but other solutes may not. because their molecules are too large to pound their way through the ultramicroscopic "pores" of the membrane. The process by which substances diffuse through membranes is known as osmosis. It is of the greatest importance to living cells, as it is by this means that dissolved gases, such as oxygen, and dis- solved food substances get into the cell, as well as the process by which waste ma- - sugar ^littion... lJ» selectively lJ.pcrroeab\^ t -'-'4* molcciclc ^ • •wat«r'mo\eculti =^ °'TS^^ -penTKoBe =1. "WCCt&V vater- .XLi» Diagram to explain osmotic pressure. Sii>:ar solution is of equal density in ea- lated cells. As these cell masses evohcd, they became more and more complex, different systems of organs appearing in more highly organized forms. In the animal series shown on ])age 146, this theory seems to be pretty well substantiated. But another theory, the organismal theory, considers the living thing as a whole, being divided into units of structure in the many-celled organism. Accord- ing to such a theory unicellular organisms would become first much chfferentiated within their own bodies, as is .seen in many of the protozoa. These theories need not concern us further at present. Both have many facts to support them, substantiatetl by the devel- opment and structure of various types of organisms. Plant and Animal Cells Differ in Size, Shape, and Structure An examination of the figure on page 140, will sliow that cells are far from uniform in size and shape. They differ in size from the smallest bacteria which can just be distinguished with an ultra-micro- scope that magnifies 3000 diameters, to cells that can be seen with the naked eye. The egg-cell of the chick, for example, includes the con- spicuous yolk, while certain cells in the human spinal cord, altluiu^h microscopic in size, may have prolongations reaching down irito the muscles of the fingers or toes. Cells are not of n(>cessity lar-jcr in large animals or plants, some of our largest cells being found li\ing isolated and alone. But under normal conditions a cell of a given size and shape always reproduces the same kind of cell as itself. 140 FUNDAMENTALS OF STRUCTURE AND FUNCTION idL "bouiillus/ Anthro^ bacillus^ As to shapes, their name is legion. A typical cell might be thought of as a spherical or ovoid body, but we find them cubical, flat, thread- like, spindle-shaped, columnar, or irregular in outline. They are often modified by being com- pressed by other cells, but frequently if given opportunity will resume their original form when released from pressure. Structural differences exist between plant and animal cells, the chief of which is the cellulose wall, characteristic of plants, which gives such cells the rigidity and yet the flexibility found in woody stems. Other physiolog- ical differences will be discussed in the following chapters. red. corynxsc^s^ .of f nog. a.no5om.: -v: :. ;;,U^ loom. Yiyoline Cartilage supporting ■tissues red. © ful. ORGANISMS ILLUSTRATING BlOL()(;i(;\| PRLNCIPLES MI BEGINNINGS: THE LARGE CROIP OF THE SMALLEST OHGA.MSMS Preview. Some forms found in a drop of fresh water: Ainoba. an animal cell ; Euglena ; Paramecium ; Diatoms : Desmids ; Bacteria ■ Func- tional differences between plant and animal cell • Suggested readings. PREVIEW Over two hundred and sixty years ago, when the Dutchman, Antony van Leeuwenhoek, examined what he called "little animals" under his homemade microscopes, he made the first real exploration of a drop of water ever attempted. His microscopes were simple affairs, consisting of a single lens. They had no tube or mirror such as our microscopes of today have. When objects were examined they had to be brought into position and focus through the use of rather coarse screws. Besides being the first person actually to see the capillary circulation of the blood (a thing that Harvey knew must be so, but which he wa.s unable to prove), van Leeuwenhoek made numerous other llhJ^sio- logical and anatomical observations which gave him the title of "founder of histology." One thinks of him most often as the first man who saw protozoa, unicellular plants, and own bacteria in standing water. Let us read his own description and judge for ourseh-es a.s to what he saw. The following extract is taken from a letter written on October 9, 1676, to Henry Oldenburg, First Secretary of the Koyal Society of London. It describes the finding of "little animals" in a drop of rain water. "Of the first sort that I discovered in the said water, I saw, after divers observations, that the bodies consisted of 5, 6, 7, or 8 ver>' clear globules, but without being able to discern any membrane or skin that held these globules together, or in which they were inclosed. When these aninuilcules bestirred 'emselves, they sometimes stuck out two little hnrns, which were continually moved, after tlie fashion of a horse's ears. The i)art l)etween H. W. H. — 11 131 152 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES these little horns was flat, their body else being roundish, save only that it ran somewhat to a point at the hind end ; at which pointed end it had a tail, near four times as long as the whole body, and looking as thick, when viewed through my microscope, as a spider's web. At the end of this tail there was a pellet, of the bigness of one of the globules of the body ; and this tail I could not perceive to be used by them for their movements in very clear water. . . . "I also discovered a second sort of animalcules, whose figure was an oval ; and I imagined that their head was placed at the pointed end. These were a little bit bigger than the animalcules first mentioned. Their belly is flat, provided with divers incredibly thin little feet, or little legs, which were moved very nimbly, and which I was able to discover only after sundry great efforts, and wherewith they brought off incredibly quick motions. The upper part of their body was round, and furnished inside with 8, 10, or 12 globules : otherwise these animalcules were very clear. These little ani- mals would change their body into a perfect round, but mostly when they came to lie high and dry. Their body was also very yielding : for if they so much as brushed against a tiny filament, their body bent in, which bend also presently sprang out again ; just as if you stuck your finger into a bladder full of water, and then, on removing the finger, the inpitting went away." His description of the cause of movement in his little creatures is amusing, yet it shows that he saw cilia plainly and estimated their size quite clearly. "But many of the things we imagine, and the natural objects that we inquire into, are very insignificant; and especially so, when we see those little living animals whose paws we can distinguish, and estimate that they are more than ten thousand times thinner than a hair of our beard ; but I see, besides these, other living animalcules which are yet more than ten thousand times than a hair of our beard ; but I see, besides, these other living animalcules which are yet more than a hundred times less, and on which I can make out no paws, though from their structure and the motion of their body I am persuaded that they too are furnished with paws withal : and if their paws be proportioned to their body, like those of the bigger creatures, upon which I can see the paws, then, taking their measure at but a hundred times less, it follows that a million of their paws together make up but the thickness of a hair of my beard ; while these paws, besides their organs for motion, must also be furnished with vessels whereby nourishment must pass through them." ' Van Leeuwenhoek was made a member of the Royal Society for his clear reports of what he saw and at his death he had sent the Society a 1 Dobell, C, Antony van Leeuwenhoek and his "Little Animah," pp. 118 and 180, Harcourt, Brace and Co. By permission of the publishers. THE LARGE GROUP OF THE SMALLEST ()IU;\NISMS m case containing 26 of his microscopes, a gift which was later lost ( )„,. of the few remaniing of the 419 lenses put up at auction after van Leeuwenhoek's death was recently examined by an expert who reported that the biconcave lens that he inspected "was very good indeed" and proved that its maker had attained "a very high degree of proficiency in grinding extremely small glasses." With the modern microscope of the college laboratory, infinitely better work can be done than with this old pioneer. The best of \an Leeuwenhoek's lenses are said to have magnified not more than 270 diameters, while the " high dry " power of the average modern micro- scope gives a magnification of about 440 diameters, so that the college freshman today has a far better physical equipment than did this famous Dutchman. He also has much more. In the years that have intervened between the time of van Leeuwenhoek and the present, patient observations of minute forms of life ha\-c been made by hundreds of scientists whose results may be found in these pages and in other books suggested for collateral reading. With this intro- duction the student might begin the study of simple organisms in some such way as Antony van Leeuwenhoek did, by examining a drop of pond water. Some Forms Found in a Drop of Fresh Water The pages that follow^ will serve to give us a slight acquaintance with some of the simplest plant and animal forms that are likely to be met in the examination of a drop of pond water or water from a laboratory aquarium. Li addition to the unicellular organisms, scores of other higher forms are likely to be seen. Countless protozoa, including the many tiny species of monads, dart across the field of the microscope ; others many times larger, with their highly specialized cell parts, as Euplotcs or Stylonychia, may be found browsing on tiny plants. Frequently one also encounters threads of the filamentous algae, Zygneyna or Sjpirogyra, while debris, consisting of tiny bits of wood, sand grains, and the glasslikc cases of diatoms and desmids. may abound. Many tiny crustaceans, water fleas, and cojx'pods are usually present, and in addition one finds the easily recognizable rotifers, with their whirling wheels of cilia, their prominent grinding organ or mastax, and their slender toelike posterior foot by means of which they often become attached to .solid objects. Sometimes a small roundworm may be found working its way through the dt^bris. while 154 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES hyaline cap.. pseuctopodiam \ ptemagsl.m o _, many types of insect larvae and pupae may also be seen. This brief list includes only a few of the many new acquaintances to be found in a drop of water. AmebOj an Animal Cell Ameba is the classic representative of a single-celled animal which illustrates the action of living protoplasm. Found in ooze taken from the bottom of small ponds or sluggish streams, it is seen to be an irregular and almost transparent cell. When in motion the protoplasm of its body apparently flows out into newly formed bulging projec- tions of the body called pseudopodia (Gk. pseu- dos, false; pous, foot). The cell body consists of two substances, an inner, more fluid, granular por- tion, the endoplasm and a more viscous area, the ectoplasm, on the outside. The whole Ameba is surrounded by a deli- -foodvacuole cate plasma membrane. When the animal moves, the protoplasm appears to flow into the pseudo- podia. According to S. O. Mast of the Johns Hopkins University, when an Ameba is mov- ing in a given direction the endoplasm sol pushes out in a pseudopo- dium and becomes changed to a gel, the "gel" at the other end of the cell becoming a "sol" that moves into the cefl body. This illustrates a characteristic of protoplasm mentioned earlier. This cell, like others of its kind, has a nucleus containing chromatin. Certain vacuoles are present, some of which are filled with a watery fluid, others hold food in different states of digestion, while a single ---■nuclexcs li.L.-fooct vacuole. Contractile, vacuole— Ameba proteus. The direction of progress of the cell is shown by arrows. What happens to the protoplasm in the extreme anterior end during movement. (After Mast.) THE LARGE GROUP OF THE SMALLEST OI\(;\Ms\is i:..-, vacuole, called the contractile vacuole, rhythmically collects and expels fluid. The function of the contractile vacuole may he to eliminate wastes from the cell, or it may have a hydrostatic function, that is, it may control the amount of water contained in the ccjl. Food particles are actually ingested or taken into the cell by the proto- plasm which flows around the food, engulfs it, and then surrounds it with digestive fluids in a food vacuole. A recent series of observations by Mast and Hanliart ' indicate that the Ameba selects certain kinds of food, ])referring, for instance, Chilomonas to Monas, although both are flagellates of about the same size, form, and activity. It was further sliown that Monas was not digested in the food vacuoles, while Chilajnonas was, and also, some organisms, such as mold spores, certain algae, and other flagellates, might be eaten but were not digested. The process of constructive and destructive metabolism may take place in a single cell. Indigestible waste materials are pa.s.sed out any^vhere from the surface of the cell body, while respiration takes place by means of an osmotic ex- change of the gases, oxy- gen and carbon dioxide, through the cell mem- brane. As a result of the taking of food, the cell gradually increases in size and then divides by a process known as binary fission. Accord- ing to a recent study by Chalkley and Daniel - the division of the nucleus shows the typical stages of mitotic division, the entire process lasting, under normal temperature conditions, about half an hour. During the process the Ameba is quiescent and the late prophorse. mid- anaphase Gccriy anaphase •metaphose Mitotic division in the nucleus of Vruflia. i, After GliiilklcN ;uni Daniel. ' iMast, S. 0., and Hanhart, W. L., " Feedins, Digestion, (Leidy)." Phusiol. -Zool.. Vol. 8, lO.'?."). Pp. 2,5,5-272. -C and the Pp. 592-619. and StarviiliiiM in .■inwrbii pmlrw Iv)." Ph„.no}. 'Zool.. Vol 8. \9^r,. Pp. 2,5,5-272. ,i„„,vll Chalkley, H. W.. and Daniel. G. E.. "The Relation between the lorn, of he I. v .« r 1 the Nuclear Pha.ses of Division in Amoeba protcu.. (Leidy). Phmol. looU \o\. rt. !..,«•«. 156 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES pspiidopodia are relatively small. After the nucleus divides, the cell body separates into two equal parts, each of which grows into a full-sized individual. Euglena Although Ameba is usually looked upon as the simplest of all animal cells, there is another group of organisms containing equally simple forms, making up a large flagsllum cytostome/. stigma .^^ b<, IM amecia in his laboratory at Yalo University lor thirty yoars and (hir- ing that period over twelve thousand generations vv.t,. I,,-,,! I.y fissi,,,, It has been observed in these cultures, however, (haf after 4() or inor.- divisions have occurred, a i)rocess called cndomixis takes plac.-. in which the old active niacronucleus is replaced by a new one made 12- Endomixis in Paramecium aurelia. The normal condition of Parainociiiin is shown in I showing niacronucleus and two niicronuclei. Follow throufrli the series pictured. What happens to the niacronucleus? How many iniiTonuclci are formed? What, happens next? Note in IV that only one daughler cell is shown. How does this cell obtain the normal number of niicronuclei? Where does the new niacronucleus come from? This rhythm of cell actixity seems to occur with considerable regularity every 10 to .^0 generations and it gi\ es the new macronucleus chromatin from the reserve sujiply held in the micronucleiis. This process does not appear in all ciliates and is not beliexed to be necessary for normal growth. (After Hegner.) from chromatin of the reserve micronucleus. This process is similar in many respects to conjugation, except that no foreign chromatin is added. Under normal conditions, another process known as amphimixis or conjugation takes place somewhat resembling the sexual procc^^ses of higher animals. Two cells come to lie with their gullet surfaces next to each other and a bridge of protoplasm forms between them, \\hile this is going on the micronucleus in each cell moves away from the 162 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES macronucleiis, elongates and divides twice in rapid succession. Three of the micronuclei thus formed in each cell disappear, but the fourth one divides again. In this last division two irregular masses of chro- matin are formed. This process has been likened to a similar division inr "Xcr Conjugation in Paramecium caudafiim. Shortly after the conjugating pair come together with their ventral surfaces opposed (I) a protoplasmic bridge is formed, the macronucleus breaks down (II) and each micronudeus divides a second time (III). What happens to three of the four micronuclei? Compare this stage with the figure on page 429 (maturation). Next the micronuclei remaining in the cell divide into two, the smaller (migratory) micronucleus passing over by the protoplasmic bridge into the opposite cell, there to unite with the larger (stationary) nucleus (VI). Trace the subsequent divisions of the fused micronucleus (VII, IX). How do we get back to the original cell condition? (X-XIV). (After Hegner.) that takes place in the eggs of animals, at the period known as matu- ration, when the sex cells are losing part of their chromatic material in preparation for fertilization of the egg by the sperm cell. The smaller mass is thought to correspond to a sperm cell of the many- celled animals, while the larger one corresponds to the egg cell. In any event, each of the smaller micronuclei migrates reciprocally over THE LARGE GROUP OF THE SMALLEST ()H(;.\MSMs i«.:{ the protoplasmic bridge, and unites with the larger niicroiiuclcus of the cell left behind. The two conjugating cells now separate, and the newly fused nucleus, composed of a male and female microinu'lcus, is left in each cell. Then a series of divisions of this nucleus takes place until eight nuclei are formed, four of which become macro- and four micronuclei. Three of the micronuclei next disintegrate, leaving the cell with four macro- and one niicionucleus. The latter divides again and with it the cell, so that two cells result, each witli a inicro- and two macronuclei. A second division leaves the daughter cells each with a single macro- and micronucleus, which, thus rejuvenateil. start off on a series of several hundred cell divisions until another period of old age comes on, when conjugation or endomixis is repetited. Diatoms These beautiful microscopic plants, sometimes called "jewels of the plant world," are among the most numerous of the one-celled plants. Over 2000 species have been identified and named. They form one of the most abundant components of plankton in both fresh and salt water, and are also found in damp earth and on moist rocks, where they may occur singly or massed together in groujis. Certain species stick together because of a gelatinous ma- terial which they secrete. Some diatoms move with a slow gliding motion when they are in con- tact with solid objects, although lacking visil)le organs of locomotion. They secrete a glasslike shell exquisitely marked by tiny ridges and rows of extremely minute holes. Diatoms have been, and still are, among the most abundant of li\ing organisms. So abundant were they in past ages that large deposits of their shells exist in the form of diatomaceous earth. In California, there are deposits of diatomaceous earth lying hundreds of feet thick over an area of many square miles, while the floor of the ocean is covered with ooze made up of skeletons of diatoms, which after death sink to the bottom of the water. This diatomaceous material is used as a basis for i^ohshnig lu.wders in the manufacture of bacteriological filters, and of certain kinds o\ porcelains and glass. The (lialoiii \(i- vinild («) Niilxt-sidf, {h^ ;:inilc side. sIkiw- inf.' IIk' rcliilion of tlic\;iK('s. ThiMiii- clfiis iiiid the two rililmiilikc chlorn- pliislsitrc iiol sliowii. (After Plil/«T.i 164 ORGANISMS ILLUSTRATING RIOLOGICAL PRINCIPLES One of the most common diatoms found in pond water is Navicula. In this form the cell wall consists of two valves, one of which fits into the other. The part that fits over the inner valve is called the girdle. The cell appears quite different in structure when seen from the valve side or the girdle edge. In the latter view, a bridgelike mass of pro- toplasm containing a nucleus appears, while in a valve view a line running down the center, called the raphe, is seen, that shows three tiny spots, one in the middle and one at each end. A mucilaginous material exudes through a series of pores which form the base of the raphe. Navicula has two chloroplasts, colored yellowish-brown by a pigment called carotin. These can be seen best when the cell is viewed from the flat side. At the time of cell division, the chloro- plasts first increase in size, pushing the two valves apart so that they barely touch. Then the nucleus, chloroplasts, and cytoplasm of the cell divide, an inner valve forming for each cell. Each of the new cells thus formed is much smaller than the parent cell. Desmids Another one-celled form common in fresh water is the bright green desmid, Closterium. Like diatoms, desmids are of various shapes and sizes. They are beautiful symmetrical structures with large, bright green chloro- plasts, which may be lobed, starshaped, or platelike. The cell wall is thin and transpar- ent, the granular protoplasm within being obscured by chlorophyll, but the nucleus, in the center of the cell, may be easily recognized. Desmids divide by a simple transverse splitting, forming two cells, each new desmid consisting of half of an old cell from which an entire cell is formed. In addition, a process of conjugation takes place, in which two cells come together, each sending out a protoplasmic protuberance that forms a connecting canal. The contents of the two cells meet in this tube, fuse, and form a single cell which grows a thick wall, whereupon it remains as a dormant spore or zygote until Closterium. Two cells undergoing conjuga- . tion. THE LARGE GROUP OF THE SMALLEST OlUiVMsMs |r,.-. conditions are favorable for germination. When the zyjrotr .l,„..s germinate, two new individuals come direetly from it. Many other forms of algae may be found in fresh and .s.h w.-.i.t. Some, like Scenedcsmus, occur in colonies, their end cells being ..ftcn provided with characteristic spines. Another colony of gr,.,.,, cells Pediastrium, made up of a flat plate of sixteen cells, is also frecpiently seen. These are only a few of the many forms of green algae that may be found in a drop of water debris tak(Mi from a (iniet poml bottom. Bacteria Various kinds of bacteria are common in a drop of i)ond water or hay infusion. They are sometimes seen moving through the water, but more often are massed together in a scum covering the surface ""a^o?. O^tfJ /f#^^' cocci op oo oo «■& S? QO QO ^ ig^Hfl GO .. ao rmcrococcA diplococci staphylococci streptococci Forms of l);icleria. of the water. Three large groups of bacteria ha\-e been established according to their shape, coccus, baccillus, and spirillum. The coccus or spherical-shaped bacteria may live singly, as micrococci. Anotlier form, the diplococci, divides and remains attaclied s(j as to form pairs ; a third, streptococci, reproduces to form chains ; while a fourth. staphylococci, forms irregular groups of eight cells or more, resem- bling a bunch of grapes; Sarcina divides in three directions to pro- duce cubical packets. The rod-shaped bacteria, or bacilli, \i\ry a good deal in size and shape, as well as in tiieir ability to form spores, some being very short, others many times longer than wide. The third type, comprising the spirilla, are cur\e(l or twisted in shape, and move through the water rapidly by spiral movement. This form can often be seen hi a droj) of pond water or hay infusion. BacilH and spirilla move by means o( Jlagdla, protoplasmic threads 166 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES which are difficult to see except under the highest power of the microscope. The cell wall of a bacterium is usually considered as a selectively permeable membrane, very delicate, and secreted by the cytoplasm. A gelatinous capsule may be formed by some bacteria, so that groujis of them clump together in masses. Although pigments are often present, bacteria contain no chlorophyll, and consequently most of them are dependent on other organisms for their food. They feed both on living and dead organisms, using not only organic foodstuffs, such as starches, sugars, and proteins, but even leather or wood. Since their food must be liquid in order to be absorbed, they form digestive enzymes within the cell which exude to digest the food out- side of the cell body. In addition to these foods, bacteria need certain mineral salts that are found in protoplasm, water, and nitrogen in a usable form. Not all bacteria are capable of nitrogen fixing, but many obtain their supply of nitrogen for tissue building as green plants do, in the form of compounds of ammonia or nitric acid. The chromatin material is scattered through the cell, there being no distinct nucleus in most bacteria. Bacteria need moisture, a favor- able temperature, and food, in order to grow. Under favorable con- ditions they multiply with great rapidity by simple fission. Under unfavorable conditions, many bacterial cells can contract, lose con- siderable water, and form resistant coats, thus making spores, which can stand extreme conditions of dryness and temperature. While bacteria are usually killed by heating to 100° C, some spores can withstand this temperature for long periods. Functional Differences between Plant and Animal Cells A comparison of the several types of unicellular organisms described might seem at first to show hard and fast distinctions between plant and animal cells. Although chlorophyll is associated with plants, it is sometimes found in borderline animals, while many plants, such as the fungi and bacteria, lack chlorophyll. Locomotion is not exclu- sively an animal characteristic. Some animal cells, as Vorticella, are fixed during a part of their life history, while many unicellular plants move freely through the water. Other plants, although fixed for part of their lives, produce sex cells that are motile in water. The greatest difference exists in methods of nutrition. In the green plant cell, for instance, food substances are made inside the cell in the presence of THE LARGE GROUP OF THE SMALLEST ORGANISMS 167 sunlight while in animal cells, food is made outside and has to be absorbed before it can be used. The method of nutrition used by the green plant is called holophytic, and that of the animal cells, holozoic. The differences between these two types of nutrition are summed up in the table below. Animal Cell Plant Cell No chlorophyll Chlorophyll present Cannot make organic foods Can synthesize organic foods out of law food materials Only source of energy is organic food Source of energy is the sun Ingests solid food Cannot ingest solid food Usually moves about after food, therefore Does not ordinarily move about, and uses greater destructive metabolism sun's energy, therefore greater construc- tive metabolism Depends on other organisms for food Supplies other organisms with food SUGGESTED READINGS Calkins, G. N., Biology of the Protozoa, Lea & Febiger, 1926. Chs. I, III, and IV, especially. Dobell, C., Antony van Leeuwenhoek and his "Little Animals," Harcourt, Brace and Co., 1932. The entire book, which contains excellent translations of most of the original letters of van Leeuwenhoek, is well worth reading. It is a most authentic picture of this interesting Dutchman and his times. Giltner, W., Textbook of General Microbiology, P. Blakiston's Son & Co., 1928. Ch. III. Holman, R. M., and Robbins, W. W., Elements of Botany, 3rd cd., John Wiley & Sons, Inc., 193(3. Ch. X. Locy, W. A., Biology and Its Makers, Henry Holt & Co., 1908. Ch. V. An excellent historical survey. Needham, J. G., and Lloyd, J. T., Life of Inland Waters, 2nd ed. Charles C. Thomas, 1930. Ch. IV. Excellent descriptions and illustrations of the life found in pond water. Singer, C. J., The Story of Living Things, Harper & Bros., 1931. Ch. IV. An interesting and authentic history of biology. Ward, H. B., and Whipple, G. C, Fresh-Water Biology, John Wiley & Sons, Inc., 1918. This book is invaluable for reference. Chapters VI, IX, and XVII arc especially useful. H. w. H. 12 VIII THE DEVELOPMENT OF SEXUALITY IN PLANTS Preview. The beginnings of sex in the algae • Oedogonium • A repre- sentative fungus • Alternation of generations in the plant kingdom • Sug- gested readings. PREVIEW The one unescapable fact that stands out in the observation of plants and animals in the world about us is the remarkable variety among living things. They range from tiny forms too small to be seen with the unaided eye to huge organisms such as elephants or trees. The biologist is not satisfied with random looking. He looks for certain things, tries to interpret what he sees, but as Thoreau once said, "We must look a long time before we can see." One of the striking facts already noted in the Roll Call of forms of life is that both plants and animals may be placed in groups having similar characters, and that these groups arrange themselves in a series of gradually increasing intricacy of structure, which goes hand in hand with an ever increasing complexity in functions. Simple plants or animals do things simply. Almost any part of the one-celled Ameba can do any part of the work of the cell although lacking organs found in higher forms. More refined ways of doing things, and a more' efficient division of work, come with increasing complexity of organic structure. The true investigator is ever alert to find forms that illustrate this increasing division of labor, and is always asking why and how such things come about. Biologists have picked out certain representative forms that clearly suggest certain facts and principles that are worth knowing. It is possible, for example, through the study of some simple forms of organisms, such as the Thallophytes, to discover the beginnings of sexuality in plants. The Thallophytes include most of the simplest plants and are divided into two great groups, algae and fungi, the latter containing no chlorophyll. Wliile there are six classes of algae, four, namely, the blue-green, the green, the brown, and the red, are classified largely on color. All of the four groups are essentially water- loving plants, showing in many ways that they are simple and rather primitive organisms. In size they range from tiny uni- 168 THE DEVELOPMENT OF SEXUALITY IN PLANTS 169 cellular forms to some of the great brown seaweeds, or kelps of the California coast which may be several hundred feet in length. Ascending the scale of increasing complexity in structure, we find the appearance first of sex cells and later of sex organs evolved to form and protect these sex cells. By selecting other representatives from the higher plant groups, such as mosses, ferns, and flowering plants, we can follow this evolu- tion of sex through the entire plant kingdom. The pages that follow will at least give us a start on the answer to the question : How and where does sex originate in plants and what is its meaning ? The Beginnings of Sex in the Algae Pleurococcus, or Protococcus as it is sometimes called, is one of the simplest of all living plants, familiar to most of us as the green "moss" usually seen on the north side of trees. Indians used it to find their direction through the forest, as persons lost in the woods do today. Its habitat suggests that the life of the plant has direct relation to moisture, temperature, and light. It would be injured by the direct rays of the sun, because some rays such as those of ultraviolet light are injurious to unprotected protoplasm. The cell of Pleurococcus is very simple as seen under a microscope. It is found single, in twos, threes, fours, or flat colonies of several cells hanging together. Examination of a single cell discloses the presence of a thin wall sur- rounding a mass of green protoplasm, the protoplast, which almost completely fills the cell. If a drop of iodine solution is placed under the coverslip, the detailed structure of the cell becomes more evident. The nucleus is completely surrounded by one large, spherical chloro- plast. The cell is a complete entity, in spite of the fact that it is often attached to other cells. Physiologically it is able to carry on all the functions of a living green plant, making food, and digesting it as well as absorbing food and water. It grows to a certain size and then reproduces by simple fission, part of the mother cell going into one daughter cell and part into the other. Theoretically the Reproduction in Pleu- rococcus. Each cell is considered as an indi- vidual, although colonies (seen above) may be formed. The protoplasm of the cell body is not shown, the single chloro- plast being surrounded by protoplasm in active cells. 170 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES protoplasm of the pleurococcus is immortal, since it passes from cell to cell by means of cell division. Spirogyra is one of the multicellular green algae. It is a slimy thread, called "pond scum," found near the surface of a pond, often buoyed up by bubbles of gas which it forms. The filamentous plant body consists of several cells joined end to end, each with a characteristic spirally-banded chloroplast. Examination of a single cell shows a colorless cell wall, the cytoplasm of the cell mostly adhering to its inner surface. Strands of cytoplasm radiate from a central colorless nucleus, which is suspended in a large vacuole or sap cavity. The most characteristic fea- ture is the large twisted chloroplast, on which are scattered many pyrenoids, bodies which contain some of the starch manufactured by the chloroplast. In- dividuals grow in size by forming, through transverse division of the cells, longer or shorter filaments, de- A Spirogyra pending upon the environmental conditions. cell showing the spiral chloro- plast containing pyrenoids, and the nucleus. At certain times in the year, the plants form resting spores called zygospores. Two adjoining filaments come to lie parallel, the cells opposite to each other sending out bulging outgrowths which meet to form a connecting tube. Meantime, owing to the dissolving of the cell wall at the end of the outgrowths, water gets inside of the cells, so that they show signs of plasmolysis, rounding up into ovoid masses. Curiously, however, the cells of one filament remain stationary, while the cell contents from the other filament move over through Conjugation of Spirogyra. Explain what happens. (After Coulter.) THE DEVELOPMENT OF SEXUALITY IN PLANTS 171 the tube and fuse with th(^ quiescent cells. When this fusion takes place, the nuclei unite so that a single resting cell is formed, called the zygote, which develops a thick wall, very resistant to drought and cold. The zygote is heavy enough to sink to the bottom of the pond when the rest of the filament dies, and under favorable conditions will germinate, giving rise to a new filament. Since these cells from different filaments join or fuse, somewhat after the manner of conjugation in Paramecium, we think of them as sex cells, or gametes. Although the two cells are of the same size, yet one is active and the other passive. In higher plants and animals, the active cell is referred to as the male gamete, or spei^m, and the non-active cell as the female gamete, or egg. A compari- son of Spirogyra with higher forms suggests a very sim- ple type of sexual reproduction, known as conjugation. In another fila- mentous form, Ulo~ thrix, certain cells are modified to be- come free-swimming zoospores, provided with four cilia which may swim about for as long as an hour before settling down. It is obvious that such a free- swimming cell may plant a new individual at some distance from the original filament. Gametes of Vlothrix are also formed as free- swimming cells, all alike, having two cilia instead of four. These gametes fuse by conjugation and produce a zygote, which, like that of Spirogyra, has a thick resistant wall, and is capable of developing even after exposure to very unfavorable conditions. In the formation of the conjugating gametes of both Vlothrix and Spirogyra a significant thing happens to the nuclei of the cells before Ulothrix: a. base of filament with holdfast; b, fila- ment producing; zoospores or gametes; c. young filament developed from zoospore; d. filament discharging zoo- spores and gametes ; e. an escaped zoospore ; /, escaped and pairing gametes ; g, zygospores ; h, zygospore pro- ducing zoospores by reduction division, (a-f/. After Coulter; //, after Dodel-Port.) 172 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES fusion they conjugate. By a series of divisions, such as is shown in the dia- gram, the number of chromosomes in the nu- clei of the zygote, result- ing from the union of the two gametes, is reduced . . to half this number. If \ spelling disappear g^j^^g g^^j^ dcvice as this ' ^ / \ were not used, every time sex cells united, the num- ber of chromosomes would be doubled. How- ever, by this so-called reduction division during the formation of the ga- Diagram to show how reduction division takes metes, which OCCUrs in place in the zygote of Spirogyra. both plants and animals, the number of chromosomes is halved. We speak of the single number of chromosomes as haploid and the double number, which comes with the union of the two gametes, as diploid. ■2.y^U first division, reduction of ci-ji-TomoSomcS from ?T-; to ri , mat.u.ra.t,ion /second cLivision, TTJitjDSiS Oedogoniiim In another of the filamentous algae, Oedogonium, there is the first appearance of two kinds of sex cells. This alga repro- duces by zoospores and in addition forms two sex organs, structures called anther- idia, which produce a number of ciliated sper7n cells and oogonia, the latter holding a single egg cell. The sperm cells swim through the water from the antheridia, one uniting with the egg cell, and almost immediately a thick wall is formed about the fertilized egg. This oospore does not produce a new plant directly, but gives rise to zoospores, which in turn eventu- ally become new plants. Life history of Oedogonium. THE DEVELOPMENT OF SEXUALITY IN PLANTS 173 Another form of Oedogonium forms antheridia and oogonia on separate filaments, the male filament being much smaller than the female filament. Thus the filamentous algae illustrate three big ideas, namely, division of labor, development of sex, and reduction of chromosomes. In the simplest plants all cells tend to do the same work, but in the more specialized algae there is a differentiation of work and an accompanying differentiation of cells to accomplish it. In the development of sex and of structures to take care of the sex cells, as found in the forms described, the contribution of the sex cells seems to be to provide a greater vigor to the offspring, especially when the sex cells come from different individuals. Most important of all is the fact that cells which fuse, as in the case of the sex cells, must have some way of reducing the number of their chromosomes, else they would be doubled each time two sex cells united. This is accom- plished by the reduction division referred to above, by which process the number of chromosomes, doubled at the time of fertilization, is halved. This reduction process occurs in both plants and animals, and although in plants it occupies a different place in the life cycle, its ultimate effect is the same in both cases. A Representative Fungus Bread mold, Rhizopus nigricans, one of the most common of the fungi, may easily be grown in the laboratory by exposing a moist piece of bread to the air for a few moments. Mold spores are so numerous everywhere that under ordinary conditions a growth of mold will be evident within one or two days, first appearing as a white, fluffy growth that rapidly covers the surface of the bread. This is the mycelium, which consists of branching tubelike filaments, or hyphae, containing many nuclei, but without cross walls. The absence of chlorophyll shows the inability of the mold to make its own foods and explains why the mycelium sends down into the bread, root- like branches called rhizoids, that secrete enzymes, by means of which the food substances in the bread are digested. Some of the hyphae form long branches called stolons, which run along the sur- face of the bread, forming new plants. At points where rhizoids are developed, there arise later numbers of erect branches, or spo- rangiophores, on the tips of which are developed sporangia, or spore- bearing organs. 174 ORGANISMS ILLUSTRATING RIOLOGICAL PRINCIPLES Great numbers of tiny spores are produced by division of the dense terminal portions of the sporangiophores. As a sporangium becomes mature an outer wall is formed and the spores turn black in color. When this outer wall breaks, the minute spores are scattered far and wide by air currents. Molds also reproduce sexually, by means of con- jugation. Rhizopus has two different strains of mycelia, one of which is called a plus ( + ) and the other a minus ( — ) strain. If hyphae of two such strains come in contact with each other, zygo- spores are formed. Short, club-shaped branches are developed from the hy- phae, the dense proto- plasmic tips are cut off from the end of each by cell walls, and these "cells," each of which contains several nuclei, unite to form a zygote. The zygote with the hyphae which develop from it proba- bly represents the diploid stage of chromosome in the life cycle, the haploid stage being reached when the spores on the sporangium germinate. The fungi are of even more interest by reason of their method of nutrition. They are typically neither holozoic nor holophytic, since they live as saprophytes on dead organic materials. This means that they must absorb food materials which are supplied to them from outside sources after digesting them by means of enzymes, when absorption takes place through the plasma membrane of the cell. Alternation of Generations in the Plant Kingdom The most important difference in the life cycle between the Bryo- phytes or Mosses and lower forms, aside from a greater differentiation of the plant body, is the alternation of an asexual with that of a sexual generation in the hfe cycle. The asexual generation, which produces spores, is called the sporophyte, while the sexual generation, which Reproduction in bread mold (Rhizopus nigri- cans). Read the text and then explain the diagrEun. THE DEVELOPMENT OF SEXUALITY IN PLANTS 175 gives rise to gametes of two different sexes, is known as the gameto- phyte. The latter generation is the conspicuous green plant that manufactures food and serves as host for the sporophyte generation which is permanently attached to it. The gametophyte of the simple moss, Funaria hygrometrica, is a short upright stalk bearing usually three spiral rows of simple leaves, sperm embryo Sfertili^ect egg rontheridiunz. mum. dbcmetophone , \ bud protonemol threocC ycrunS gb-metophyt' The life cycle of Funaria, a moss. Which stage is more prominent, gametophyte or sporophyte? each containing numerous chloroplasts. At the lower end, a group of small brown rkizoids furnish the means of attachment to the sub- stratum. The moss plant is dioecious, having separate sex organs on different plants. The male gametophytes are shorter than the female gametophytes and bear at the upper tip a cluster of structures known as antheridia. Each mature antheridium looks like a tiny club with a wall formed of rather large, thin cells, which forms a recep- tacle for numerous motile sperm cells. The female gametophyte bears at the apex of the short stem, although in the mature plant 176 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES hidden by leaves, a cluster of flask-shaped structures called archegonia, at the bottom of each of which is a single rather large egg cell. Fertilization of the egg can take place only when the antheridia and archegonia are wet from rain or dew. In such an event the sperm cells ooze out in a mucilaginous substance secreted from the walls of the antheridium and pass in drops of water to the necks of the flask-shaped archegonia. Here they are chemically attracted by a substance exuded from the inside of the archegonium and swim down the tubular neck until one meets the egg cell, when fertilization takes place. The gametophytic phase of the moss is the haploid stage of the chromosomes, fertilization of the egg restoring the diploid number characteristic of the sporophyte. This generation begins with the cell division which follows the fertilization of the egg in the archego- nium and results in the growth of a tiny stalk, bearing at its upper end a capsule, that in the adult sporophyte is filled with asexual spores. During the formation of the spores within the capsule, the formative tissues produce a number of large, rounded spore mother cells, from each of which by nuclear divisions tetrads, or groups of four spores, are formed. During this tetrad formation, a reduction division takes place so that the spores contain only the haploid number of chromosomes. The moss capsule is quite a complex structure with a cap, or oper- culum, that covers an urn-shaped affair bearing at its upper end a circle of teethlike structures collectively called the peristome. As the sporophyte ripens it dries up and the numerous ripe spores are scat- tered by the action of the peristome teeth, the latter being very hygroscopic, or sensitive to moisture. When the weather is humid or wet, the teeth of the peristome curl up and when dry they straighten out, thus expelling the spores, which may then be scattered by the wind. The germinating spore does not grow directly into a leafy plant, but first forms a protonema or algalike filament from which upright stalks later arise, while rhizoids grow downwards from it, thus forming again the moss plant. This life cycle with its alterna- tion of gametophytic and sporophytic stages is characteristic of the life cycle of mosses and liverworts, as well as the higher group of the ferns (Filicinae). In the flowering plants (Angiospermae), one finds an almost com- plete suppression of the gametophytic generation, the sex cells or gametes being produced in modified leaflike parts of the flower. The floral parts — sepals, petals, stamens, and carpels — are thought of THE DEVELOPMENT OF SEXUALITY IN PLANTS 177 as leaves which have become metamorphosed from their vegetative form and function to hold the sex structures. The stamens and pistil (carpel) contain spore-forming tissues which, by means of reduction division, produce pollen grains containing microspores (sperms), while ovules produce a female gametophyte and its egg. The sperm cells are formed in the pollen grains, while the egg cells germinating poller tube osUs of anLher form pollen grains with— ^ sperm 2,-^ ^; tuba nuclau^ TRe embr/o sac contains a dividimg nuclaus ^ v '/ bolUn eight cxne jlnally formed, tuJo forn-i fusion rnAcleu^ /^tL-ubo Development of male and female gametophyte in the flowering plants. Only the cells which actually form these structure are shown. The parts of the sporo- phyte upon which the gametophyte is parasitic are omitted for the sake of clarity. Read the text carefully and then use the diagrams. are held within the ovary of the pistil as has been previously stated. In the angiosperms or flowering plants the male gametophyte is so much reduced that it consists of only three cells, a tube nucleus and two generative cells (see figure). Just previous to the formation of the pollen grains (male gametophyte) reduction di\ision takes place so that its cells contain the haploid number of chromosomes. The female gametophyte is also greatly reduced. After reduction divi- sion, the megaspore divides (see figure) one nucleus migrating to each end of the etnbryo sac (female gametophyte). The nuclei continue to divide until eight are formed in two groups at opposite ends of the embryo sac. From each group a single nucleus then unites with the other to form a fusion nucleus (see figure). At this stage the egg nucleus is ready for fertilization by the sperm nucleus. A double ferti- lization now takes place, the sperm nucleus fuses with the egg nucleus and the second sperm nucleus unites with the fusion nucleus. The 178 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES former gives rise to the young plant, the latter to its food supply, the endosperm. The transfer of pollen in flowers of the same species may result in the fertilization of the egg and subsequent growth of Division I Thallopbyta algcxe. "^ Division I Bryophyta Division BE Tr-ccchsopViyta.- vasculcti- T=lcL"ts subdivision A*B;vc ^abdVvi/ion D PteropsicCcc ferns ^^rnnospe4*ms angiospcrrrjs primiuve plants Lvcopsida ,Sl*enopJic£a ^ener-cction Diagram showing relation of sporophyte and gametophyte generations in the plant kingdom, the plant body (sporophyte generation). The evolution of sporo- phytic and gametophytic generations in the plant kingdom is shown in the above chart. SUGGESTED READINGS Coulter, J. M., Barnes, C. R., and Cowles, H. C., A Textbook of Botany, Vol. I, American Book Co., 1930. This text gives an excellent foundation for the understanding of sexu- ality in plants. Gager, C. S., General Botany, P. Blakiston's Son & Co., 1926. A general botany which gives much information on economic questions, as well as sex development in simple plants. Robbins, W. J., and Rickett, H. W., Botamj, D. Van Nostrand Co., 1929. Chs. XV-XXIV. Excellent diagrams help in the understanding of the development of sex. Sinnott, E. W., Botany, Principles and Problems, 3rd ed., McGraw-Hill Book Company, 1935. Chs. XI and XIV-XXIII. A thoroughly up-to-date treatment of the subject. Wilson, C. L., and Haber, J. N., Plant Life, Henry Holt & Co., 1935. An interesting and well-written elementary text. I IX DIVISION OF LABOR IN THE COELENTERATES Preview. The Hydra, a representative of the phylum Coelcnterata ; the ectoderm and its functions ; the endoderm and its functions ; reactions to stimuli ; reproduction ; regeneration ■ Hydroids • Suggested readings. PREVIEW It has already been shown that unicellular animals may exhibit considerable complexity of structure, and that associated with this complexity, there is a separation of functions in different parts of the cell, but we have not traced this division of labor into the many- celled animals or metazoa. The colonial forms, such as Pandorina, Eudorina, and Volvox, claimed by both botanists and zoologists, are interesting exam]iles of aggregations of many cells showing little evidence of organization or division of labor. Even in the colony of Volvox, most of the cells have common functions, only the reproduc- tive cells being set off from the others. The Hydra, a tiny animal little higher in the scale of life, gives every evidence in its structure of being a simple organism and not just a collection, or colony, of cells. It shows, in a convincing manner, how a simple, many-celled organism lives. It answers the question of how division of labor might arise among the cells of a simple organism, For this reason it is chosen as a type in most courses in biology and so has a place in this text. The Hydra, a Representative of the Phylum Coelenterata Hydras are quite abundant in many ponds or slow-moving streams, where they may be collected on the stems and leaves of aquatic plants. In an aquarium, they often leave these plants and become attached to the glass walls of the aquarium, where they appear as tiny brown or green cylinders one-half of an inch or more in length. At the free or so-called oral end, a circle of tentacles surrounds a conelike area, the hypostome, in which the mouth is found. The opposite, or aboral, end forms a disklike structure which is provided with mucous cells that aid it in sticking to a surface. Hydras are able to move slowly by a looping motion of the body. The green ones, which are much more 179 180 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES active than the brown ones, frequently change their position if food is not abundant. They respond to chemical stimuli of food, to light, and to unfavorable temperatures, food being the chief factor in their environment. The color of green hydras is due to the presence of Hydra is able to change its position both by turning "handsprings" as shown in the diagram and also by contraction and expansion of the basal portion of the body. minute green algae, called Zoochlorellae, that live in a symbiotic relationship within the endodermal cells. The term, Coelenterata, which is the name of the phylum to which the common Hydra vulgaris belongs, comes from the Greek words koilos, hollow, and enteron, intestine, which may be translated "hav- ing an internal digestive cavity," an apt title, since a Hydra is really a hollow, double-walled bag. The Ectoderm and Its Functions The bulk of the outer layer of cells (ectoderm) is made up of large epitheUo-muscular cells, having a layer of muscle fibers placed lon- gitudinally at their bases, that enable the animal to lengthen or shorten its body. A similar layer of fibers on the inner layer of cells which run circularly around the body allows it to expand or contract in diameter. Between the epithelio-muscular cells and near the inner margin of the ectoderm are found numerous smaller interstitial cells from which are derived numerous other cells, including the cnido- blasts. Nerve cells are likewise scattered throughout the ectoderm, forming a nerve Jiet at the base of the epithelial cells. Cnidoblasts are most abundant on the tentacles, although they are found on all parts of the body exclusive of the basal disk. They hold four kinds of stinging capsules, nematocysts, by means of which the animal paralyzes living prey that comes in contact with its tentacles. The nematocysts are capsules containing a hollow inverted thread which under certain conditions can be thrown out, together with a poisonous substance, hypnotoxin, that has the power to paralyze any other small animal which it touches. The nematocyst reacts to cer- I DIVISION OF LABOR IN THE COELENTERATES IJil tain chemical stimuli that apparently cause a change of osmotic pres- sure within the cell, thus forcing out its threadlike portion. After a nematocyst is protruded, the cnidoblast dies and is soon replaced by another. stinSina "nerve cell "muscular absorbing cell ^ .flagellum -sensory cell -^ cell ® cxxnthmd cell The Endoderm and Its Functions By cutting a section through the body of a Hydra its similarity to a two-walled sac is evident. Between the ectoderm and the inner layer of cells (endoderni) a thin, structureless layer called the mesoglea forms as a secretion from the cells of the «^toclerm j e«dod^m inner and outer layers. Mesoglea forms much of the bulk of other coelenterates like the jellyfishes. The endo- derm consists principally of large vacuolated cells that have flagella at the free or inner end, al- though they are also capable of developing pseudopodia at this end. Circular contrac- tile fibers are developed at their basal end. Thus they are endothelial-muscular cells. In the third of the body nearest the basal end, gland cells develop, which secrete digestive enzymes. Nerve and sensory cells are also found in the endoderm. For a simple animal, the Hydra seems to have many kinds of cells. What is the use of so many ? The answer is found in the way it gets food, ingests it, and finally absorbs it into the body cells. By watch- ing a hydra in the aquarium it will be seen that its tentacles are con- stantly moving as if seeking food. If a tiny bit of raw beef is placed within reach, the animal will bend over and carry the meat to the mouth, the edges of which soon close around it, forcing it inside. If the piece is too large to be taken in, the Hydra actually turns inside out in an attempt, usually successful, to put the meat inside the gastrovascidar cavity. Once inside the cavity, digestive enzymes from the glandular cells act upon the food, gradually breaking it down into Sections through the body wall of hydra showing the two layers of cells separated by the striated lamella secreted by the basal parts of the ecto- dermal and endodermal cells. 182 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES smaller and smaller fragments. Digestion appears to be aided by the churning movements caused by expansion and contraction of the body wall. Ultimately some of the food is reduced to a soluble state, and absorbed into the endodermal cells. Meanwhile some of the large vacuolated cells put out pseudopodia and engulf some of the undigested food particles, finishing the digestive process inside their own cell-bodies. Thus Hydra has two types of digestion, one intracellular, like that found in all unicellular animals and, there- fore, more primitive ; the other, extracellular, that is, taking place in the digestive cavity. Most of the food of the Hydra is digested in the latter way, the cells lining the cavity absorbing the digested food before passing it along to the cells of the ectoderm. According to Hegner, part of the absorbed food is in the form of oil globules which are passed over to the cells of the ectoderm and stored there for future use. Unusable or undigested material is thrown out of the digestive cavity by a sudden contraction, there being no other way of eliminat- ing such wastes except through the surface of the body, as in lower forms. Hydra like other animals uses oxygen to release energy. Respiration probably takes place through the surface of the entire body, the cells receiving oxygen and giving off carbon dioxide by diffusion through the cell membranes. Reactions to Stimuli Hydra show very definite reactions to certain stimuli, most of which have to do with obtaining food. Hungry Hydra are much more active than well-fed ones, and respond to various chemical stimuli besides reacting to mechanical stimuli, to heat, to light, and to electricity, all of which indicates the possession of some sort of simple nervous system, since the movements made are more or less co-ordinated. If touched lightly on a tentacle with a needle, only the tentacle contracts, but with increased stimulation, the other tentacles contract, until finally, the whole animal draws down into a little ball. Its physiological condition, according to Jennings,^ determines whether it " shall creep upward to the surface and toward the light, or sink to the bottom ; how it shall react to chemicals and to solid objects ; whether it shall remain quiet in a certain position, or reverse this position and undertake a laborious tour of exploration." The nervous system of Hydra forms a nerve net. It consists of a concentration of primitive nerve cells about the base of the hypostome 1 Jennings, Behavior of the Lower Organisms. Columbia Univ. Press, 1915, p. 231. DIVISION OF LABOR IN THE COELENTERATES 183 and the foot. This network of cells lies in the ectodermal layer of the animal, and receives impulses from sensory cells as well as trans- mitting them to the muscle fibrils. The sensory cells of the ectoderm .vary in their location ; one type occurs on the tentacles, one on the hypostome, and a third on the foot (base). Neuro-sensory cells which are located in the mid-body area .also resemble nerve cells, except that they send processes to muscle fibrils and so become intermediate between those receiving stimulation and those making the response. Some nerve cells appear in the endodermal layer but are not, so far as can be deter- mined, connected with the ecto- dermal nerve net. Reproduction Probably the most important function of the interstitial cells is their growth into sex cells. Most Hydras are hermaphroditic, that is, have both kinds of sex cells present in the same individual, but since the sperm cells and ova ripen at different times, fertilization is accomplished by sex cells from different indi- viduals. Sperm cells are produced by the mitotic division of interstitial cells, each of which first produces a number of parent male cells, contain- ing the somatic number of chromosomes. The nerve net in a young hydra as seen with an intravitani methylen- blue stain. Note the ringlike ar- rangement in hypostome and foot. What effect might such an arrange- ment have on movement? (After J. Ilodzi.) These cells divide four times and in the process a reduction division takes place, leaving the sperm cells with just half as many chromosomes as the body cells. A somewhat similar process takes place in the formation of the ova. One interstitial cell becomes larger than the others, rounds into a sphere, and is surrounded by other interstitial cells, which serve as an ovary for the growing egg. The latter continues to grow in size, form- ing yolk from the surrounding cells. Just before the egg becomes mature, the process of maturation takes place (see page 429), dur- H. w. H. — 13 184 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES ing which the number of chromosomes is reduced to half the body- number. Spermaries and ovaries can be seen in the Hving Hydra as little lumps on the ectoderm. The spermaries are always found near the free end of the body, the ovaries, when present, being nearer the base. The egg is fertilized while still at- tached to the parent and develops into an embryo surrounded by a protective chitinous case, in which stage it sinks to the bottom of the pond for a resting period before emerging as an adult. Asexual development also takes place. A small bulging area, formed by the interstitial cells, ap- pears on the side of the body, which more or less rapidly grows into a short column surrounded by tentacles, depending on the food supply available for the parent Hydra. When * fully developed the bud may separate from the parent and lead a separate existence. A Hydra fre- quently produces more than one bud on a single animal. young hixd sperrr? — cells forming ec.todJ2.rm endocferm- jonriing" Longitudinal section through the body of a Hydra, showing both sexual and asexual repro- ductive structures. Regeneration Although regeneration takes place in other groups of animals it is best seen in the phylum, Coelenterata. The primitiveness of Hydra is shown by the fact that it can regenerate or replace lost parts by growth of the body cells. It may be cut lengthwise or crosswise, or even into small pieces, and the fragments will, under favorable con- ditions, give rise to complete individuals. DIVISION OF LABOR IN THE C0ELENTE1\ATES 185 Hydroids Hydra vulgaris is a fresh water form, but many more representatives of the Coelenterate group are found in salt water, the most famiUar being the hydroids found attached to the piles of wharfs and other submerged objects. Among the most common hydroids are members hydranth. -gbnobVzsca ^9 ^onacC medusa /;<^^^»v^^",'^^ ^^^ (^..fertiTe asexual *~-^-Viyctrorhi3a. - y Stage / ^..blastula ^T-planula Life cycle of Obelia — showinff alternation of generations.' Compare with text pages 18.5-186 for explanation of diagram. of the genus Obelia. These animals form colonies, in which the indi- viduals, called polyps, or zooids, are attached to each other by means of hollow stalks, covered with a chitinous, cellophanelike perisarc. At the tip of each branch, the covering expands into a cuplike hydro- theca, which surrounds the living polyp. As in Hydra, each individual polyp of Obelia is hollow and two layered, with a circle of tentacles about the raised hypostome, in. which the mouth is located. The tentacles are provided with nematocysts that act in the same manner as in the Hydra. The food cavity, however, extends down each stalk- like branch or individual and is continuous with that of the other polyps, thus forming a common gastrovascular cavity in which food 186 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES is digested. There are also cells, as in Hydra, which perform intra- cellular digestion. Obelia gives rise to another type of polyp than the nutritive individ- ual just described. This is the reproductive polyp, or gonangium that grows out as a bud, expands into a knoblike central axis known as the hlastostylc within a chitinous, closed vase, called the gonotheca. On the sides of the blastostyle budlike structures, called medusa buds, develop. These break off and swim away as tiny bisexual jellyfish, or medusae, representing the sexual stage in the life history. A sperm cell from one of these medusae fertilizes an egg from another, which, after a developmental period, becomes a free-swimming ciliated larva, called a planula. After a short time the planula settles down and produces a new asexual colony of Obelia. Other related forms as the jellyfish, Aurelia, possess a predominating free-swimming stage, while the sessile, non-sexual generation is reduced. This life cycle is reminiscent of a similar condition in plants, which also have an alternation of generations. During the maturation of the sperm and egg cells, reduction division takes place in which the chro- mosomes of the sex cells are reduced to half the body number. In alternation of generations of plants, all the cells of the gametophytic generation are haploid, but as in animals only the mature sex cells are haploid, the body cells having the same number of chromosomes as the body cells of the sexual generation. The end result accomplished in both plants and animals is the same. SUGGESTED READINGS Curtis, W. C., and Guthrie, M. J., Textbook of General Zoology, 2nd ed., John Wiley & Sons, Inc., 1933, pp. 278-301. Guyer, M. F., Animal Biology, Harper & Bros., 1931, pp. 197-206. Hegner, R. W., College Zoology, The Macmillan Co., 1936. Ch. X. An authentic description of Hydra and its activities. X BEING A WORM Preview. A typical worm ; external structure of the earthworm {Lum- bricus terrestris) ; the digestive tract and its functions ; how blood circulates, the blood and its functions ; organs of excretion ; the muscles and their work ; reactions to stimuli ; the nervous system and its functions ; the reproductive system and reproduction • Regeneration • Suggested readings. PREVIEW Passing from the simple two-layered development of the Hydra, in which division of labor among the cells is slight, we come to the earth- worm, another lowly animal, but one which represents the big idea of a typical three-layered, segmented form. In Hydra, the egg develops into an adult form having two layers, namely, edodertn and endoderm, but in the earthworm, a third layer, the mesoderm, appears, w^iich is characteristic of all the higher animals. These three germ layers are of great significance in the study of animals, for all of the complex tissues of the body are derived from them. Another reason why the earthworm is chosen for study is because it represents a very simple type of segmented or metameric animal of which a great variety is found not only among worms but also among insects and crustaceans. Judging by the insects, segmented animals are the most abundant and successful of all animals, since they out- number all other species. The pages that follow will concern them- selves chiefly with the "hows and whys" of the activity of the common ''night crawler," some of which are: How far has division of labor progressed ? What organ systems are well developed ? How does co-ordinated movement take place, and how do worms become aware of their surroundings ? A Typical Worm External Structure of the Earthworm (Lumbricus terrestris) The body of the earthworm is divided into segmented parts, or metameres, which in adult worms may number over one hundred. The body tapers bluntly at each end, the anterior end being easily 187 188 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES distinguished by the rounded mouth which is just ventral to or under a small protuberance, the 'prostomium, while the anus, or posterior end of the digestive tract, is a tiny slit in the last segment. The posterior end is also flattened, and between segments 32 to 37, not counting the prostomium enclosing the mouth as the first, there is found a swollen region, called the clitellum, important in reproduction. The upper or dorsal side may be distinguished by its darker color, while the ventral side is slightly flattened and contains four double rows of tiny projections called setae, which give the worm a grip on the ground when in locomotion. The dorsal side is devoid of any The common earthworm, Lunibricus ierresiris. Wright Pierce Note the swollen area, or clitellum. openings except some very minute dorsal pores that communicate with the body cavity, or coelom, but the ventral side has several paired openings, difficult to find, which lead to the reproductive and excretory organs. The surface of the body is covered with a delicate iridescent cuticle, secreted by the living epithelial cells of the skin, but which is itself dead. Its iridescence is caused by the presence of numerous grooves (striae), and its surface is pierced with small holes, which are openings for the mucous gland cells of the skin. The coelom or body cavity is cut up into small compartments by partition walls, or septa, that are absent or incomplete in the extreme anterior region, between the 18th and 19th segments, and in the region posterior to the reproductive organs. The coelom in the living worm is filled BEING A WORM 189 Septum Vnuscle,--- hear-ts also 3,4-.S seroinal — receptacle with fluid which passes from one segment to another through single perforations in each of the septa. The fluid contains ameboid cells, that probably serve as scavengers, and it acts as blood, bathing and nourishing the tissues and carrying away wastes. The Digestive Tract and Its Functions The food of earthworms, bits of animal or vegetable matter mixed with soil, is taken into the mouth by means of suction. A muscular pharynx, previously moistened by the fluid poured out from small glands in its wall, is able to pull the material into the esophagus, a thin-walled part of the tube which extends from the 6th to the 15th segment, beside whose walls, between segments 10 to 12, there are embedded three pairs of whit- ish structures, the calciferous glands. These glands produce a limy secretion supposed to neutralize the food materials. The esophagus leads into a thin-walled crop, occupying the 15th and 16th segments, which opens into a thick- walled, muscular gizzard ex- tending over segments 17 and 18. The latter organ has an internal chitinous wall, and is probably used to macerate bits of undigested food by means of muscular contraction. The remainder of the food tube, ex- tending from the 19th segment to the anus, is called the intestine. Its inner surface is increased by a fold on the dorsal side (typhlosoJc) , while surrounding it there is a layer of yellow-brown tissue cMorogogen cells, which are thought to aid in excretion and possibly digestion of food. The wall of the intestine contains gland cells that secrete at least three kinds of enzymes, which digest starches, fats, and proteins. The digested food is absorbed Seminal vesicle.—. cConsal vessel .pViarxnx .p'hag"tc5 .Caldfe.r<3US glancfs .crop intestine t/pbloSole rzerve CorcC ventral vessel three ofhen-.-r^-l vessels *■ The earthworm {Lnmhricns ierrestris) opened from dorsal side to show internal structure. (After Sedgwick and \\ ilson.) 190 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES through the walls of the intestine, most of it passing into the blood and directly into the coelomic fluid, where it may continue to the muscular wall outside the coelom. Unusable material, mostly earth, is passed off by muscular contraction through the anus, and may often be seen on lawns as little piles of "castings." How Blood Circulates Since in the earthworms there is a very different arrangement than in Hydra, where food is directly available to all the cells, we would expect to find some means of distributing it to the tissues where it may be used. This is accomplished by means of a closed system of blood vessels. Some idea of the circulation may be derived by a study of the accompanying diagrams. Five large blood vessels run lengthwise through the body, one dorsal vessel, close to the food tube, into the walls of which it sends two pairs of lateral vessels in each segment ; another, the ventral vessel, runs just ventral to the digestive tract and also sends lateral branches into its wall. There are also three others, the paired lateral neural vessels and the suhneural vessel, which run longitudinally, the latter directly under the nerve cord, and two other smaller ones lying parallel one on each side and above the nerve cord. Five "hearts," so called because of their frequent contrac- tions, encircle the esophagus in the region of the 7th to the 11th segments, connecting the dorsal with the ventral vessel. Blood passes into the dorsal vessel especially from a long typhlosolar vessel which helps drain absorbed foods from the intestinal walls, flowing forward until it reaches the " hearts." Its forward movement is caused by slow, regular contractions of the dorsal blood vessel. The blood passes posteriorly through the "hearts" and then flows into the ventral blood vessel. Here it passes poste- riorly, although some of it moves from the hearts toward the nerves buccal cavity Esuprcicsophatfeol , tiixurnssoiohigeal Sub esopho^ol .„psai, vessel lateral vessel ■esof>hogus ..ventral VGSjel - - (trop .nsrve ccnet ■with. IntM-al neurai vesssis intastina. The circulatory system of the earthworm. BEING A WORM 191 anterior end of the body. Blood also passes tliroush two intestino- integumentary vessels which pass off at the 10th segment to supply the walls of the esophagus and the skin, and to nephridia of that region. Parietal vessels connect the dorsal and subneural vessels, cross Section of typyosolar vessel / '>_Jat^rccl-y2eu:ral vessel V nerve CorcL The '"hearts" of the earthworm. How do they function in circulation.^ that branch from the ventral vessel to supply the body muscle walls and nephridia. Blood also passes from the ventral vessel to the body walls, and to nephridia, and returns to flow, after passing through capillaries, into the lateral neural trunks. In the subneural vessel, the blood flows posteriorly and thence up by way of the parietal vessels into the dorsal vessel. Both dorsal and ventral vessels supply the anterior part of the worm. The Blood and Its Functions The blood of the earthworm consists of a liquid plasma, carrying colorless corpuscles which are flattened spindle-shaped bodies. The red color is due to hemoglobin, the same oxygen-carrying substance found in the blood of man. But in the earthworm the plasma is colored rather than the corpuscles. The exchange of food and oxygen, which the blood picks up in the intestine and body walls, respectively, occurs in the tiny lymph spaces around the individual cells. Respiration takes place through the moist outer membrane 192 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES of the skin, where the oxygen is picked up and combined with the hemoglobin, to be later released in the cells of the body where work is done. Carbon dioxide and wastes are here taken up by the blood and carried back to the skin and to the nephridia or excretory organs. One can easily demonstrate the network of tiny capillaries in the skin where this exchange takes place. Organs of Excretion The paired nephridia are essentially coiled tubular organs, made up of a ciliated funnel or nephrostome that opens into the coelom, a thin ciliated glandular ^-^epttcin like region tube, that loops on itself about three times, and a pore, the ncphridiopore, through which the excre- tory products pass to the exterior. Some excretory materials are probably taken directly from the coelomic fluid by means of the currents caused by the cilia, while other wastes may be taken directly from the blood- capillaries which cover the surface of the glan- dular tubules. One characteristic feature of the nephridium is that it always passes through the septum separating two segments. A nephridium of an earthworm. Trace the passage of fluid from the coelom to the exterior of the worm. Note the ciliated surface of the neph- rostome. What is its function .!^ (After Wolcott.) The Muscles and Their Work Movement is brought about by muscular contraction. As an earthworm crawls, a wave of contraction from the posterior toward the anterior appears to move up the body of the worm. A careful examination shows that movement is brought about by the contrac- tion and relaxation of two opposing groups of muscle fibers and by the movement of the rows of setae on the ventral surface. The muscles are arranged in two layers just under the skin, an outer circular layer running around the body and an inner longitudinal layer. When the worm lengthens, the longitudinal muscles relax BEING A WORM 193 and the circular muscles contract, while a shortening of the worm results from a contraction of the longitudinal muscles and a relaxing of the circular muscles. Each stiff seta is placed in a little sac, from which it extends out beyond the surface of the body. Inside the sac, attached to the seta and to the outer body wall, are two pairs endocterra ^ •muscle.-, peritoneum ^^^ TOphridium ^Cuticle ectoderm Circtxlar ^."peritoneum. muscle ^nephricCiTopore <-Seta / verztro-l vessel lataml vessel 'wentral rjerve- cord. subnsLcral vess-©! Cross section through earthworm. Compare this with cross section of Hydra. What advances in complexity of structure flo you find:' In the earthworm the most noticeable difl'erence is seen in the coelom. which is formed by a sphtting of the mesodermal bands in the embryo (seen on page 197). Note that the coeiom is completely lined by a delicate membrane, the peritoneum. Notice also the longitudinal fold or typhlosole which gives more surface to the inner wall of the intestine. What is its function .^ In the diagram, the funnels of the nephridia are not shown. Explain why this is so. of muscles by means of which the seta can be directed forwards or backwards, depending on the direction the worm is traveling. When the worm is moving forward, the anterior end is extended, the setae, that are pointed backward, are set into the ground, serving as an- chors, while the posterior end of the worm is pulled forward by means of the contraction of the longitudinal muscles. 194 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES Reactions to Stimuli Earthworms live in soil and make burrows which extend from a few inches to several feet under ground. They are nocturnal and lie in their burrows not far from the surface during the day time, coming out at night to forage for food. In winter, they go below the frost line, remaining there inactive. In hot and dry weather, they go as far down as possible into the earth, while a heavy rain will bring them out of their burrows in great numbers. Earthworms react positively to mechanical stimuli. A vibration on the earth will send them down into their burrows. They are positively attracted to surfaces of solid objects, as can be seen if worms are placed on moist blotting paper in a covered pan. They will soon be found lying along the edges of the pan, where two surfaces are in contact with the body. This response to contact apparently keeps them quite constantly in their burrows. They react positively to certain chemical substances, like foods, and move away from others. A match that has been dipped in ammonia and placed near the anterior end of an earthworm will demonstrate this reaction. They respond positively to moderate moisture, which is needed for respiration through the body covering, and to different intensities of light, by withdrawing from bright areas and moving toward weak illuminations. Like Hydra, however, reactions to stimuli depend largely on the "physiological condition" of the worm, that is, upon internal rather than upon external factors. The Nervous System and Its Functions The earthworm has a simple type of central nervous system con- sisting of a ventral nerve cord, with thickenings, called ganglia, in each segment, a dorsal "brain" or supraesophageal ganglion, made up of two ganglia, and a "ring" of nervous tissue, called the circum- esophageal connectives, which extends around the esophagus, connect- ing the "brain" with the ventral nerve cord. Lateral nerves, which leave the "brain" and cord to end in muscles, skin, and other organs, form a peripheral nervous system. The worm does not have visible organs of sensation, but the skin, especially at the anterior and posterior ends, is dotted with groups of tiny sensory cells. Some of these are sensitive to light, and still others probably to odor. Stimuli received by these cells are transmitted to the central nervous system by means of nerve fibers. Those which lead from the sensory cells to the central nervous system are known as afferent fibers, while out- BEING A WOIIM 195 going fibers which originate in nerve cells within the cord are known as efferent or motor fibers, since they end in muscle cells and stimulate them to contract, thus causing motion. The unit over which these impulses travel is called a neuron, which is the term given to the nerve cell and its prolongations. (See page 340.) In the earthworm sensory anterior- SerjSory c<=ll5{r<2cepto«) epidermis.': ■muscle cells ;e|^fecton$) , ^—Septu.rrL \j •postsrior The nerve cord of the earthworm showing neurons concerned in the reflex arc. Explain how adjustment to an unfavorable condition might be affected. How might movement in another segment of the worm be co-ordinated with the one shown in the diagram.'' (After Curtis and Guthrie.) impulses are passed longitudinally, both anteriorly and posteriorly, by means of the peripheral nervous system, and these impulses are modified by means of adjustor neurons in the central nervous system. This accounts for the co-ordination between segments as the worm crawls toward a desirable object or suddenly withdraws from a harm- ful situation. The Reproductive System and Reproduction Earthworms have both testes and ovaries in the same animal, and are therefore hermaphroditic, but they are not capable of self- fertilization. Two pairs of testes lie attached to the anterior walls of 196 ORGANISMS ILLUSTRATING RIOLOGICAL PRINCIPLES segments 10 and 11, and are enclosed by the ventral unpouched portion of two of the three seminal vesicles. Dorsally the three pairs of large pouches of the seminal vesicles in segments 9, 11, and 12 are light-colored structures easily seen in a dissection. Immature sperm cells are passed from the testes to complete their development in the seminal vesicles. Two pairs of vasa efferentia in somites 10 and 11 fuse to form the paired vas deferens that carry the sperm to the exterior through the male openings on segment 15. A pair of tiny ovaries are attached to the anterior septum of segment 13, the eggs i^..-^ Semirzal rsceptocle --.'tes'tis .--fur\T\©l ^^..)... seminal vesicle ovctr^ ovicCuct spsrm. duct^ Reproductive organs of the earthworm. The seminal vesicles are cut away on one side to show the funnels of the sperm ducts. Read your text carefully and explain how reproduction takes place. passing from this into the oviducts which open to the surface on seg- ment 14. Fertilization of the eggs is accomplished by the process of copulation in which two worms, placing themselves in opposite directions, become "glued" together on their ventral surfaces by means of mucus secreted from the glands of the clitellum region. While they are thus placed a mutual transfer of sperm cells from the seminal vesicles of one worm to the seminal receptacles of the other takes place, rhythmic muscular contractions of the body helping to force the sperms along. Then the worms separate. Later, when the eggs are to be laid, a cocoonlike band of mucus is formed by the clitel-. lum, which is forced forward by movements of the worm, and as it passes by the oviducal pores, receives the ripe eggs. When it passes over the opening of the seminal receptacles on the ventral surface of BEING A WORM 191 sperm ...mesooCarm. mesoderm onus V. segments 9 and 10, it receives sperm cells from the other worm that have been stored there. The girdle is passed down over the anterior end of the worm, slipped off, forming a closed case which contains the eggs, sperms, and a nutritive fluid. These capsules may be found in late spring under stones, boards, logs, or in manure heaps. After fertilization, the egg of the earthworm divides first into two, then four, then eight cells, and so on, continuing until a hollow ball of cells, called a hlastula, is formed. These cells are not all the same size, larger cells appearing on the lower pole of the sphere, which begins to flatten and show a depres- sion, forming eventually a hollow cuplike affair, called the gastrula. This process known as gastrula- tion places the larger cells of the lower pole on the in- side of the cup where they become the endoderm, leaving the outer cells of the sphere to form the ectoderm. Meantime a third layer of cells which lies between the other two layers buds off and be- comes the mesoderm. This latter layer gives rise to the musculature, blood vessels, and most of the excretory and reproductive tissues ; the endoderm forms the food tube and much of the glandular material con- nected with it ; the ectoderm gives rise to the epiderms, the nervous system and sense organs, and the outer portions of the nephridia, repro- ductive ducts, and digestive tracts. The young worms remain in th(^ egg case until they are about an inch in length. When first hatched they have no clitellum, since this organ appears only in mature worms. gostrola Stages in development of earthworm. Fig- ures II-V. Segmentation of egg and formation of blastula. Figures VI-VIII. Sections, show- ing formation of mesoderm as a band of cells. IX. Late stage of gastrula, showing coelomic spaces in mesoderm bands. X. Longitudinal section of young worm showing food tube, mouth and anus. (After Sedgwick and Wilson.) 198 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES Regeneration Earthworms, like other members of the lower phyla of the animal kingdom, have the ability, under certain conditions, to grow new parts. Experiments have been made by Hazen, Morgan, and others that show if a sufficient number of segments are present a worm may regenerate a new posterior end, or even a new anterior end. Earth- worms have even been successfully grafted end to end. SUGGESTED READINGS Curtis, W. C, and Guthrie, M. T., Textbook of General Zoology, 2nd ed., John Wiley & Sons, Inc., 1933. Excellent chapter on the Annulata, pp. 350 to 375. Darwin, Ch., Formation of Vegetable Mould, D. Appleton & Co. An easily read classic which ought to be known to every student of biology. Hegner, R. W., College Zoology, 4th ed., The Macmillan Co., 1936. Chapter XV is a well-written and authentic chapter on the Annulata. XI THE POPULAR INSECT PLAN Preview. The insect body plan; the head and its appendages; the thorax and its appendages ; honey manufacture ; digestion ; circulation, respiration, and excretion ; the nervous system • Reproduction and life his- tory • The life in the hive • Suggested readings. PREVIEW It would seem right in a text on biology that a representative of the largest and most successful group of animals should be described and that more than a passing glance be given to this enormous group, which contains far more than half of all living animals. We are always meeting insects, because they are so plentiful rather than from choice. They annoy us when we are in the woods, they bite us when we are lolling on the beach at the seashore, they get into our foods and render them unfit for use, or they eat our stored clothes. Worse than this, they defoliate trees, and sometimes destroy forests, and take their tithe of the nation's food crops. A good many have been implicated in the transfer of disease and some have actually rendered regions uninhabitable by man. Biologists have a good reason for a study of representatives of the great phylum, Arthropoda, because the arthropod plan of structure is the one employed by the majority of the species of the animal king- dom. In its simplest form, it represents an organism made up of segments, each body segment bearing a pair of jointed appendages. The head always bears at least one pair of jointed antennae or feelers, jointed mouth parts, and usually compound eyes. The body is pro- tected by an exoskeleton composed of chitin secreted by the cells beneath. A digestive tract passes straight through the body and there is a nervous system such as we saw in the Annelids, consisting of a ventral nerve cord, a dorsal "brain," and a nerve ring about the esophagus. Dorsal to the food tube is an elongated heart, there being no closed system of blood vessels. Such a simple arthropod would be difficult to find for laboratory purposes, so we have to use other more specialized forms. From the strictly biological point of view there is another reason for the study of an insect. It offers an example of a segmented ani- H. w. H. — 14 199 200 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES mal that has gone in for specialization in a big way. The insects are a subdivision of the Arthropods, animals that have jointed legs and jointed bodies, and as such show definite repetition of similar parts, or metamerism, a phenomenon previously noted in the Annelids. As a group they have become differentiated to such an extent from their not so distant relatives that, like the man on the flying tra- peze, they ''fly through the air with the greatest of ease." In no other group except the birds has this ability been so exploited. In addition some forms, such as the bees, ants, and wasps, show an astonishingly complex social life. As a successful group insects show numerous adaptations, not only in structure but in life habits. They are not only active but often so inconspicuous as to pass unnoticed by their enemies. Insects are characterized by a rapidly growing larval period associated with an abundance of food. The protected pupa is characterized by internal changes fitting the organism for the active reproductive life of an adult. They deserve our careful consideration as a type for study. The Insect Body Plan Adult insects are readily identified because the body is made up of three parts, an anterior head, a mid region or thorax, and a posterior region, the abdomen. The body may be further subdivided into Wright Pierce The large vagrant grasshopper {Schistocerca vaga Scudder) normal size. A typical insect. Give all the distinguishing marks of an insect as shown in this photograph. THE POPULAR INSECT PLAN 201 segments and has three pairs of jointed thoracic legs. These charac- ters distinguish any insect. If you will refer to the "Roll Call" you will see that the various orders of insects are distinguished by still other characters, such as the presence or absence of different kinds of wings, or differences in the structure of the mouth parts, which may be modified for various purposes. All insects breathe through tracheal tubes and have a body .^./Vclypeus upper Up mandibla rTncuciUotry pcdptxs hypoph tarsus ^-articularis ■poiten cojtib middle leg' of worker honey beer- „f)dten Ijasket vnelatorsxTS hind leg" o^-workei" Money bee -^lanta. inner .surf a Smith, T., Parasitism and Disease, Princeton Univ. Press, 1934, p. 111. THE ART OF PAIUSITISM 219 worm, and the spiny-headed worm, for example, there is no trace whatever of a digestive tract in the adult. Such worms, however, have access to various digested foods which are ready for absorption by the host and it appears certain that these gutless forms must be able to absorb and utilize materials from the alimentary canal of their benefactor. Other worms, such as the flukes or trematodes, and roundworms, possess a well-developed alimentary canal, the secretions of which, in some instances at least, cause a liquefaction of the tissue in the immediate vicinity of the parasite, thus making it available as food for the organism. Another problem which parasites have had to solve is that of respira- tion. In the case of cellular or blood-inhabiting forms the parasite obviously has access to plenty of oxygen, whereas intestinal parasites face a difficulty, since the alimentary canal is known to contain little oxygen. Many investigators now believe that these worms secure their energy from the breakdown of dextrose. This substance results from the hydrolysis of more complex carbohydrates and is the form in which it is absorbed from the intestine into the blood stream. Presumably oxygen is secured during the process of anaerobic fermen- tation that results in the splitting of dextrose or glycogen (if the carbohydrate has been converted into glycogen during the metabolism of the parasite) into fatty acids and carbon dioxide. This type of metabolism is characteristic of some bacteria and yeasts. One of the most striking effects of the parasitic habit lies in the tremendous development of the reproductive capacity of the parasite, a process undoubtedly correlated with the numerous hazards which must be met if its life cycle is to be completed. The development occurs in two ways, — first by the production of enormous numbers of eggs, and secondly by the interpolation of asexual stages in the cycle. Thus it has been estimated that a single free-swimming, ciliated stage {miracidium) of a fluke may be the indirect parent of as many as 10,000 free-swimming, tailed larvae {cercariae). External or ectoparasites also show marked evidence of adaptation to their type of existence, as shown by the piercing and sucking mouth parts of the parasitically inclined arthropods or the degeneration of the mouth parts in the case of the adult botflies, as well as by the laterally compressed body of the flea, and the loss of wings in lice and bedbugs. Limitation as to the host and as to the location on the host shows specialization among this group. These factors tend to illustrate stages in the development of ectoparasitism. 220 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES Keeping the Cycle Going The chief problem of any species centers about maintaining itself, a statement to which there is no exception in the world of parasites. Obviously those organisms which have become adapted as ectopara- sites are not faced with complicated problems relating to the transfer from host to host. By means of simple contact a new host may be reached, or if a portion of the life cycle of the parasite is free living, it may leave the host to deposit its eggs. Even in cases where the eggs are laid among hairs of the host they usually fall to the ground to develop. A more difficult problem of maintaining the species must be faced when internal parasites are involved. Bacteria which are capable of producing protective capsules or spores of one sort or another are tided over unfavorable periods and so aided in reaching new hosts. They are adapted also for rapid reproduction. One worker has esti- mated that if the multiplication of bacteria were unchecked one cell would be the parent of 281,500,000,000 bacteria in two days. Such a mass at the end of the third day would weigh about 148,356,000 pounds. Many parasitic protozoa as well as metazoa are adapted to be transferred from one host to the next by means of resistant cysts secreted by the organism. Others, like the blood-inhabiting try- panosomes or malarial organisms, secure transference by adapting themselves to insects which act as wholesale distributors for the parasites. Some produce harmful toxins which occasionally kill the host. In such instances, however, one may be sure that the host is abnormal and the parasites have not become adapted to it. In the case of the trypanosomes of man in Africa, antelopes are their natural hosts and are quite tolerant to these blood parasites. Since man and domestic animals are unnatural hosts, they are consequently much more severely affected by them. The Complexity of Parasitic Relationships The most satisfactory way to secure a general idea of the surpris- ingly varied adaptations to a parasitic existence is by a study of a few examples. Such a study emphasizes clearly the almost uncanny adaptations which have been made by parasites to insure the com- pletion of their life cycles. While various types of parasitism clearly exist, nevertheless the line that demarks one kind of parasite from THE ART OF PARASITISM 221 another may not always be sharply drawn. However, for the sake of convenience an attempt will be made to outline briefly a few examples of such relationships. External Parasites External parasites are found throughout the plant and animal king- doms. Even among the minute protozoa, ectoparasitic organisms occur, such as Cyclochaeta, a parasite on fishes, which may cause an appreciable economic loss under epidemic conditions. The lam- prey eel among the chordates is a large external parasite on certain fishes. For the sake of con- venience, external par- asites may be classified as to whether they are temporary, periodic, or permanent. Some forms, like the house fly, do not really belong in any of these cate- gories. Yet the house fly certainly deserves mention, since it serves as a mechanical carrier from one host to another for the transfer of numerous bacteria and their spores, as well as the cysts and eggs of various other parasites. Temporary Parasites. As an example of temporary parasitism may be mentioned the parasitic Hymenoptera that lay their eggs on the eggs, larvae, or even the adults of other insects. During the developmental interlude they remain as true parasites within the body of the host until they eventually destroy it, at which time they cease their parasitic existence and become free living. The ichneu- mon flies, that belong in this group of parasitic Hymenoptera, each year attack and destroy great numbers of injurious as well as some beneficial insects. Another example of a temporary parasite is the ox botfly, the free living adult of which attaches its eggs to hairs on the legs of cattle. Upon hatching, the larvae penetrate the hide and 1^^ "-^l^'^^^LjEPmS^^^^^Mnr ll^Sj^Slmr' 1^' '^HP ^^^^^S.1 fW^'^^J American Museum of Natural History These brook lampreys are close relatives of a larger form which frequently attacks fish and remains as a temporary external parasite until the host is destroyed. What type of mouth is characteristic of this group ? 222 ORGANISMS ILLUSTI\ATING BIOLOGICAL PRINCIPLES wander through the underlying tissues of the host until in the spring of the year they come to lie beneath the skin, which is soon punctured to serve as an air vent. Finally, when the larvae are full grown they burrow out, fall to the ground, and there pupate, finally emerging as adult free living flies, destined to ruin many million dollars' worth of hides annually. i>e;Comes larva fall? to drouncC , pupatss rnatas, Iccys becomes j lodgecC xxndzr biole. tovaroC spring penetratss hide of cattle and ,^^ tissue during the. vinter Life cycle of the ox botfly. Periodic Parasites. Other arthropods definitely fall into the group of those that are periodically parasitic. Such forms are predators, and most of them are blood suckers, in which manner they may serve as a link in a chain of parasitism. Thus the female mos- quito serves as the carrier for organisms that cause malaria, yellow fever, and filariasis. Others like the tick or rat flea may not only secure a meal of blood from one host but at the same time be the means of transmitting Rocky Mountain spotted fever or bubonic plague to some other host. Certain species fall into the realm of parasites by their own right, the tick and botfly clearly belonging in this latter group. thp: art of parasitism 223 \\ ri(jhi I'iirct Longitudinal section showing mistletoe invading the tissues of its host. Permanent Parasites. Comparatively few organisms belong in this category. Some of the flukes with a continuous life cycle like the marine Epidella melleni, or the gill fluke, Ancyrocephalus, pass their entire cycle upon the same host, adding their progeny to the same ani- mal and so on ad infinitum. Similarly, the female head louse that cements her eggs, or "nits," to human hair from which newly hatched lice appear within six to ten days is another example of a permanent parasite. The new addi- tions to the community of head lice must soon feed upon the roots of the host's hair or else they will die. The parasitical mistletoe is practically permanent in habit, since it not only taps the life-giving fluids of its host but also lives for many years upon the same tree. Internal Parasites The food cycle plays a vital role in the dispersal of all internal animal parasites. It frequently happens that animals which suck the juices of plants or the blood of other animals play an important part as an intermediate host. It should be borne in mind that when carnivores are included in the chain of parasitism, the cycle tends toward greater complexity. A few examples will serve to illustrate this point. Parasites Requiring One Host. The adult hookworm, Necator americanus, lives in the small intestine of man, where the adult female is attached to the walls of the intestine and produces great numbers of eggs which are eliminated from the digestive tract in the early developmental stages. Under proper conditions of soil, temperature, and moisture, development of the larvae proceeds rapidly, so that hatching may take place within 24 hours. The small larval form is only about 0.25 mm. in length, but by the end of the third day it has nearly doubled in length and soon molts twice, then being in the 224 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES infective stage. Hookworm larvae may enter the body in food or drink, but the normal method of entry is by actively boring through the skin of the human hand or foot. For this reason the disease is called "ground itch" because of the inflamed areas caused by the to stomach egg passed, in fexies larva ready'' to infect -^ human iTost Life cycle of the liookworm. r-yo entrance of the larvae. Once liaving effected entrance into the host, the minute worms are passively carried through the blood stream to the heart and thence to the lungs, where they migrate out from the capillaries of the lungs and work their way through the delicate walls of the air sacs into the lung cavities. They next migrate up THE ART OF PARASITISM 22:, the lung passages over the "saddle" to the esophagus, and there are swallowed, reaching the stomach and eventually the intestine. Within the next fortnight two more molts occur, after which the parasites reach maturity, copulate, produce eggs, and continue the cycle. The large roundworm, Ascaris lumhricoides, lays eggs which de- velop into infective embryos within three weeks under proper con- ditions of temperature and moisture. After reaching the digestive tract of the host together with food or drink, the newly hatched larva burrows through the mucous layer and starts on a "10-day tour" following essentially the same itinerary as that of the hook- worm. Among the protozoa the Ameba, Endamcha histohjtica, the cause of amebic dysentery, is transmitted from one human host to another and thence to the outside world, and back again to the human large intestine by means of resistant cysts carried in contaminated food and drink. Parasites Requiring Two Hosts. The dread pork roundworm, Trichinclla spiralis, while a permanent parasite having a relatively simple life cycle, nevertheless requires two hosts to complete its cycle. The encysted larvae occur in a variety of hosts, but are normally secured by man through eating insufficiently cooked pork. The parasites mature rapidly in the small intestine and reproduce within twenty-four to forty-eight hours of their arrival. Each viviparous female produces between 10,000 and 15,000 larvae, which are depos- ited directly in the lymph or capillaries lining the intestine, and are thus circulated by the blood until they reach the voluntary muscles of the body. There, these minute roundworms leave the blood stream, enter the muscle fibers, where within a month a lemon-shaped cyst is deposited about them. Since man is not cannibalistic, the introduc- tion of these parasites into his body becomes a blind alley so far as completing the life cycle is concerned. Unfortunately, when these parasites are once established in the body, there is no way of getting rid of them. In due course of time, calcium carbonate is deposited about the cyst and eventually the parasite dies, but the obnoxious cyst remains to remind the infected person of his injudicious meal by frequent muscular pains which may accompany this infection for years. The normal hosts of TrichineUa seem to be the rat, mouse, and pig. The former are commonly found in numbers about slaughter houses and the percentage of their infection is usually 226 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES liigh. A great number of other animals have been experimentally infected. Nearly all of the taenioid tapeworms have a rather simple life cycle. In the case of the beef and pork tapeworms, for example, the infective stage occurs in the flesh of the host in a milky white cyst. When this larval tapeworm, or cysticerus, consisting of an inverted head or scolex and its outer cyst wall, is ingested by man, the head if imppcfparly cooked. , pork with "^ cysLs In musda. f ibens may develop vhan eaten 'by encysts in pork if hog' is "host- vulva oviduct tacome adults in smoll intestine, vithin afev dajs- ■females burro*^ into U5C mucosa , depos-it over 10,000 larvae^ intestine of hog- encyst in human Yntcscl© if man is host larvae enter blooeC stream , are carried to vol untoj'y muscles •muscles ^ , . , , of xnan which larvae penetrate and then The life cycle of Trichinella. becomes everted, and then attached to the intestinal wall, where the worm starts budding segments or proglottids and soon reaches sexual maturity. Proglottids of Taenia saginata, or proglottids together with free eggs in the case of T. solium, are passed with the feces and, when eaten by the proper intermediate host, develop into cysticerci. Cattle, buffalo, giraffes, and llamas may harbor the larval form of the beef tapeworm, while the hog, camel, monkey, dog, and man are the only known hosts for the pork tapeworm. The chief difference between the cycle of these two parasites centers around the possibility of auto-infection in the case of the latter. This occurs by ingesting the eggs destined for the outside, which hatch in the intestines, THE ART OF PARASITISM 227 lorozoitsi ifeetecL sctUvory glcmcd gametoejtc migrate to the blood stream and so reach various parts of the body, there producing cysticerci. As in the case of Trichinella, human infection really becomes a blind alley for the parasite. Malaria. One of the most economically important parasites is the causative organism of malaria, a minute spore-forming protozoan of the genus Plasmodium. The infective stage, or spo- rozoite, reaches the blood stream of man in the saliva of the mosquito, which is poured into the wound im- mediately after the victim is punctured. This minute parasite promptly pen- etrates a red corpuscle and starts to de\'elop asexually, growing until it fills about one half of the corpuscle. It is now ready to undergo the asexual reproductive cycle. The chromatin mate- rial is gradually separated into a number of tiny masses, each one of which finally becomes surrounded by a bit of cytoplasm. Growth continues until the red corpuscle is filled with e eye e o a number of new indi\-iduals called merozoites. Soon the corpuscle bursts, liberating these merozoites, each one of which seeks out a new corpuscle and begins the asexual cycle all over again. This asexual cycle recurs regularly, the intervals depending upon the species of parasite infecting the blood stream. Thus in the case of tertian malaria, schizogony is completed every twenty-four hours, while in the quartan type it takes forty-eight hours to complete it. The periodic chills and fever so characteristic of malaria occur at the time of the bursting of the red corpuscles with the subsequent release of the asexually formed merozoites and the accompanying waste matter. Quinine is the most widely used drug to combat the infection as it destroys the newly "hatched" merozoites. jneTO*oiteS^&S® 228 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES After a number of asexual generations have been produced, special larger, sausage-shaped crescents appear within the red corpuscles. These are the gametocytes, or sexual forms. If a female mosquito sucks blood from a person having mature male and female stages of the parasite in the blood, such i^a, parasites are taken mto the diges- tive tract of the mosquito, where union of the male and female gametocytes takes place. After conjugation the resulting zygote forms an ookinete or cyst that enters the lining of the stomach of the mosquito, in the outer walls of which a complicated de- velopment then ensues for about twelve days, ending with the formation of a large number of spindle-shaped structures called sporozoites. The cyst then bursts and the sporozoites migrate to the salivary gland of the mosquito. After that time, if the female mos- quito bites an uninfected human host she infects him with the sporo- zoites, which enter red blood cells. Animals are not the only group having complicated parasitic cycles. The various smuts, mildews, and rusts are plant parasites that annually take their toll throughout the country. Wheat rust is probably one of the most destructive of the parasitic fungi. This rust has been the most dreaded of plant diseases because it destroys the harvest upon which the civilized world is most dependent. Wheat rust has long been associated with barberry bushes. As early as 1760, laws were enacted in New England providing for the destruction of barberry bushes near wheat fields, although nothing was actually known of the relationship between the barberry and rust until com- paratively recent years. It is now known that wheat rust may pass part of its life as a parasite on the barberry, whence it migrates to the wheat plant and there undergoes a complicated life history. Since the nourishment and living matter of the wheat are used as food by Diagram of eggs, larva, pupa, and adult of Culex (left) and the malarial carrying Anopheles (right). How could you tell the eggs, larvae, and adults of these two genera apart ? THb: AllT OF PAllASITISM 229 the parasite, the plant is weakened and Httle or no grain is produced. A few of the wheat rusts do not require two hosts but complete their life cycle on wheat alone. Such rusts pass the winter by means of thick-walled spores which may remain in the stubble or in the ground until the young wheat plant appears the follow- ing year, or the spores are carried by the wind from other regions. Parasites Requiring More Than Two Hosts Tapeworms show a va- riety of adaptations and exhibit a unique and deli- cate balance that permits the completion of their various cycles. Roughly they may be divided into two groups, one in which the eggs reach water, sub- sequently passing through some aquatic organism, and a second in which ova are scattered in the soil and reach the intermedi- ate host by means of food or drink. In the first group are the broad tape- worm of man, the bass tapeworm, and many others, while the second includes the various tae- nioid worms and their relatives. All of these par- asites show a remarkable degree of specialization. N. Y. State Conservaiion Depi. The life cycle of the bass tapeworm {P. arnblo- plilis). (1) The mature tapeworm occurs in the intestines of the large- and small-mouthed hass. (2) Contact with water causes the proglottids to liberate the eggs which are eaten, (.3) by various copepods. \\ hen infected copepods are eaten by many species of plankton-feeding fish (1) a larval tapeworm (plerocercoid) develops in the mesenteries, liver, spleen, or gonads of these fish. Heavy infections in the small-mouthed bass affect reproduction. The tapeworm reaches maturity when fish infected with the larval stage are eaten by larger ones. How could this cycle be controlled in fish hatcheries!' The broad tapeworm of man, Diphyllobothrium latum, was brought to this country sometime during the last century by immigrants from the shores of the Baltic Sea. The worm matures in the digesti\e 230 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES tract of the host, producing a string of as many as 3000 to 4000 seg- ments or proglottids, often reaching a length of ten meters. Mature proglottids, passed from the host with the feces, must reach water, where the eggs are shed. After a developmental period in the water, the eggs hatch into free-swimming larvae (coracidia), which to continue development must be eaten by a copepod. The parasites penetrate the intestinal wall and so reach the body cavity of this host, where they develop until the copepod is in turn eaten by a fish, when they usually penetrate to the flesh of the host and grow to approximately six millimeters in length. Various fishes, such as the northern pike, Esox lucius, wall-eyed pike, Stizostedion vitreum, sand pike, S. cana- dense griseum, as well as the burbot, Lota maculosa, may all serve as second intermediate hosts for this important parasite. Man and other carnivores acquire the infection by eating improperly cooked fish. The bass tapeworm which matures in large- and small-mouthed black bass also requires three hosts — copepods, small fishes which carry the larval stage encysted in the viscera, and the final host, or adult bass. The life cycle of this parasite illustrates very clearly the interdependence of organisms necessary for the completion of the Adult yellow grub, enlarged from mouth cavity of hejxn« •i>;^@ e-Maturtegg ^o^' — "— "^^--tS N. Y. State Conservation Dept. Diagram of the life cycle of the yellow grub of bass (C. marginatum). (1) The adult fluke in buccal cavity. (2-4) Embryo within egg hatches as free living miracidium which, upon entering snail, produces a mother sporocyst and two generations of rediae (5-8), cercariae (8-9), liberated by the daughter redia, penetrate many species of fish (10-11) and mature when eaten by various herons (12). THE ART OF PARASITISM 2:{i cycle. The adult tapeworm matures sexually in the spring of the year, the mature eggs being shed into shallow water where the fishes come inshore to spawn. The eggs of the parasites are soon eaten by copepods and the developmental period necessary for the larval parasite to reach its second infective stage is closely correlated with the time interval between the laying of the bass eggs and the absorp- tion of the yolk sac of the bass fry. At the time the young fishes begin feeding upon plankton, the copepods in the vicinity of bass nests are found to be much more heavily parasitized than at other seasons of the year. It is adaptations such as these which enable parasites to complete complex life cycles. Flukes, or trematodes, probably undergo more complicated cycles than any other group of parasites. In considering the complex cycle of a trematode one should keep in mind that there are usually Diagram explaining the life cycle of endoparasitic trematodes. two free-living stages, — the miracidium and the cercaria. The variations that may be expected in such a cycle are apparent upon inspecting the above diagram. The frequent presence of a second intermediate host suggests a characteristic of most trematodes. For example, the great blue H. W. H. — 16 232 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES heron harbors an adult fluke, Clinostomum marginatum, in its mouth cavity. Eggs discharged by the parasite reach the water and soon hatch, the miracidia penetrating snails. After several generations in the snail, fork-tailed cercariae emerge to penetrate under the scales into the flesh and sometimes on the fins of many species of fresh-water fish. Here they grow into the typical yellow grubs commonly found surrounded by a cyst formed by the connective tissue of the host. As a result of the above discussion of parasitism it is hoped that some concept of the elaborate food chains and interrelationships and interdependence characteristic of the various groups of parasites and their hosts may be gained. Because these relationships are so com- plicated and form so intricately woven a pattern, it becomes prac- tically impossible "to predict the precise effects of twitching one thread in the fabric." SUGGESTED READINGS Cowdry, E. V., et al., Human Biology and Racial Welfare, P. B. Hoeber, 1930. Ch. XVII. Popular discussion, resistance, etc., from the bacteriological point of view. Elton, C., Animal Ecology, The Macmillan Co., 1935. Chs. V, VI. Excellent readable discussion of parasitism from an ecological view- point. Massee, George, Diseases of Cultivated Plants a>}d Trees, The Macmillan Co., 1910, pp. 1-23, 59-77. A good discussion of parasitic plants. Needham, J. G., Frost, S. W., Tothill, B., Leaf-Mining Insects, The Williams & WUkins Co., 1928. Ch. I. Deals with natural history of group. Smith, T., Parasitism and Disease, Princeton University Press, 1934. Excellent general, but somewhat technical, discussion of the parasitic habit. XIII ADVANTAGES OF BEING A VERTEBRATE Preview. Vertebrate cliaracteristics • Skeletons • Invertebrate attempts • The vertebrate endoskeleton • Suggested readings. PREVIEW How fortunate it is that we are vertebrates, not only vertebrates in general but mammalian vertebrates of the royal primate line which has blossomed finally into human beings ! When one thinks over the myriads of lowly, less endowed animals scattered along the devious highways of evolution, who might have been our near relatives, it is a real privilege to claim relationship with such highly endowed primates as monkeys and apes. With the inclusive vertebrate type, to say nothing of the specialized Pri- mates, there are certain outstanding structures and qualities which we as mankind are thankful to possess. They are so famihar to us, however, that we are apt to forget how far our fortunate biological heritage is dependent upon them. Only a few of these distinctive vertebrate characteristics that give us occasion for self-congratulation may be pointed out here. A consideration of the Vertebrates as such forms a biological science in itself, set forth in a voluminous Hbrary of descriptive and inter- pretative books. Vertebrate Characteristics Even a partial list of the distinctive vertebrate endowments would include the following: 1, a highly developed nervous system, based upon a hollow dorsally-located nerve cord ; 2, a unique embry- onic skeletal axis, called the notochord, which is the foundation for a living internal skeleton, adaptable to the changing demands of growth ; 3, a peculiar kind of blood, that in the higher forms makes the mainte- nance of a constant body temperature possible regardless of the sur- rounding temperature ; 4, various devices for effectually transporting the blood to every part of the body, devices that are as superior to the methods employed by non-vertebrates as modern highways and means of transportation are better than the conditions encountered in the days of the trackless wilderness ; and 5, locomotor organs for 233 234 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES getting about on land, in water, and in air, far surpassing those employed by lower animals. Skeletons Let us consider briefly just one vertebrate feature, namely, the living inside skeleton, which gives the name "vertebrate" to the group. It is the culmination of an endless array of experiments and adapta- tions that have been going on since the beginnings of life on this planet, and there is every indication that the end is not yet. The skeleton of man, for example, is by no means the final mechanism of its kind. There are to be expected in the future other models nearer to perfection, though based upon all that has gone before. Invertebrate Attempts The microscopic protozoa made brave experiments with the idea of a skeleton, in their case an armor mostly for protective pur- poses and consequently found located on the outside of the animal itself. In fact, protection seems to be the primary service of skeletal structures in general, although secondarily supplanted largely by the function of support and muscle attachment. It still plays an important role even in the vertebrates, since the brain and cord, being ex- tremely liable to injury, are enclosed within a protective skull and enveloping vertebral arches, while the viscera are in part stowed away within a bony thoracic basket. In the great group of arthropods, that includes both crustaceans and insects, the skeleton is plainly a protective external covering which, being a lifeless excretion of the skin, does not change in size after it has been laid down. As the anima'l grows, the dead inelastic skeletal armor thus formed fits more and more tightly over the enlarging body until finally it has to crack open in order that the animal may emerge and become refitted, after an interval of rapid bodily expansion, with a new and larger skeletal garment. This complicated process is called molting. To elaborate and then periodi- cally to reject all this material is not only a physiologically expensive performance but it is also a hazardous one, since while shifting into a more commodious suit of armor, the animal may lose a leg or two, and is always exposed for some time to enemies while in its defenseless shell-less condition. Insects, caught in the same evolutionary blind alley with their crustacean cousins, have taken an upward step by secreting a much ADVANTAGES OF BEING A VERTEBRATE 235 thinner chitinous envelope than the more cumbersome "crust" of the crustaceans. Instead of molting at repeated intervals throughout life, they have hit upon the idea of metamorphosis, whereby they do all their molting early during the growing larval stages. Then, as adults of established and unchanging size, they live happily ever after without being troubled by the inconveniences and perils of growth within an unada])tive external encasement. Another and paramount objection to a protective exoskcleton is the increasing burden of a heavy armor which soon becomes insup- portable, necessitating a limit to the size of the body encased within it. The largest known representative of the enormous group of the insects is probably smaller even than the smallest adult vertebrate. The mxoUuscs have gone at the problem of evolving a skeleton in another way. Although the skeleton is still on the outside, excreted and consequently lifeless, it is never wastefully molted after the crustacean fashion. The parsimonious molluscs keep every particle of their old dead shells and simply add new layers on the inside, as growth demands. The layers, being a little more extensive with each addition, form by their edges the familiar "lines of growth" showing as parallel ridges on the outside of the shell. This particular experi- ment in skeletons, however, has cost the group of molluscs dear, for the heavy shell, together with the accompanying policy of passive defense, has either impeded the power of locomotion with all attendant advantages that would accrue for the evolution of the nervous system, or has brought about its complete abandonment. The clams and their allies, therefore, have stuck conservatively in the mud and lagged behind in the race for life, while other animals without the incubus of a molluscan shell have toiled successfully on to higher levels of attainment in working out their organic salvation. The Vertebrate Endoskeleton The vertebrates alone exploit a fundamentally different model in skeletal structure. An increase in size being necessary for dominance in the struggle for existence, an adequate supporting scaffolding for the body is de- manded, and as a result the skeletal function of protection now be- comes secondary. Levers and muscles to work them to attain locomotion, with ample skeletal surface for their attachment, are also in- dispensable for animals that are to develop a successful nervous sys- tem. The vertebrate skeleton provides for these adaptive advances. 236 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES The fact that the vertebrate skeleton is inside the body makes it a changeable living structure which, by reason of its capacity for continuous growth, keeps pace with the increasing demands of the enlarging organism. With the introduction of such a scheme of mechanical support, the ban upon size imposed by a lifeless exoskele- ton is lifted, so that during the Age of Mesozoic monsters there were dinosaurs and similar beasts, for example, that were able to lift tons of flesh into the air upon majestic bony scaffoldings. These prehis- toric giants proved impracticably large, however, and vanished forever from the face of the earth after recording by means of their fossil remains the results of these colossal experiments in the mech- anism of living inside skeletons. There still remain today, elephants on land and whales in water as living illustrations of how far it is possible to go in the matter of size when an adequate living internal support is provided. The remarkable superiority of the vertebrate endoskeleton over all other skeletal devices is evident. It would be possible to go much further and to unfold some of the marvelous details of adaptation which every separate part of the vertebrate skeleton presents. That would call for many pages. It is the task of the comparative anatomist to assemble and elucidate the innumerable facts about the vertebrate plan of structure, of which those involved in the skeleton are a sample, and to point out wherein we are fortunate to be constructed as we are. This is a fertile field, inviting intellectual adventure for those who have the curiosity to explore it. SUGGESTED READINGS Adams, L. A., An Introduction to the Vertebrates, John Wiley & Sons, Inc., 1933. A fine text. Keith, Sir A., The Engines of the Human Body, J. B. Lippincott Co., 1919. Parallels intriguingly worked out for the mechanically minded. Neal, H. V., and Rand, H. W. Comparative Anatomy, P. Blakiston's Son and Co., 1936. Written by two masters of the subject. Newman, H. H., Vertebrate Zoology, The Macmillan Co., 1920. Just what the title indicates. Walter, H. E., Biology of the Vertebrates, The Macmillan Co., 1928. Many illustrations. Bibliography. Wilder, H. H., History of the Human Body, Henry Holt & Co., 1923. Told with literary grace without sacrifice to accuracy. THE MAINTENANCE OF THE INDIVIDUAL XIV THE ROLE OF GREEN PLANTS Preview. Structure of green plants • The raw food materials used by plants • The root and its work • The stem, structure and function • The structure of the leaf • How green plants make food ; carbon dioxide as raw material ; the role of water ; chlorophyll and light ; relation of artificial light to food making ; what goes on in the green leaf in sunlight ; chemistry of food making • Enzymes and their work • How food is used by the plant body • Respiration • Transpiration • The rise of water in plants • Produc- tion of oxygen by plants • Suggested readings. PREVIEW It is a trite statement to say that the destiny of man on the earth depends upon green plants. All living stuff is made up of the ele- ments found in air, in water, and under the earth's surface, yet no laboratory technician has ever been able to put this material together and make protoplasm. That energy is displayed by plants and animals is obvious, but no man has ever been able to energize matter and create a living organism. We know that the units of structure, the cells, do release energy and that this energy comes, as does all other energy, from the oxidation of fuel substances. Such fuels used by living things we call foods. Moreover, these foods, be they from plant or animal, in the long run depend upon energy derived from the sun. The Biblical declaration that ''all flesh is grass" is literally true, for without green plants animals would have no food. We do not think of plants as very dynamic objects compared with animals. Nevertheless, if we look at the soil pushed up by growing seeds, the pavement broken by the growth of trees, and even the hardest rocks split apart by the wedge action of growing stems and roots, we realize that plants are very much alive. They respond to the various stimuli in the environment, reacting like animals to temperature changes, to gravity, to various chemical substances, or to the directive force of currents of water. Unlike animals, whose metabolism is catabolic, the green plant's metabolism is more completely anabolic. It builds up materials to 237 238 THE MAINTENANCE OF THE INDIVIDUAL a greater extent than it tears them down in the metabolic process. Compare the growth of an animal with that of a plant such as a big tree. While the animal is more fixed in size and limited in age, the tree grows for a longer period of time and grows to a greater size. These differences are due to a continuous growth of the meristemic tissue already mentioned, and also to the fact that new tissues and organs grow continuously from this area of meristem that is found in growing buds, stems, and roots. The most important difference between green plants and animals lies in the fact that the green plant can make use of the sun's energy to manufacture foodstuffs on which not only it, but also the animals which eat it, depend. In an investigation of a living green plant, two methods of study present themselves. We can rather carefully dissect and study each system of structures which makes up the living organism and direct our attention to the microscopic make-up of each part. In this way a fairly complete picture will be had of the organism in its entirety. But such a picture will lack vitality. If the plant is a living thing, then why not study it from the point of view of function, of what it does and how it lives, using only so much reference to the structures as will make intelligible the work of the parts of the plant? This is the viewpoint adopted for this unit. The plant is to be thought of as a living, working organism, performing the same metabolic functions as any other organism, but in addition doing a different kind of work from that of animals, that of synthesizing organic foodstuffs out of chemical raw materials from the air, the soil, and the absorbed water. This unit, then, will bring up a number of important points. Among them will be such questions as these : What are the adaptations which enable the green plant to do its work? Where does the raw material from which food is manufactured come from and how does the plant get it ? Under what conditions is the work of food manu- facture performed? Where is food made and how does it get to the cells where work is done? Why is light necessary for green plants and why do they bleach in the darkness? Are green plants really as important as is here indicated ? These and similar questions will be answered in the pages that follow. Structure of Green Plants It is not easy to give a general description of a green plant. In the higher plants it is obvious that there are several well-defined regions which are called root, stem, leaves, flowers, and fruits. In these Wright Pierce (1) Eucalyptus trees, natives of Australia , which have found California so well suited to their needs that they are a dominant form there. (2) Rose, shrub showing bushy habit. (3) Snapdragon, a common annual. (4) Carrot, a biennial; note the food storage in the root. Of what value is this to the plant ? 239 240 THE MAINTENANCE OF THE INDIVIDUAL e e i regions several different methods of growth occur which will be described later. Some plants that grow more or less continuously, forming a woody body which resists cold and storm, are called trees or shrubs. Others die down at the end of the year, although they have some wood fiber in the body. These are the herbaceous plants, examples of which are peas, beans, and a variety of garden plants and roadside weeds. Herbaceous plants DiCotyledoiv MoaocotyledoR that produce seeds and die before the following winter are called annuals. A second group of herbaceous plants, called biennials, store food in the roots or underground portion of the stem. After the upper part of such a plant is cut down by un- favorable weather conditions, the following spring the underground portions send up a new shoot from the subterranean food supply. This gives rise to flowers and seeds at the close of the second year. Ex- amples of biennials are carrots, parsnips, and beets. A third type of herbaceous plants is the perennial, which grows each spring from the underground parts that remain alive during the winter. Many of our common weeds have this habit, which makes them difficult to eradicate. Woody plants, such as trees and shrubs, as we have seen in the unit on classification, are grouped either as conifers (the softwoods, pines, firs, hemlocks, and their relatives) or as deciduous hardwoods. The latter are placed with the flowering plants, and may be either monocotyledons or dicotyledons. These two groups have differences in the structure of leaf, stem, and seed. The monocotyledons usu- ally have parallel-veined leaves, like those of grass or lily. Their stems have scattered "closed" woody vascular bundles and a single cotyledon in the seed. The dicotyledons have netted-veined leaves, iS tern Differences between monocoty- ledons and dicotyledons; c, cotyle- don ; e, endosperm ; fb, fibrovascular bundles; h, hypocotyl; p, plumule. THE ROLE OF GREEN PLANTS 241 sudh as are seen in the elm, oak, or sassafras; stems with "open" vascular bundles which usually appear as a ring of growing tissue; and seeds with two cotyledons or seed leaves. These structures will be referred to in more detail later. The Raw Food Materials Used by Plants For a good many centuries after the time of the Greek philosophers who first hold this theory, it was thought that green plants absorbed food from the soil, but it was not until the time of the Belgian philoso- pher van Helmont, who lived in the sixteenth century (1577-1644), that it was clear that water played a very important part in the growth of a plant. One of van Helmont's experiments consisted of placing a willow slip weighing five pounds in a vessel containing two hundred pounds of dried soil. For five years he watered the tree with distilled water, making careful observations on it until it had grown to be a sapling weighing one hundred and sixty-nine pounds and three ounces. But when he weighed the soil in which the tree had grown, he found it had lost only two ounces. Clearly then, the gain came largely from sources other than soil, and he rightly concluded that water was largely responsible for the increase in weight. In the first half of the eighteenth century, an English clergyman, Stephen Hales, worked out the daily water consumption of a plant by ascertaining the relation between leaf and stem surface and the quantity of water absorbed. He went a step further than van Helmont in suggesting that plants take something from the air as well as the soil with which to build up their body material. In 1779, Ingen-Housz, a Dutch phy- sician, who was a co-worker with the famous surgeon, John Hunter, showed that the green part of a plant, when exposed to light, uses the free carbon dioxide of the atmosphere, but that it does not have this power when kept in darkness. A little later, in 1804, de Saussure, by a series of experiments, proved that carbon dioxide and water were both used by plants in the sunlight and that as carbon dioxide was taken from the atmosphere, about the same amount of oxygen was returned to it. He, however, missed the use of the green coloring matter of the leaf in its connection with the sun's energy in building living matter and food. The real explanation of the function of this green substance (chlorophyll) was left for Julius von Sachs, a famous botanist of the nineteenth century. He was the first investigator to demonstrate the fact that green plants make food for the world. Just how they do this is still not fully known, although plant physiologists 242 THE MAINTENANCE OF THE INDIVIDUAL have been experimenting and are still experimenting in the attempt to solve the problem. With this background, our point of view is to consider the living green plant as an organism, faced by the same kinds of problems as a living animal, taking a living from its environment, storing up food for the inevitable time of food shortage, and eventually forming fruits to hold the seeds which are necessary to pass the stream of life on to the next generation. Unlike an animal, the green plant takes raw food materials from its environment and, under certain favorable conditions, synthesizes them into organic foods, a process effected by means of a number of adaptive structures, in certain, favorable environmental conditions, the chief of which is sunlight. By burning the body of a hving plant until nothing but ash remains, and then making a careful analysis of this residue, frequently as many as thirty chemical elements are found. Twelve are nearly always present, eight of which are essential to plant growth. The latter are boron, calcium, iron, magnesium, manganese, phosphorus, potassium, and sulphur. It will be noticed that this list does not agree exactly with the previous list of elements usually found in the protoplasm of Hving things (page 131), but the implication is clear. The chemical elements found in living matter, as previously noted, are also found in rocks or soil, air, and water. The stage is set and it remains for the scientist to discover just how these elements, found in the environ- ment, can be made into food and living stuff by the green plant. A good many experiments have been made with plants to determine more exactly the function of these elements. It has been shown that if green plants are placed in a nutrient solution containing the necessary elements,^ growth will take place. If, however, certain elements are subtracted from the solution, the plants will not develop, or their growth will be considerably slowed down. Such experiments give us our first clue to one important use of the root. It is evidently an absorbing organ through which the plant takes in not only water, but some of the essential mineral materials necessary for its growth. 1 A list of the most commonly used nutrient solutions for plant growth are given below. Crone's solution : Water, 2.0 1. ; KNO3, 1.0 g. ; FeP04, 0.5 g. ; CaS04, 0.25 g. ; MgSOj, 0.25 g. Detmer's solution: Water, 1000 g. ; Ca(N03)2, 1.0 g.; KCl, 0.25 g. ; MgS04, 0.25 g. ; KH2PO4, 0.25 g. ; FeClj, trace. Knop's solution: Water, 1000 g. ; Ca(N03)2, 1.0 g. ; KNO3, 0.25 g. ; KH2PO4, 0.25 g. ; MgS04, 0.25 g. ; FeP04, trace. Pfeffer's solution : Water, 3-7 1. ; Ca(N03)2, 4 g. ; KNO3, 1 g. ; MgS04, 1 g. ; KH2PO4, 1 g. ; KCl, 0.5 g. ; FeCls, trace. Sach's solution : Water, 1000 g. ; KNO3, 1.00 g. ; NaCl, 0.50 g. ; CaS04, 0.50 g. ; MgS04, 0.50 g. ; Ca3(P04)2, 0.50 g. ; FeCls, 0.005 g. THE ROLE OF GREEN PLANTS 243 The Root and Its Work Recent experiments made by Weaver ^ and others show that plants have extremely comphcated root systems. The roots of an old oat plant, for example, although extending through only about two cubic yards of soil, were found to have a total length of over 450 feet. Weaver found that hardy wheat plants sent their rootlets into the soil six feet below the surface ^,CeJ7tml Cylinder- -_>v&ocf^ bundle -root "hciiT~ ictermis of the ground. In the bush morning-glory, a common plant of the mid-western plains, the roots may extend ten feet into the ground and a distance of twenty-five feet away from the parent plant. The roots of corn extend laterally three to four feet from the stem and sometimes over seven feet into the soil. All this is evidence for the great importance of the root as an absorbing organ. Examination of longitudinal sections cut from growing roots shows that the body of a root is made up of a central woody cylinder surrounded by layers of softer cells, collec- tively called the cortex. Over the lower end of the root is found a collection of cells, most of which are dead, ar- ranged in the form of a cap covering the growing tip. As the root pushes through the soil, the outer cells of this root cap are sloughed off, and are rapidly replaced by growing cells of meristem that are just under the root cap. The root cap proper is evidently a protective adaptation. In the woody region of the root are vascular tissues consisting of xijlc77i and phloem. These tissues form a series of tubelike structures which together with . >■-' J~OOt/ cctp Root of a dicotyledon, greatly magnified. Find the functions of each part labeled. How might soil water get from the outside of the plant into the woody bundles.'* ' Weaver, Root Development of Field Crops, McGraw-Hill Book Co. 244 THE MAINTENANCE OF THE INDIVIDUAL strong supporting woody cells constitute the vascular bundles that put the root in connection with the stem and leaves above it. If mustard seeds, for example, are germinated in a moist chamber, a few days after germination the lower part of the root will be found to be covered with a delicate, fuzzy growth. Ex- amination of the root at this stage shows an actively growing area of meristem, an elongating zone of tissue directly back of it, with a zone of maturing tissue above, which together make a zone of growth coincid- ing more or less directly with an area covered with fuzzy structures known as the root hairs. Root hairs vary in length according to their posi- tion on the root, the longer ones being found at some distance from the tip. They are outgrowths of the outer layer of epidermis. A single root hair examined under the compound microscope is found to be a threadlike, almost colorless structure. The delicate cellulose wall is lined by the protoplasm of the cell, the outer layer of which forms a selectively permeable mem- brane. Inside the root hair are found numerous vacuoles filled with cell sap. A nucleus is always present and may be found in the basal part of the cell, or in the hairlike portion itself. The root hairs are evidently living epidermal cells. An examination of a young plant growing in moist soil shows that the root hairs reach out between the particles of soil, apparently being closely cemented or attached in places to them. Each particle of soil is surrounded by a delicate film of water, which, with the dissolved minerals found in it, is absorbed into the root hair by the process of osmosis. The wall of the root hair is covered with a delicate layer of mucilagelike pecten formed by the outer layer of the cell wall and is also in contact with the moist protoplasm within the cell, which forms a delicate membrane Root hairs of corn, showing their relation to the root tip. Root hair, showing its relation to an epidermal cell. How do you account for the attachment of the soil particles to the surface of the root hair ? THE ROLE OF GREEN PLANTS 245 just under the wall. Diffusion takes place following the laws of osmosis, according to which water passes through a selectively per- meable membrane from a point of its greater to a point of its lesser concentration. This means that water passes from the soil into the cell sap, which has a higher concentration of solutes than does the water. Since the cell sap within the root hair has received a greater quantity of water, it in turn becomes a point of higher concentration of water than the cells lying next to it interiorly, and consequently, the flow continues from these outer cells to the adjoining cells which have a higher concentration of solutes. In this manner water is passed through the cells of the root i)ito the woody cylinder inside the cortex. Once having reached this region it passes up the tubes into the stem and on into the leaves as will be shown later. The Stem, Structure and Functions In thinking of the tree as a li^'ing organism, we are not so much concerned with the internal structure of the stem as with the way it functions. For many centuries it has been known that water passes up through the wood. If a tree is girdled — that is, if a narrow strip of bark extending inward as far as the wood is removed — the tree will keep its leaves for some time, indicating the upward passage of water which keeps them from wilting. If, however, a strip of wood directly under the bark is removed, enough of the bark being left intact to allow for passage of fluids, the leaves will wilt within a very few mo- ments. A cut branch of apple or willow placed in red ink after a few hours shows by a red circle, visible in sections cut across the stem, that the colored water has passed up through the outer layers of the new wood. In order to understand better the pathways for the rise of sap in the dicotyledon stem, one must study its growth. When seen in cross section, the vascular tissues of such stems are arranged in a circle. In some herbaceous stems, the woody bundles are separated by a parenchyma, but in trees, shrubs, and a good many herbs, the bundles are united to form a complete ring around the stem. These vascular bundles are open at each side and grow more or less continuously from a single row of meristem or embryonic cells which form a layer around the stem. This layer is called the cambium, and the growth of the wood and bark of our large trees is due to the activity of this always youthful layer of cells which, like the cells of embryonic tissue, continually divide and multiply to form internally the xylem or wood 246 THE MAINTENANCE OF THE INDIVIDUAL and externally the phloem tissue. In the spring when this tissue is very active, it forms a soft layer of cells that allows of the easy sepa- ration of bark from wood, a fact well known to any small boy who has made a willow whistle. It is not necessary to go into the details of stem structure, except to note that the cambium layer gives rise each year to new layers of Cross section of stem of Ricinus communis, a dicotyledon, showing cambium ring. In what area of the diagram does growth take place ? tissues, both internally and externally. The inner layers made up of secondary xylem are from the annual rings of a tree. In spring the growth of the tissue is rapid, while in winter it is very slow indeed or stops entirely, thus making the differences in the cross section shown in the figure. As the tree ages, changes may be noticed in the appearance of the older woody area forming the interior of the trunk. This wood becomes darker in color, its chemical composition changes, and it loses its ability to conduct water. It is known as the heart- wood as distinguished from the outer rings of wood called sap-wood. THE ROLE OF GREEN PLANTS 247 The latter conducts water, while the heart-wood functions merely as a supporting tissue. As the tree increases in diameter, the area of ,_bark .-Cambium layer- annual pith rays Section through a dicotyledonous stem. Explain its method of growth. heart-wood increases while the sap-wood, although greater in cir- cumference, gets proportionately smaller in extent. The bark, or area outside the cambium, is made up of several different tissues, which have a somewhat different arrangement in conifers than in deciduous trees. The area known as phloem is formed immediately outside the cambium. This area contains many living sieve tubes through which elaborated food is carried down from the upper part of the plant. The sieve tubes in the conifers are more or less regular in arrangement while in deciduous trees they are scattered. In both stems they are all surrounded by parenchyma. Scattered through the bark of deciduous trees are masses of tough, stringy schlercnchyma cells of two types, phloem fibers — fibrous, elongated cells that give strength and elasticity to the trunk — and thick- walled, hard stone cells. Outside the latter area is formed the corky layer, produced by a layer of growing cells known as the cork cambium. Cork cells, which have their walls impregnated with an insulating sub- stance called suherin, are of great value to the tree because they prevent a rapid loss of water from the tissues. It is this layer in the Spanish cork oak which is of commercial value. In some trees, such as the redwoods, the bark forms a coating highly resistant to fire. H. w. H. — 17 Above, sieve \ (' s s e 1 (of phloem) with c()mf>anion cell; below, sieve plate, with section of com- panion cell. (After Stras- burger.) 248 THE MAINTENANCE OF THE INDIVIDUAL Wright Pierce The characteristic lenticels of the white birch {Betiila populi folia). Note the placement of the lenticles. Scattered over the surface of twigs and young tree trunks are found many lenticels, openings in the corky layer which become filled with loose masses of cells. They are found both on roots and stems and act as pores which allow for the exchange of gases be- tween the living cells of the cortex and the me- dium outside. Lenticels are often spoken of as "breathing jDores" and experimental evidence seems to make this title valid. As the stem or trunk of a tree grows larger in diameter, there is an in- creasing area that uses water and foods. Cells cannot grow without food, and food in a growing plant cannot be made without water. The structures which put the water-conducting tissue of the inside of the stem in connection with the phloem of its outer part are known as vascular rays. They may be seen in almost any cross section of a tree which has produced secondary xylem and phloem. Here the cambium has rows of irregularly placed cells that instead of forming xylem and phloem produce ingrowing masses of more uniform parenchymatous cells making vertically placed strings of tissue. These bands act as conducting pathways for water from the xylem to the phloem and also as chan- nels for elaborated food from the phloem to the xylem, thus dis- tributing these materials to the growing trunk. Experiments by phi .oem (i:am\:)i.imri \ — •^yiein pith Note the bands of living parenchym- atous tissue that grow inward toward the pith. THE ROLE OF GREEN PLANTS 219 Aiichtor ^ have shown that food and water are not transferred from one side of a tree to the other, but instead that ahnost all of the water taken in is used directly above where it is absorbed, while food passes down from the leaves on the same side of the tree. There is seemingly little cross transfer of food or water in a plant stem. Vascular rays must not be confused with the so-called pith rays which are formed in herbaceous stems such as Ranunculus or in the stem of Clematis where, as the primary wood bundles grow in the pith, the pith forms narrow plates between the bundles. These appear as the pith rays in a cross section. Conditions of growth upon which the passage of food and water depend differ in monocotyledons from those in dicotyledons. If a stalk of celery or asparagus is placed in red ink over night, the color is seen to be located in little fibrous bundles of tissue which are scat- tered throughout the stem. If such a stained stem is examined in cross section under the microscope, it is found to be made up of pa- renchyma or pith which is dotted with little groups of woody cells of irregular size and shape. These are the vascular bundles which, Transverse section of stem of corn, a monocotyledon, showhiK the " scattered " vascular bundles which are cut in cross section. ■ Auchter, E. C. in Woody Plants?' " Is There Normally a Cross Transfer of Foods, Water, and Mineral Nutrients Univ. Maryland .\gric. Exp. Station, Bull. 251, Sept. 1923. 250 THE MAINTENANCE OF THE INDIVIDUAL instead of being located in a ring as in the dicotyledons, are scattered through the pith although more concentrated toward the outer edge of the stem. Examination of this outer edge or rind shows that there is no true bark, but that this outer area is made up of these same woody bundles closely massed together. Under high power, the bundles are seen to have outer strengthened walls of wood cells enclosing tubelike cells of different diameters of which the larger have pitted surfaces. The area containing these tubes is the xylem. Other elon- gated tubular cells having their ends perforated with small holes like a sieve, form the sieve tubes, w^hich are the conducting tissues of the phloem. In the phloem, the tubes pass foods down from the leaves, while the xylem A cross section through a closed monocotyle- carries water up from the donous bundle. Note that the thick-walled roots to the leaves. The xylem cells completely enclose the cells of the entire WOody bundle is en- ^ °^™' closed w^th a tough wall of sclerenchyma which gives strength and resiliency to the stem. Since this hard tissue binds the entire bundle, it is called a closed bundle. Monocotyledonous stems grow, then, through an increase and lengthening of closed bundles in the parenchyma of the stem. The end result in both monocotyledon and dicotyledon stems is the same. The vascular bundles put the root, stem, and leaves in direct communication. The root hairs at one end and the cells of the leaf at the other end are the opposite terminals of long communicating woody tubes. These tubes carry water and solutes up from the soil to the cells of the leaf, and, as will be shown presently, carry elaborated food materials down from the leaves to various parts of the plant, where they may be stored for future consumption or used immediately to liberate the energy needed in growth and in destructive metabolic changes. The vascular bundles which leave the stem to enter the leaves do so by way of the petiole or leaf stalk. As they enter the blade THE ROLE OF CxREEN PLANTS 251 of the leaf, they branch into bundles of ever smaller and smaller diameter to form the veins of the leaf. In the monocotyledonous leaves, these veins run in a more or less parallel direction as seen in grass blades or palm leaves. In the case of the dicotyledonous plants characteristic irregular and netted veins are found, reminding one of the branch- ing of the capillaries in the human body. These veins are made up structurally of tracheids and tracheal vessels, ser\'ing as water-conducting tissues ; sieve tubes, which carry out food materials from the leaf; and supporting tissue, which makes up the mechanical framework of the veins. Thus the veins act as a sup- porting skeleton for the leaf as well as conduits. The Structure of the Leaf The outer covering of the leaf (epi- dermis) is composed of a layer of irregularly shaped cells, usually rather flattened. In some plants, like the mullein, these cells are prolonged into hairs, or again the layer, as a whole, is frequently covered with a waxy cuticle which is impermeable to gases and water. The under surface of the leaf, as seen through the compound micro- scope, shows many tiny oval openings, which are called stomata. The position of the stomata varies in different leaves. Some plants, as, for example, water lilies, whose leaves float on the surface of the water, have them in the upper epidermis. Others have them on the under .side, and .still others have them on both surfaces. The estimated num- ber of these openings varies. Mac- Dougal estimates that as many as two million are on the under surface of an Stomata from the loaf of an Easter lily (Lilium lonyiflorum) : Above, a stoma, as seen in sur- face view, showing the two kidney-shaped guard cells {g), which enclose the stomatal aper- ture (s), the more deeply shaded portion representing the central slit ; note the chloroplasts in the guard cells; (b) subsidiary cells. Below, a stoma, as seen in cross section ; note the guard cell (g) next to the subsidiary cell (6) ; the outer slit (o) is enclosed between the cutinized outer guard-cell ridges (r), the enlarged area just below being the outer vestibule (o') ; below the central slit (s) is the inner vestibule (('), which here opens directly into the cavity (c) underneath the stoma. 252 THE MAINTENANCE OF THE INDIVIDUAL oak leaf of ordinary size, while four or five hundred thousand to a leaf is a common estimate. Surrounding the opening of each stoma are found two kidney-shaped cells, the guard cells, which can easily change their shape under certain conditions. They are of great importance in the life of the plant, since they control to a great extent the amount of moisture that may be lost from the leaf's sur- face. The guard cells are noticeably greener than the epidermal cells, the color being due to many tiny green chloroplasts. If the leaf is cut in cross section and examined under the microscope, it will be found to be made up largely of a tissue known as mesophyll. Lying close to the epi- dermis are one or two layers of elongated cells with the long axis placed at right angles to the sur- face of the leaf. These layers of cells are collec- tively called the palisade layer. Each cell of this layer contains numerous chloroplasts which are found in the protoplasm close to the cell wall. It has been estimated that a square inch of a sunflower leaf contains as many as thirty million of these chloroplasts, which are most important structures in the plant so far as food making is concerned. Below the palisade layer is a layer of numerous irregular cells containing fewer chloroplasts. These cells are known collectively as the spongy parenchyma. Be- tween them are found air spaces connected with the exterior of the leaf through the stomata. We have already noted that the veins form the framework of the leaf and in a cross section are often found occupying part of the area of spongy parenchyma. These veins connect the vascular tissue of the root and stem with the leaf. The petiole, or leaf stalk, is made up largely of vascular and supporting Cross section through a leaf; e, upper epider- mis, e', lower epidermis, showing stomata (s) ; I, intercellular spaces in the spongy parenchyma. Note the cross section of the vein (v). Why is the palisade layer (p) so placed ? THE ROLE OF GREEN PLANTS 253 woody tissue. At one point on the petiole, usually close to the main stalk, a little time before the leaves drop from deciduous trees in the fall, a layer of delicate, thin-walled cells is formed which extends completely across the petiole. This is called the separation or ab- scission layer, and it is at this point that the leaf is cast off. How Green Plants Make Food The general biologist is concerned not so much with the structure of the organism or with detailed minutiae as with the general metabolism of an organism as a whole. He wants to know how plants and animals act as living things, both alone and in relation to each other. We have examined the green plant from the standpoint of structure and are ready to consider it as an organic whole, as a living organism that releases en- leof on live plant + light- boilecL ■— r^ in -^oodi ■\>^ alcoViol positive reaction "where sta.r-cVi was locctte<:C ergy, respires, feeds, repro- duces, and in time dies. But we must remember that in addition, the green plant makes food, and it is this process upon which we will now focus our attention. It is a relatively simple matter to prove that sun- light is necessary for starch making in a leaf. Place a healthy green plant in dark- ness for a couple of clays. Then pin strips of black cloth over parts of some of the leaves and expose the plant to bright sunlight for a few hours. Later, remove the leaves and boil them to soften the tissues, adding alcohol to extract the chlorophyll, and finally, place them in a solution of iodine. A blue color will appear in those parts of the leaves exposed to sunlight, while the covered areas will be colorless. The appearance of the blue color in the presence of iodine is the regular test for starch, thus showing clearly that sunlight is necessary for starch making. Another simple experiment may be performed to show that air is also a necessary factor. Place a healthy green plant in darkness for two or three days, then carefully smear vaselin(> on th(> ui)i)(>r and lower surface of two or three l(?aves, leaving the others uiitoiiclicd. Proof that light is necessary for starch forma- tion in green leaves. 254 THE MAINTENANCE OF THE INDIVIDUAL Place the plant in full sunlight for a few hours, then remove the vaselined and untouched leaves, and treat both in the manner de- scribed in the last experiment. The leaves to which no air penetrated will be shown to have no starch. The need of carbon dioxide in the process of starch making may also be demonstrated by a relatively simple experiment. If plants are grown under similar conditions in two bell jars, but in one case the carbon dioxide in the atmosphere is removed by means of soda lime, while the other plant is left in the bell jar containing normal air, the latter continues to grow while the one lacking carbon dioxide does not increase in size. By burning a plant in a hot flame, it can be ultimately reduced to mineral ash equaling about 4 to 5 per cent of the entire weight. Ac- cording to Raber, from 1 to 55 per cent of the plant is consumed, while from 40 to 95 per cent, roughly speaking, consists of water. Since a green plant is immobile and since it has no way of obtaining material except from the air, water, and the soil that surrounds it, it may be safely assumed that if food is found in the plant body, it must be made there. That foods are found in plants is common knowledge. We eat roots, stems, fruits, and leaves of plants. Grains form our staples of food. Roots and various types of fruits form part of our dietary, while herbivorous animals live upon grasses and fodder crops. This brings us then to the sources of the raw materials out of which these elaborated foods must be formed. Carbon Dioxide as Raw Material Carbon dioxide is not only a product of respiration of animals but of plants as well. A man gives off about nine hundred grams of carbon dioxide daily into the air. Carbon dioxide also gets into the air from the combustion of inflammable materials. Volcanic eruptions and other sources of combustion increase the amount, while decaying organisms and the oxidation of rocks and soils add a very appreciable amount daily to the store. While it is estimated that there are only two grams of carbon in each ten liters of air, nevertheless the fact that carbon dioxide is universally available in the air and oceans close to the surface of the earth shows that it may readily be made use of by growing plants. Its need in food manufacture is well illustrated by the statement that the world crop of wheat requires annually one hundred and fourteen million tons of carbon dioxide in order to pro- duce the seventy million tons of carbohydrates which form this crop. Wrijjla Pierce The role of water. Upper photograph : The Mohave River near Victorville. This river rises in the San Bernardino Mountains and loses itself in a desert sink. What effect does it have upon the desert .►> Lower photograph: An irrigated orange ranch in the desert near Clareniont, California. Thousands of acres of trees now grow where desert conditions existed before irrigation. 255 256 THE MAINTENANCE OF THE INDIVIDUAL The Role of Water Water as a raw material needs little mention. The soil always con- tains more or less water, and the original source of water in its cycle through the oceans, the air, the clouds, and rain gives the earth a never ending water supply. When mm aids Nature in carrying water to dry areas by irrigation the desert literally is made "to blos- som as the rose." Certain chemical elements find their way into the plant body with this water. If the green plant is to manufacture organic food substances, it is evident that the elements carbon, oxy- gen, and hydrogen must come from the water and air. Various mineral salts, taken in by the root, furnish the necessary amounts of calcium, iron, potassium, sodium, and other elements, which leaves only nitrogen to be accounted for. Although nitrogen makes up approximately four fifths of the atmosphere, it is nevertheless unusable in that free form. It is an extremely inert gas and does not unite readily in combination with other substances. By means of the proc- ess of decay, however, and particularly through the nitrogen-fixing bacteria found on the roots of certain types of plants, this highly im- portant element is made available to plants. So much for the raw materials. Now let us turn to the machinery of food manufacture. Chlorophyll and Light Common observation shows that there is a relation between light and the green color of plants. We are familiar with the bleaching of celery stalks, with the curious blanched elongated shoots of a potato which sprouts in darkness, and with the fact that young seedlings are devoid of chlorophyll until after they have sprouted. Seedlings grown with light coming from one side turn to the source of light, while plants grown in a dark box having a hole on one side work their way toward the light. Obviously light has a very potent effect on the plant. Sunlight passed through a prism is broken up into seven primary colors ranging from violet to red, but passed through a spectroscope shows numerous dark lines traversing different areas in the spectro- prism. The most conspicuous are used as landmarks by physicists and for convenience have been designated by the letters A to H by Fraunhofer, their discoverer. These several wave lengths of light can be measured and it has been fovmd that they vary from 0.00076 mm. at the red end of the spectrum to 0.00039 mm. at the violet end. THE HOIJ-: OF r.REF^.N PLVNTS 257 Rays of greater and shorter length are also found at eaeh end of the spectrum forming the ultraviolet and infrared portions. The heat of light rays varies, Ijcing greater at the r(>d end of the spectrum. Since all life depends upon this I'adiant energy whose source is the 1 z ^ 4 I n m EA When a green leaf is placed in the path of light passing through a {)risni. dark strips appear, due to the partial or conipleh^ blocking of the light energy. These are shown in the absorption spectra above. .4, chlorophyll of Alliumiirsi- mim in alcohol; B, chlorophyll of English ivy {Iledera helix) in alcohol; C, chlorophyll of OscUlatoria in alcohol; D, carotin. 1, 2, 3, 4. absorption bands of chlorophyll; /, //, III. absorption bands of carotin; EA, end absorption. The lettered broken lines mark the position of the principal absorption hnes of the solar spectrum (Fraunhofer lines); the numbered solid lines form a scale from which wave lengths (X) in nullionths of a millimeter may be found by adding a cipher; note the increasing dispersion from left (red) 1o right (violet). (After Kohl.) sun, the green plant is no exception to this rule. Certain parts of the plant, however, are more susceptii)le than other portions to ra- diant energy. While the green leaf as a whole needs sunlight, it is only chlorophyll in the chloroplasts that is al:)le to utilize it for food making. If a chloroplast is examined under a very high magnification of the microscope, it is found to be a mass of living matter somewhat denser than the protoplasm surrounding it. In its disk-shaped struc- ture the green coloring matter is arranged around the outer part of the chloroplast, while the central portion usually contains a clear area 258 THE MAINTENANCE OF THE INDIVIDUAL filled with fluid. Chlorophyll is a very complex protein, apparently made up of two substances known as Chlorophyll A, having the chemi- cal formula C55H7205N4Mg, and Chlorophyll B, C55H7o06N4Mg. It is found to be somewhat like the hemoglobin of the human blood except that it has an atom of magnesium instead of iron and the property of fluorescence, its color being different in transmitted or reflected light. Chlorophyll in solution, when extracted from the leaf by means of alcohol, appears green as light passes through it, but red when light is reflected from it against a black background. Other pigments are closely associated with chlorophyll, a group of yellow pigments called carotins, which give the yellow color to carrots and other fruits or vegetables, and xmithophylls, pigments that help give color to leaves in the fall. Numerous experiments have been made to dis- cover how chlorophyll does its work. It has been found that if light is passed through this sub- stance and then broken up by a prism, that part of the light which is absorbed by the chlorophyll may be detected by the presence of absorption bands in the spectrum where the chlorophyll has taken out the light. By this means we learn that the red band of the spec- trum is most active while parts of the blue, violet, and indigo regions of the spectrum are also absorbed. A classic experiment by Engel- mann illustrates this in another w^ay. A filament of an alga was placed in a culture of bacteria which were active only in the pres- ence of oxygen. The filament was then put in darkness until the bacteria had used up all the oxygen present. Then the slide con- taining filament and bacteria was placed on a microscope under a solar spectrum. In a short time the bacteria were found to mass themselves in abundance at the red end of the spectrum and to a lesser extent at the blue end, because at these points more oxygen was given off by the alga, thus indicating activity in starch formation. al 3 C 5 E l\ D F '.■.'/• ii;| y.*''"^'--"'.v-/ ■'■*. «%.,. '-■■■.' ■'«■- •-•'.■'..•?-'- ^/A_ _ _ : •.•.-'•* .■-*■.■.'.■?... . m 1 ^ :::;•:.;; ■■^.■'■'•'.•.■■-- X-' ;p^^'-" ."*■'■■ -,•': ;■;v■■iv^,v:;:: ^^^'^^^S:0' /\ •' ''Z^'J^' I"." Engelmann's experiment to show the areas in the spectrum most favorable for oxygen release in a green alga. The dots represent bacteria. THE ROLE OF GREEN PLANTS 259 Relation of Artificial Light to Food Making We have already noted that there are great differences in the amount of sunhght required by plants. As a matter of fact, very strong sunlight may cause harm since it overheats the protoplasm, thus endangering the life of the plant. Moreover, it increases the rate of transpiration so that water is evaporated too rapidly. Experi- mental evidence with growing plants shows also that too much sun- light may retard growth. Some plants are shade loving, as may be Shade loving plants on a forest floor. Note the leaf arrangement with reference to light. seen in any field trip to a forest. The differences in illumination are correlated with differences in the structure of the leaf, the ])lants which are exposed to bright sunlight having a well developed palisade layer, while the spongy parenchyma is not so well developed. The reverse is true in shade-loving plants. In addition, plants that live in the shade are apt to have a very thin epidermis and usually ha\-e dull leaf surfaces which do not reflect the light as reatlily. Contrary to common belief, it is possible to grow i:)lants without sunlight as pro^'ed by recent experiments (Harvey) with a large number of different crop plants such as grains, tomatoes, squash, peas, potatoes, and others. Plants exposed continuously to the light 260 THE MAINTENANCE OF THE INDIVIDUAL of nitrogen-filled tungsten lamps of from 200 to 1000 watts produced both viable fruits and seeds. The bearing of this experiment upon growing crops in areas where the days are short and the intensity of sunlight not great is readily seen. Lamps have been put on the market for use in the home which provide space directly underneath the bulb for stimulating plant growth during the winter season. What Goes On in the Green Leaf in Sunlight When we examine the green leaf to see how it is adapted to use the energy of sunlight, several interesting facts are discovered. One is that a plant places its leaves so that they get the largest possible amount of sunlight, in a given period. Petioles and even stems of plants turn with the sun so that a maximum amount of green surface is exposed to its rays. Looking at a tree from above as the bird sees Diagram to show the cells of the palisade layer of a leaf at two different times during the day. Which of the two receives full sunlight ? it, leaves are found to be so arranged that there is a minimum amount of overshading, the leaves forming a sort of mosaic or pavement on which the sunlight falls. Examination of the internal structure of the leaf also shows that the palisade layers which contain the greatest number of chloroplasts per cell are massed close under the upper part of the epidermis. It is this layer of palisade cells wdiere most of the work of starch or sugar making takes place. In the cells themselves, the green chloroplasts are so placed that a maximum amount of light falls upon them. When the sun's rays are slanting during the morning and afternoon, light can reach all of them readily, while at the period of greatest illumination, when the sun's rays are direct, less light reaches them as they lie one above the other. Their position may be changed in the protoplasm, their movement being controlled by the THE ROLE OF GREEN PLANTS o^.i liviii■ \h + O CO + H2 — ^ CH2O (formaldehyde) 6 CH2O — ^ C6H12O5 (sugar) One objection to this theory is that carbon monoxide is extremely poisonous and is almost never found free in plants, while the product formaldehyde is also a poison. Later theories postulate that by first reducing carbon dioxide and water to carbonic acid, then to formic acid and hydrogen peroxide by the addition of a molecule of water, formaldehyde and hydrogen peroxide result, the peroxide being finally reduced to water and oxygen : C02+H20 = H2C03 (carbonic acid) H2C03+H20 = HCOOH (formic acid)+H202 (hydrogen peroxide) HCOOH+H2O = CH2O (formaldehyde) +H2O2 2Ho02 = 2H20+02 The last step in this process is brought about by an enzyme, known as catalase. Plant physiologists believe that although formaldehyde 262 THE MAINTENANCE OF THE INDIVIDUAL is a poison, it is probably changed into sugar so rapidly that at no time is there much present in the cells of the leaf. The last part of this process, that of changing the formaldehyde to sugar, seems to be brought about by the action of the two chlorophylls, A and B. One recent writer, Gordon, > has given the following suggestive formula: 6 C55H70O6N4Mg + 6 H2O = 6 C55H7205N4Mg + 602 (Chlorophyll B) (Chlorophyll A) 6 C55H7205N4Mg + 6 CO2 = 6 C,r,H7o06N,Mg + CeHisOs (Chlorophyll A) (Chlorophyll B) (sugar) To the amateur chemist this means very little, but it suggests the double action of the two chlorophylls in the formation of sugar. All we really know is that sugar is first formed in the green leaf and that later this is changed to starch and stored in that form in various parts of the plant. Of the manufacture of foods other than sugar very little is known. There are tiny droplets of fat in the vacuoles inside the chloroplasts. We know that fats can be synthesized out of carbohydrates by animals. Therefore, a similar process may take place in plants. Fatty tissue is undoubtedly manufactured out of the carbon, oxygen, and hydrogen contained in the sugar molecule. Probably a like situation exists in the chloroplasts of the leaves, although we do not know just how this process takes place. Proteins are even more complex than carbohydrates and fats. Their molecule contains nitrogen and a number of mineral salts, in addition to carbon, oxygen, and hydrogen. Protein foods are found not only in leaves, but in most of the storage organs of the plant. Apparently proteins can be synthesized out of the sugar plus the elements nitrogen, sulphur, and phosphorus, wiiich combine with the carbon, oxygen, and hydrogen of the glucose. Proteins are probably manufactured in other cells than those containing chloro- phyll, wherever .starches, sugar, and the essential salts are found, although light does not seem to be a necessary factor in the process. Proteins are undoubtedly used in any of the cells of the plant, just as they are in animal cells, for the making of protoplasm, since the plant is a living organism composed of cell units each of which is doing a common work for the plant as a whole. 1 Gordon, R. B. : " Suggested Equation for the Photo-synthesis, Action." Ohio Journal of Science, 29: 131, 1929. THE ROLE OF GREEN PLANTS 26:5 Enzymes and Their Work The changes just described which take place in food making as well as in food storage, all belong to a series of oxidative and reducing changes that are presided over and brought about by enzyme action, another indication of the importance of these omnipresent substances. We have already spoken of enzymes and their work, but reference to them again may not be amiss at this point. They are found practically everyw^here in the living cells of plants and animals, being much more numerous than was at first believed. Although their nature is not fully known, we do know that they are colloidal sub- stances, because they will pass through porcelain filter, but not through membranes. We also know that some of them are doubtless proteins, and that they are sensitive to light and ultraviolet rays as well as to heat, acid, alkali, and substances which are toxic to proto- plasm. They are powerful catalyzers, as is shown by the fact that a single gram of the enzyme invertase, for example, will quickly hy- drolyze one million times its weight of sugar. Enzymes are found in all living cells and are specific in action, that is, one enzyme will only do a certain type of w^ork. In general, they may be divided into a number of groups, depending upon their function, such as the hy- drolases, that act in the digestive processes of plants and animals by hydrolyzing materials ; the oxidases, which enable cell respiration to take place ; the fermentases, as, for example, remiin, that is used in cheese making, and the coagulascs, to which pedasc belongs that is used commercially in substances sold for use in jelly making; and finally, the carboxylases, which cause organic acids to split into carbon dioxide and other simpler substances. Specific examples of these various plant enzymes include the en- zyme, diastase, that causes the digestion of starch. Another enzyme, maltase, aids in the digestion of maltose to glucose, a still simpler sugar. A similar action takes place by means of the enzyme, ptyalin, in our own salivary digestion. Bacteria carry on a slightly different type of digestion in which cellulose or wood fiber is broken down and used as food. Here another enzyme, cellulase, causes this digestive change. Still another enzyme, called lipase, is instrumental in the digestion of fats. In fruits and seeds rich in fat, such as the avocado, Brazil nut, walnut, almond, or pecan, the fats are broken down into fatty acids and glycerine just as in animals where lipase is formed by the pancreas. H. w. H. — 18 264 THE MAINTENANCE OF THE INDIVIDUAL Protein digestion is l^rought about by a different group of enzymes, called proteases. These enzymes are found in abundance in leaves and germinating seeds of plants and to a lesser extent in practically all plant tissues. In the living plant, the digestive enzymes carry on a necessary and important work. If plants make foods in the green leaf, and they do, and if they store foods in the root, stem, fruit, and seed, then there must be some way to transfer the foods made in the leaf in a soluble form to those parts of the organisms where the food is finally used. This work of changing insoluble foods to soluble foods is obviously performed by enzymes. A still more interesting phenom- enon sometimes takes place. Many of these enzymes under certain conditions are capable of reversing their actions, that is, of converting a soluble substance like sugar into an insoluble one such as starch, or of changing proteins to soluble forms so that they can be transported through the vascular system of the plant and stored in insoluble form in seeds, nuts, and roots. The changes from sugar to starch may take place in leaves wherever certain plastids known as amyloplasts exist. These bodies have the power to form starch in the presence of a series of enzymes which first bring about the transformation of simple sugars to more complex sugars, and then to an intermediate substance between sugars and starches, called dextrin. Dextrin is changed into soluble starch by the enzyme, amijlase, and finally the soluble starch is converted into insoluble starch by the enzyme, coagulase. Thus we see that the work of enzymes is absolutely essential to the life of the plant. Al- though plants and animals obtain their foods in different ways, they probably assimilate it in much the same manner, for foods serve exactly the same purposes in plants and in animals, namely, they are oxidized to release energy and they build up living matter. How Food Is Used in the Plant Body Although, basically, the uses of food are production of energy and making of protoplasm, certain substances are produced by plants which are not found in animals. For example, the plant cell is charac- terized by its cellulose wall which in old cells is strengthened by the addition of a complex substance, known as lignin. This forms the useful substance we call wood. In addition, other products charac- teristic of plant activity should be mentioned : the fatty substances, known as cutin and suherin, as well as waxes which give the "bloom" to certain fruits ; the essential oils in resins, such as lemon, pepper- THE ROLE OF GREEN PLANTS 265 mint, wintergreen, menthol, eucalyptus, camphor, and the like ; va- rious alkaloids ; poisonous substances such as nicotine and strych- nine ; acids such as mahc, citric, and tartaric. Plant protoplasm, in addition, as we have seen, manufactures many characteristic enzymes and produces pigments like the chlorophylls and carotins al- ready mentioned. The carotin present in green grass fed to dairy cows gives the deeper color so much desired in cream and butter. Another interest- ing substance found in carotin is a precursor of Vitamin A which exists in plant bodies as a form of carotin and is prob- ably transformed by the liver of animals into Vitamin A. This is another example showing how closely the lives of plants and animals are interwoven. (See pages 277-279.) 00 A diagram of the outer portion of a cross section of a wheat grain showing the various layers of tissues : h, the different integuments of the ovary and seed which make up the husk ; o, the cells of " tileu- rone layer" of the endosperm, which are loaded with protein grains : and b, the layer of starch-bearing cells. (After Cobb.) Respiration Respiration is essentially the same process in plants as in animals. In its simplest terms it is the release of potential energy from foods by means of the process of oxidation, whereby oxygen is used and carbon dioxide is given ofT. Glucose is perhaps the chief fuel of the plant body, although fats also serve this purpose. The latter are probal^ly changed to sugar before actually being utilized in the respiratory process. In order to have respiration take place, there must be an exchange of gases through a selectively permeable membrane. This means that there will be an exchange of oxygen and carbon dioxide in the cells where the oxidative process is taking place. Sin.ce respiration occurs in all living cells and since there is a greater volume of carbon dioxide and oxygen in parts of plants that are growing rapidly, it is obvious that growing roots must have a supi)ly of oxygen. This is a reason for the loosening of soil particles around plants in cultivation to allow air to have access to the root hairs. The actual oxidative |)rocess is con- 266 THE MAINTENANCE OF THE INDIVIDUAL siderably influenced by external conditions. Low temperatures slow up the process as do very high temperatures, there being an optimum temperature for each organism at which the rate of respiration goes on best. Seeds have survived a temperature of —250° C. Experiments with leaves show that the respiratory rate increases rapidly from 0°-40° C, from which point it falls slowly until the death of the organism. The amount of food present in the plant is a second factor influencing the rate of respiration, while the rate also varies with the amount of protoplasm in the cells. Light usually increases the respiratory rate, probably because of a parallel increase in food and temperature. It is also found that wave lengths which increase photosynthesis also increase the respiratory rate. Finally, the rate of respiration is greatly affected by poisons or anesthetics, at first being increased, but later slowing down rapidly. In brief, respiration in plants, as in animals, is induced by the action of enzymes, and results in the release of energy. Transpiration If a healthy potted plant is placed in a dry bell jar and left in the sun for a few minutes, drops of water are seen to gather on the inside of the jar. By covering the pot with a rubber tissue to exclude the large. battery jar-. (Jovcrect vith 5heet rubbe'T.... moifture star-t- 24 Viours later Experiment to show transpiration. Read your text and explain what has happened. possibility of the evaporation of water through its surface and return- , ing it to the jar under similar conditions, drops of moisture are again found after a time on the inner surface of the jar. Obviously, water must come out through the leaves or stem of the plant, a fact which can be demonstrated by weighing it before placing it in the jar, and THE ROLE OF GREEN PLANTS 267 again after a brief period of exposure to sunliglit, when it will be found to have lost weight. This loss ol' water takes place through the sto- mata and to some extent through the lenticels of the stem, a loluniom- enon closely associated with the process of i)hotosynthesis, for which a relatively enormous amount of water is required. The reasons for this are that living matter is largely composed of water ; that the pro- cess of food making cannot take place in plants unless the interior of their leaves are moist ; and, in the third place, because water is one of the raw materials used in making sugar. The amount of water given off by plants through transpiration is very great. Early in the eight- eenth century Stephen Hales (see p. 241) estimated that an average crop of cabbages loses from three to four tons of water per day per acre in warm weather. An acre of pasture grass is said to give off over 100 tons of water on a hot, dry day. A medium-sized tree will evapo- rate about five to six tons of water on a hot day. One writer, von Hohnel, estimated that an acre of large beech trees would transpire 30,000 barrels of w^ater in one summer. Such figures show that a green plant loses water very rapidly during hot, dry days. The amount of water lost differs greatly under different conditions. If the air is humid, or if the temperature is lowered, or if the tempera- ture of the plant becomes low, the rate of transpiration is greatly Diagrammatic cross section through a stoma to show movement of guard cells. The dotted lines show the closed position. Closure is brought about by the guard cells becoming more elongated and flattened, while the outer wall (w) remains in place, the ventral wall (/) and dorsal wall (V/) assume the positions (/') and id') moving toward the central slit (s) of the opening of the stoma. This movement is largely brought about through the change in position of the base or hinge {h) {h') of the guard cell. (After Schwendener.) reduced. The stomata tend to close under certain conditions, thus helping to prevent evaporation. The opening and closing of the stomata depend on changes in turgor of the guard cells. The stomata 268 THE MAINTENANCE OF THE INDIVIDUAL open when the guard cells become more than normally turgid, but if the turgor of all of the living cells of the leaf is reduced by water loss, then the stomata seem to close automatically. Light increases the amount of sugar formed in the guard cells because of the chloroplasts present, which results in a concentration of sugar, thereby causing a change in turgor. When the leaf is not illuminated by direct sunlight, or at night, the amount of sugar con- centration in the guard cells becomes less, and consequently the stomata close. They usually are closed at night but remain open from shortly after sunrise until late in the afternoon. Toward the middle of the afternoon they begin to close, thus decreasing the amount of water lost in the latter part of the day. Plants wilt on hot, dry days because they cannot obtain water rapidly enough from the soil to make up for the loss through the leaves. Many adapta- tions are found in the leaves which help prevent this water loss, such as waterproofing of the outer cells, hairs on the leaf surface, the absence of leaves, as in the cactus where the minute leaves are early replaced by spines, or the actual turning of the leaves in order to place a small surface to the sun, as in the compass plant, thus causing the rate of evaporation to decrease. - 1 ; ] 1 ■■ 1 i B :. I ' v ! 1 ■■ i i > 1 1: L ■ J -LI u mmd ■WSfflTslSlpiJI W riylu J'icrcc Capillary tubes of various sizes. Is there any relation between the size of the bore of the tube and the water level in the tube ? Explain. THE ROLE OF GREEN PLANTS 269 The Rise of Water in Plants We have spoken of the passage of water from the root up the stem into the leaf. Osmotic pressure has been shown to be sufficient to start this column of water on its way up the stem, but it is not enough to account for the rise of water sometimes hundreds of feet into the air in the stems of trees. Several theories have been advanced to account for this phenomenon. The most satisfactory of these is the theory that such a column of water is held together by the force of cohesion. Experimental evidence shows that the cohesive quality of water in capillary tubes is very great. The core of water acts as a fine, extremely ductile wire. When we realize that a core of water in a tube 2^ of an inch in diameter will withstand a pressure of over 4600 pounds to the square inch, it will be seen that such resistance is a factor in the rise of water through the very tiny tubes found in the vascular bundles of a tree. Another factor in the rise of water in a plant or tree is the evaporation that takes place through the leaves, causing a pull on the cores of water in the tubes of the vascular bundles. During the daytime this is undoubtedly the chief factor in causing the rise of fluids in the stem. Production of Oxygen by Plants A good many years ago the botanist Sachs proved that a green plant placed in the sun- light will give off oxygen, an experiment easily shown in the laboratory. If an aquatic plant such as Elodca is placed under an inverted funnel in a bell jar of water, and an inverted test tube of water is placed over the mouth of the funnel, bubbles of a gas are seen to leave the plant and gradually displace the water in the test tube. If a sufficient amount of this gas is collected, it can be tested with a glowing splint of wood and proved to be oxygen. The amount of the gas can be shown to depend approxi- mately on the amount of sunlight and consequently the rate of photosynthesis. Going back to the formula which shows the making How would you pro\e that the gas on the test tube w as oxygen ? 270 THE MAINTENANCE OF THE INDIVIDUAL of sugar in the leaf, we find oxygen is given off as a by-product. The reaction may be expressed by the following formula : 6 CO2 + 6 H2O + energy from sunlight = CeHi^Oe + 6 O2 (glucose) The value of this reaction to mankind is obvious. The by-product oxygen, which is poured into the air by green plants, is used by animals as well as plants in their respiratory processes. This exchange of oxygen and carbon dioxide by plants and animals gives us one of the most significant and far-reaching interrelationships seen in the organic world. Briefly summing up the process of food making in plants we find that raw materials pass in the form of water and soil solutes from the soil through the root hairs and up the vascular bundles of xylem into the leaf, where water is taken into the individual green cells. Carbon dioxide reaches the cells from the air through the stomata and to a lesser extent probably *in the water stream through the roots. In sunlight, the process of photo.synthesis takes place. Elaborated foods made in the form of sugars may be changed by enzymes to starches and immediately stored in the leaf, or may be passed down through the sieve tubes of the phloem to various parts of the plant where they may be used or stored. Fats are probably synthesized from carbohydrates in the green parts of plants, while proteins seem to be formed in the cells irrespective of the presence of chlorophyll. Enzymes play a very important role both in the manufacture and in the use of food and are essential to respiration and oxidation. The digestive processes which go on in the leaf and other cells of the plant are also due to enzymes. All that has been said in the preceding pages leads to the most important plant function, the reproduction of the species. With vegetative propagation by means of budding, runners, underground stems, tubers, or some of the other asexual means of continuing life, plants would not go far. To establish outposts in far-flung dominions they must have means of travel. These can only be obtained through free moving parts. Such plants are seeds and fruits, which may be dispersed by outside agencies far from the parent plant. The life of the flowering plant culminates in the production of seeds and fruits. As growth progresses and food is accumulated, a time comes, sooner or later, when the energies of the plant are directed to the rapid production of the reproductive organs. Often this growth - -a ? CI ~ r - ^ - - ic .z .a. c ll C q; 2^ £ C it - x a. > o t- o ^ > c o •- Q 271 272 THE MAINTENANCE OF THE INDIVIDUAL is much more rapid than vegetative growth, and almost overnight, flowers appear. The flower, as has been previously shown, holds the gametophyte generation of the plant and produces from fertilized eggs the seeds which hold the embryos or future plants. The fruit arises from the ovary of the flower, together with the parts that may be attached to it. HHI ■ j^^^^^^^^^^V ^ij^^^^^^^^^^^.<^ ■1 BB^^' iBHb\ HP jR I^^^^V^I^^^^Hh tl^^^HpiriJ il ^^^B^^^^.^^^^^^^^^^^^^1^ si ^K^Sk^^^^^'f^ -^RI m!^ * ■' \'^m ^i^^^^H ^^^K\ (' , Jm R^^J^^^H Ij^Em^I ^^^^H[|' ^^^h\ ^jS^M i wK^v w^wf^^^^L J^HSVIjI^I i^H i^fe^l^a^^K i^Sf/^H ^^^^K.Bfe. ''^'^^^^H^^^^B ^^^^^^^H \^KKrj^^ ^i^^^^^^^l I^Hi^nH ^^^^KWe.' .*^^^^^^^^^B ^^^^H l^lMjSi si^^^^^l F^HB^^l^^H ^^^^H ^H I^^I^H ^1^^ ^^^^^^^^^^^^P^ ^^^B^^v^^^^^^^^^^^^^^l ^^B ^^^R^^4^J<| l^^^^^^^^^^^l iH^iBfl ^^^^K' * !|^^^^^^| ^^^1 ^^^^» fl^^^^E ^^RaJf^H ^^^^^^^^K^^lf ''^fl^^^^^^^^^^^^^^^l ^H ^^^^^^^^^^^K i^^^ ,^^^^^HH li^Hf |%^H H^^^ ^sljj^^l ^^1 l^^feylVI Wright Pierce Adaptations in certain fruits for seed distribution. Can you describe the specific adaptation in each case? Sometimes the parts are fleshy, forming edible fruits such as apples, pears, or plums ; occasionally they form hard coverings such as the shefls of nuts, and often they are prolonged into feathery outgrowths which aid in the distribution of the fruit and seeds. Enough has been said of distribution for us to grasp the significance of such adaptations, the ultimate purpose of which is to place the embryo in new areas so that when the seed germinates it may develop into a new plant and thus complete the life cycle. SUGGESTED READINGS Biisgen, M., and Miinch, G. (translated by Thompson), The Structure and Life of Forest Trees, John Wiley & Sons., Inc., 1929, Interesting and authentic, THE ROLE OF GREEN PLANTS 273 Ganong, W. F., The Living Plant, Henry Holt & Co., 1913. A not too technical account of how plants Hve. Holman, R. M., and Robbins, W. W., Textbook of General Botany, 3rd ed., John Wiley & Sons, Inc., 1933. Excellent chapter on photosynthesis. Macdougal, D. T., The Green Leaf, D. Appleton & Co., 1930. A fascinating account of the work of the green leaf. Readable and authentic. Raber, 0. L., Principles of Plant Physiology, The Macmillan Co., 1928. A readable, but thoroughly scientific, book of reference. Especially valuable are chapters IV, VI, XVI, XIX, XX, XXI, XXII, and XXIV. Sinnott, E. W., Botany, Principles and Problems, 3rd ed., McGraw-Hill Book Co., 1935. Chapters IV, V, VI, VII, and VIII are useful for reference. Note the many suggestive questions at ends of chapters. Wilson, C. L., and Haber, J. M., Introduction to Plant Life, Henry Holt & Co., 1935. A general botany with a new point of view. Readable and usable. XV THE METABOLIC MACHINERY OF ANIMALS Preview. Section A . Intake devices and how they function • Foods and their uses ; energy producers ; non-energy producers ; vitamins ■ The acti- vators — enzymes • Digestion in lower animals • Digestion in higher ani- mals ; methods of increasing digestive surfaces ; parts of the digestive system : The oral cavitj^, the pharynx and esophagus, the stomach, the small intestine, the large intestine ; the digestive glands and their enzymes : The salivary glands, the gastric glands, the intestinal glands, the pancreas, the liver, the secretions of the small intestine ; absorption and the fate of absorbed foods • Section B. The how and why of circulation • Why a transportation system • Unspecialized transportation systems ■ Open cir- culatory systems • Closed circulatory systems : Among invertebrates ; among vertebrates • The blood • The lymph • The conduits — arteries, veins, and capillaries • The heart • The aortic arches • The course of blood in the body ; functions of the blood • Section C. Respiratory devices • Respira- tion ; the protein, hemoglobin ; external respiration : Respiratory papillae, respiratory pouches or trees, lung-books, the body surface, gills, tracheae, lungs, internal respiration ; respiratory system in man • Section D. Ex- cretory mechanisms • Excretion ; types of excretory devices : Contractile vacuoles