mdls and Man MORGAN <\ 3^^^^^^^^^^^^^HE Marine Biological Laboratory Library Woods Hole, Mass. »/'^♦^V, Presented by Prof, Ann H. Morgan Aug, S, iSSi ss^^^^^^^^^^^^^ai f- 70, 7 Kinsliips or Animals and M an /\- To\'fhn^r^U nf A iJtninl P\\^ \h ujlj By ANN H. MORGAN, Ph.D. Mount Holyoke College McGRAW-HILL BOOK COMPANY, Inc. New York Toronto London 1955 KINSHIPS OF ANIMALS AND MAN Copyright © 1955 by the McGraw-Hill Book Company, Inc. Printed in the United States of America. All rights re- served. This book, or parts thereof, may not be repro- duced in any form without permission of the publishers. Library of Congress Catalog Card Number 55-6859 To Enzabeth Adams Professor oi Zoology m Mount Holyoke College Pref ace Both living and non-living things are composed of strikingly similar sub- stances. The living ones are not only composed of like substances but all living matter is put together in the same unique way. Kinships based on these similarities form the central theme of this book. The author's experience has indicated that the study of these relationships makes zoology more vital and interesting to the student. The first chapter, "Relationships of the Living World," presents the gen- eral plan of the text and is its introduction. Part I, "The Foundation," tells of matter and energy as they occur in plants and animals, and familiar natural processes. It includes a discussion of atoms and molecules and references to the newer knowledge concerning them. This leads logically to the cell, as a sample of the complete organization of living matter and the focus of a great wave of contemporary investigation. Relationships are persistently evident in our world and universe. Many of them are suggested in the Kinships of Animals and Man, and they are emphasized in the special discussions of Part II, "Ecology." Among these relationships are the competitions and unconscious cooperations of animals, the associations of animals in communities, and photosynthesis, the most im- portant food-making process in the world. Protoplasm must have water wherever it is. The inside of the animal body is wet. Structures and functions of tissues and organs are affected by the fact of their dependence upon fluid. In Part III, "The Internal Environment of the Body," the main systems of the animal body, invertebrate and vertebrate are studied with respect to their basic similarities. It is well known that modern medicine is the internal ecology of the human body. Traces of history remain in animals and parts of animals showing the broken story of their evolution. "The Evolution of Animals" is a series of chapters which recount the great contribution of one or another group. Above all, evolution appears as a story of the continuity of life. Vi PREFACE '.'Evolution and Conservation," or applied ecology, belong together in a final summary. Conservation is already taking its place as a part of Evolution. I am grateful to all those who have made this work first of all a pleasure to me. There are more of them than I can name here. First are my colleagues in the Department of Zoology in Mount Holyoke College; Professors A. E. Adams, E. M. Boyd, J. W. Kingsbury, C. Smith, I. B. Sprague and K. F. Stein. Dr. Adams has been generous beyond my telling in giving her time, scholarship and keen critical sense. Others have read parts of the book and given suggestions that have greatly helped. These are: Dr. H. M. Allyn, former Dean of Mount Holyoke Col- lege; Dr. E. P. Carr, Professor of Chemistry, Mount Holyoke College; Dr. M. P. Cloud, Librarian of Peabody Museum, Yale University; Dr. E. T. Eltinge, Associate Professor of Plant Science, Mount Holyoke College; Dr. M. P. Grant, Professor of Zoology, Sarah Lawrence College; Dr. E. K. Moyer, Associate Professor of Anatomy, Medical School, Boston University; Dr. F. A. Saunders, Professor of Physics, Harvard University. The original drawings except a very few of my own have been made by Shirley P. Glaser, Biological Artist of the Peabody Museum of Yale Uni- versity. I have been fortunate in having the benefit of her ability and ex- perience. Other illustrations, exclusive of my own photographs, are used through the courtesy of those whose names are written beneath them. The generosity of many authors in particular is a cause of my warm thanks. There are many others whose interest and good wishes I have appre- ciated. Among them are Andrew Bihun of the National Audubon House; J. P. Hughes of the W. B. Saunders Company; my sister, Christine M. Kenyon; and my brother, Stanley D. Morgan. Finally I give my hearty thanks to those who have helped to make my manuscript into a book. At first, Helen L. Goodwin, and later, Irene Moss- man have typed the revisions with exacting accuracy. Fortunately, the index of Kinships has been made by a biologist who is also a librarian. Dr. M. P. Cloud of the Peabody Museum at Yale University. Ann H. Morgan Contents Preface v 1. Relationships of the Living World 1 Part I. THE FOUNDATION 2. Life Is a Concern of Matter and Energy 9 3. Living Matter and Cells 25 Part II. ECOLOGY 4. Plants Provide for Themselves and Animals 51 5. Animals and Their Environments 67 6. Mutual Relationships of Animals 91 Part III. THE INTERNAL ENVIRONMENT OF THE BODY 7. Tissues 107 8. An Agent in Evolution — The Body Covering 126 9. Protection, Support, and Movement — Skeletons 135 10. Movement — Muscles 155 1 1 . Foods and Nutrition 168 12. Circulation and Transportation — Body Fluids 195 13. The Release of Energy — Respiration 224 14. The By-Products of Metabolism — Excretion . , 242 15. Chemical Regulation — Endocrine Glands 255 16. Conduction and Coordination — Nervous System 279 17. Responsiveness — The Sense Organs 309 18. Reproduction 331 Part IV. THE NEW INDIVIDUAL 19. Development 359 20. The Physical Basis of Heredity 388 vii 71142 Vlll CONTENTS Part V. EVOLUTION OF ANIMALS 21. The Protozoans — Representatives of Unicellular Animals 425 22. Sponges — A Side Line of Evolution 454 23. Coelenterates — Simple Multicellular Animals 465 24. Ctenophores — Comb Jellies or Sea Walnuts 493 25. Flatworms — Vanguard of the Higher Animals 498 26. Roundworms — The Tubular Plan 519 27. An Aquatic Miscellany 533 28. Annelids — Pioneers in Segmentation 552 29. Arthropods — Crustaceans 572 30. Arthropods — Insects, Spiders, and Allies 589 31. MoUusks — Specialists in Security 630 32. Echinoderms — Forerunners of the Vertebrates 651 33. Introduction to the Vertebrates — Lower Chordates and Fishes . . 662 34. Amphibians — The Frog, An Example of the Vertebrates 681 35. Reptiles — First Land Vertebrates 713 36. Birds — Conquest of the Air 729 37. Mammals and Mankind 752 Part VI. EVOLUTION AND CONSERVATION 38. Organic Evolution — Conservation 777 Appendix 795 Scheme of Classification 795 The Plant Kingdom 795 The Animal Kingdom 796 Equivalent Measurements 799 Suggested Reading 801 Index 819 1 Relationsnips or tne Living Worla This book is about animals, those that are regularly called animals and others, the human animals. The human ones descended from some now un- known ancestors of the apes, developed language and mind with ideas and became unique among all animals. It is about the relations of animals to one another, and to the plants upon which they depend, to water, to the sun, and to the earth about them. The organization and relationships inside and out- side of animals are the keys to their existence. Inside, the secretion of a gland in one part of the body is carried by the blood and stimulates the heart and muscles in other parts. Outside, the seasons change, the woodchucks go into their holes for the winter and the bobolinks fly south. Like the sun and the atmosphere and the soil, living organisms — the wood- pecker in its hole in the pear tree and the fisherman and the fish — are composed of atoms and molecules. For every organism, life is a concern of matter and energy. It is not that its substances are so unusual; it is the way they are put together that makes living matter different from everything else. Living matter occurs in cells. They are samples of its composition and activity, units of the architecture of plants and animals, rosebush and man. To the passing glance cells appear disarmingly simple although they are complex far beyond our present understanding. In many-celled animals the bridge over which all inheritable qualities pass to the next generation is in the con- tent of two microscopic cells. By their union and the divisions which follow it, the billions of cells in the body receive their quotas of inheritance. Plants and animals are bound together in a multitude of ways and the same fundamental processes of living are common to both. A cactus is nearer to human kin than a stone; the starvation of corn is not as spectacular as the starvation of cattle, but it also is a disaster. 1 2 RELATIONSHIPS OF THE LIVING WORLD Chap. 1 On every hand animals depend upon plants directly and indirectly, for food, for shelter, even for decoration. Long before mankind made bouquets, the bowerbirds of Australia scattered blossoms on their courting grounds. Green plants carry on the great business of making the food that is essential to themselves and to animals. In spite of the schemes for providing the world with synthetic food, a cow will keep her mouth to the grass for some time to come. Plants also profit from the animals; many of them, including large numbers of fruit trees, do not produce seeds without insect pollination. The two main ways to study animals are: with emphasis on their asso- ciations in groups of other living organisms, and with emphasis on the indi- vidual. As associated organisms animals are considered among others of their own kind or of different kinds in environments of soil, water, or air, within a complex web of influences. The environment of the butterfly on the flower includes the sun, the rotating earth, and the atmosphere as well as the flower (Fig. 1.1). Ecology is the study of plants and animals in their home environ- ments. It is discussed near the beginning and again at the end of this book, Fig. 1.1. Escaping energy, the heat and light of the sun. Left, the sun in total eclipse by the moon. The sun's corona of light streaming out great distances from behind the darkened moon. Right, part of the profile of the sun showing its promi- nences, great flames that extend hundreds of thousands of miles into space. All processes of living are related, directly or indirectly, to the capture of the radiant energy of the sun. (Courtesy, Mount Wilson and Palomar Observatories.) but the relations of living organisms to their surroundings pervade all of its chapters. The animal body itself is a portable environment; the lungs and the heart carry on unique activities in their own special surroundings. The evolu- tion of animals is a history of relationships. Everybody has had experience with an animal in its home territory: clothes moths in flannel, skunks along the byways, or robins on the lawn. Everyone knows that plant lice suck up plant juices, that robins eat hugely of earth- Chap. 1 RELATIONSHIPS OF THE LIVING WORLD 3 worms which in turn eat heavily of leaves and of soil rich in microscopic plant cells. The watery homes of .animals are exciting because they are relatively primeval. Wade into the border waters of a lively pond and you look down into a world in which animals are swimming, climbing, burrowing, eating plants, eating one another, mating, laying eggs, floating, and doing nothing but the basic business of living. The pond is affected by surrounding condi- tions but its swarming population is primarily adjusted to an ancient world of water. The tidal and surface waters of the sea contain populations which dwarf those of fresh water, but ponds and seas bestow similar benefits in the same great boon of water. Living substances must be wet. Life began in the water and all plants and animals are still bound by their need of water, even though many of them have moved into deserts. All plants and animals are sub- ject to the chemical and physical features of their environments. The carbon cycle begins when plants take carbon dioxide from the air and build it into carbohydrate food. The atmospheric pressure in high and low places and the amount of oxygen in the air or water continually affect animals. Ecological relationships — the fish to the sea, the bird to the air — pervade the evolution of animals and plants. They are apparent in a survey of the main groups of animals arranged with respect to their structures and activities. They also appear in special studies of certain animals as representative types, such as those of the ameba, hydra, grasshopper, honeybee, and frog. In this book each of these has been included with its own group of relatives instead of being considered as an isolated creature; no plant or animal lives unto itself alone. In the systems of the body and in their fundamental patterns animal groups show resemblances and relationships. The circulatory systems of all vertebrates are built on a similar ground plan. Except in protozoans and the simplest of multicellular animals, kidneys are tubular organs closely associated with circulating blood. From earthworm to man the body is a tube within a tube; in invertebrates the nerve cord is on the ventral side of the body; in all vertebrates it is on the dorsal side. Environment has been a sculptor. In envi- ronment and outward form a whale is fishlike; in internal anatomy it is closer to a squirrel. Conservation is applied ecology. Not until good things are going or gone do we appreciate what they used to be. A stream runs clean and cold and well- fed trout cut through its currents. This home is right for them. No alterations are needed. Presently an upstream paint shop is established, the waste warms and poisons the water, supplies it with scum, bad smell, and gases that kill the trout. The need of getting back the clean, cold water is urgent for whatever fishes may still be alive. If the paint shop and the bad smell had not become a part of their environment, there would be many more alive. Conservation of our natural resources is growing daily more important. The kinships of ani- 4 RELATIONSHIPS OF THE LIVING WORLD Chap. 1 mals and man extend in every direction and include all living organisms and the times and places which have made them and are still making them what they are. The second way of studying animals is with emphasis on the individual. It is the study of the structure and function of tissues and organs, by examination and experiment. Every animal has an internal environment inherited from its ancestors through ages of evolution. Within the body all cells live in a watery environment as truly as do animals in a pond. The amount of water is con- tinually regulated; chemical conditions — acidity, alkalinity, enzyme, and hor- mone actions are constantly balanced, unbalanced, and rebalanced; physical conditions are changed; temperatures shift, and pressures vary. Every animal body holds a special environment of which there is no duplicate and probably nothing in existence that is at once so complex, delicate, and generally durable. The release of energy in respiration, chemical regulation by the endocrine glands, and the excretion of the by-products of metabolism especially empha- size the balancing associated with these processes. As animals are examined, it becomes more and more clear that there are not thousands of separate facts to be learned, but a few associations and principles that apply to essentially similar things. The Fields and Subdivisions of Zoology. The science of biology includes all living organisms. The term, actually meaning the science of life, Gr. bios, and logos, discourse, is used commonly and loosely, often with little understanding of its meaning. It may include only the plants and be called the biology of plants; it often deals only with animals, the biology of animals. In either case it is concerned with the general facts and principles of plant or animal life. Zoology is the study of all aspects of animals, including their relations to each other and their environments in time and space. Other associated sciences are those particularly concerned with the environment, such as geology, physi- ography, oceanography, and meteorology which is concerned with conditions of the atmosphere. All of these are supported by physics, treating of the prop- erties of matter, and by chemistry which deals with its constitution. There are many subdivisions of zoology, the science of animals (including man). The principal ones are the following: Subdivisions of Zoology Name Description Anatomy Gross structure of the animal Histology Function and microscopic structure of tissues and organs Cytology Function and structure of the cell and its contents Physiology Function of the whole animal, or of its parts Embryology Development of the new individual Chap. 1 Name Genetics Ecology Taxonomy Zoogeography Paleontology Sociology Parasitology Psychology Zoology is animals, such Entomology Ornithology Protozoology Herpetology RELATIONSHIPS OF THE LIVING WORLD 5 Description Science of heredity dealing with characteristics arising from the behavior of genes Relationships of animals to one another, to plants, and to the environment; their home life Classification of animals and its principles Distribution of animals in space Distribution of animals in time; fossils Societies of animals and man Study of animals that live and subsist upon other animals or plants to their harm Study of the mind also divided into branches for the study of special groups of as: Insects Birds One-celled animals Amphibians and reptiles Parti Tne Founaation 2 Lire Is a C s a v^oncern or Matter and Energy We live in a universe of substance and force. Everything that we can discern with our senses is either one or the other, matter or energy. So far as they are known, matter and energy are always associated. They are in the grass beneath our feet, the wind and the rain, our food and our use of food. Even a little understanding of the character and relationships of matter and energy throws light upon the lives of plants and animals; it may be the eyeshine of a cat in the dark, the song of a wood thrush, the drip of sweat from the skin, the heat of fever, the chnch of muscles. Matter Our bodies are composed of matter. It is all around us: books, plants, ani- mals, sugar, smoke, gasoline, the earth, the planets, and the far-off galaxies, each of them Uke the Milky Way of which our own solar system is a part. What are these things? What is matter? A good deal has been learned about its structure mostly during the last part of the nineteenth and the first part of the twentieth centuries. In its analysis all the roads have led toward electricity. But nobody knows what matter is because no one yet understands electricity. All matter is composed of invisible atoms; there are millions of billions of them in a drop of water, each one containing extraordinarily minute electrical particles. The electrical nature of living matter has been known in one way or another for a long time, but in recent years more and more evidences of it have been discovered. The Italian anatomist, Luigi Galvani (1737-1789) was observing a freshly killed frog hung from an iron fence by a copper wire hooked under the sciatic nerve when he noticed that the muscles twitched whenever the wind-blown legs touched the iron fence. Thus a century and a half ago Galvani discovered that living matter conducts electricity and re- 9 10 THE FOUNDATION Part I corded his observations in his essay, "Force of Electricity on the Motion of Muscles." Less than half a century later it appeared that living tissues not only conduct electricity but also produce it. Now rhythmically repeated waves of electrical charges are received over wires connected with metal plates placed against the human head, and the records of them are taken by recording mech- anisms (Fig. 16.23). The existence of electrical brain waves is clearly estab- lished. Energy Energy is the capacity for action, the ability to do. Expressions of it are the jumping of fleas, the wriggling of a baby, the leap of a rabbit, the response of a tear gland. Just as life is known only through matter, so energy is measured only by its effect on matter, the size and the speed of a flea's jump. Characteristics of Energy. Heat is the commonest form of energy. This is so generally true that measurements of energy can be stated in units of heat. The small calorie is the amount of heat required to raise one gram of water one degree centigrade at sea level pressure of nearly 15 pounds per square inch. Since the gram is too small to be a convenient unit, a large calorie has been adopted for general use. It is the amount of heat required to raise one kilo- gram (1000 grams or 2.2 pounds) of water one degree centigrade, also at sea level pressure. Potential and Kinetic Energy. Usually energy can be in two forms, potential in the rabbit's readiness to jump, and kinetic in the actual jump. Atomic energy is seemingly of a different sort. Potential energy is that contained in any object because of its position or shape or substance. Kinetic energy is that of motion. A fish hawk (osprey) hovering aloft over a lake has potential energy of position. This becomes kinetic energy as the hawk cuts downward to pick a fish from the water. The wiry threads wound around the eggs of certain mayflies have potential energy that becomes kinetic (Fig. 2.1). They are tightly coiled as long as the eggs are in the body, but they spring loose and catch on plant stems as soon as the eggs are laid in the water. Living cells hold potential energy of substances such as fat which may be transformed into the energy of heat. In a more particular sense the energy of substances is usually called chemical energy (Fig. 2.2). Energy is either stored or liberated in all chemical reactions. A coal fire is a chemical reaction in which chemical energy stored millions of years ago is liberated from the coal: coal -f O- (oxygen in the air) = CO2 (carbon dioxide gas) + energy (heat). Catalyzers are aids in chemical reactions, hastening them without entering or being affected by them. Many of them are known as enzymes or ferments and each one acts upon particular substances and under certain conditions. The Chap. 2 LIFE IS A CONCERN OF MATTER AND ENERGY 11 Fig. 2.1. Change from the potential energy of position to the kinetic energy of motion in the threads of a mayfly egg, the size of a sand grain. Before the egg is laid a wiry thread is coiled like a watch spring around each end of it; the energy in their coils is potential. Mayflies strew their eggs on lakes and streams. As the eggs touch the water the coils spring loose; in so doing their potential energy becomes kinetic. The threads catch on submerged twigs and the eggs are suspended above the mud that otherwise would smother them. respiration of every living cell is a chemical reaction in which the chemical energy in the cell's substance is transformed into the energy of activity and heat. Transformations of one kind of energy into another are constantly going on about us. The radiant energy of the sun becomes that which is stored in the simple sugars of green grass. Cows feed on grass and its stored energy is even- tually transformed into milk for calves or babies. Atomic Energy. The energy within the atom shows itself in qualities of cohesion. It is liberated when under special conditions one kind of matter is changed into another, e.g., nitrogen into oxygen. Such a change generally occurs in atoms in which the particles in the nucleus are numerous. They may be unbalanced for a long period and relatively unstable as in radium, uranium, and thorium. Such atoms cannot hold themselves together and their radio- activity is a long, continued breaking apart. The beginnings of the knowledge of radioactivity moved rapidly. In 1895 Rontgen concluded that some active radiation emitted spontaneously from 12 THE FOUNDATION Part I Fig. 2.2. Chemical energy stored in a globule of fat. Fat cells from connective tissue underlying the skin of a rat. Fat stained black. (After Maximow. Courtesy, Gerard: Unresting Cells. New York, Harper & Bros., 1940.) uranium had fogged photographic plates protected by light-tight envelopes. He coined the word radioactive to describe the activity. In February of the next year Henri Becquerel read a paper before the Academy of Science in Paris in which he announced that compounds of uranium were able to affect a plate through an envelope that was proof against light. The radiations were called x-rays because they were not understood. Following Becquerel's dis- covery his one-time student, Marie Curie, succeeded in isolating minute quan- tities of two highly radioactive new elements from uranium minerals, to which she gave the names polonium and radium. In 1899 Becquerel showed that the rays from uranium could be separated into two types, alpha rays easily ab- sorbed by a few sheets of paper, and beta rays able to penetrate thin alumi- num. In 1900 Villard discovered still a third and more penetrating radiation from uranium minerals, the gamma rays. By 1913 Rutherford and Soddy had coordinated the various processes and proposed a theory that the nucleus of the atom was spontaneously disintegrating. They suggested that the nuclear disintegration was explosive and showed that during the process particles of matter and energy were lost. Since that time the knowledge and use of atomic energy have become important in many fields of biology; x-ray photographs are routine items in medical practice; exposure to controlled quantities of x-rays is a common treatment of cancer; Muller's experimental radiation of fruit flies produced inheritable differences in generation after generation of their offspring; and the use of radioactive tracers has opened a new era in biological investigation. Atomic energy is now a tool in world politics; perhaps it is more true that Chap. 2 LIFE IS A CONCERN OF MATTER AND ENERGY 13 world politics is a tool of atomic energy. The most startling display of energy that had ever been known to the world occurred on August 6, 1945, when an atomic bomb exploded over Hiroshima, Japan, and uranium atoms (U-235) broke apart and unloosed their extraordinary power. Structure of Matter The physical states of matter are more or less easily changed by conditions about them. In shifting temperatures, the state of water may be a gas, fluid, or solid, i.e., vapor, rain, and sleet in quick succession. The composition of mat- ter is not thus easily changed, the elements and their compounds, the atoms and molecules. Atoms are the incredibly minute, organized units of matter that are the building blocks of elements. An element is composed of one kind of atom for which it is named, oxygen, carbon, calcium, and so on. One hundred elements are known, mainly dis- covered in nature: certain radioactive ones have also been created experimen- tally. The elements are distributed unevenly. Four of them, oxygen, carbon, hydrogen, and nitrogen constitute 96 per cent of living matter; less than 20 make up 99 per cent of the atmosphere, the ocean, and the earth's crust. Molecules are usually the units peculiar to an element or a compound. Molecules of elements contain two or more atoms of the same kind. Molecules of compounds have two or more different kinds of atoms. The molecule of water has two atoms of hydrogen and one of oxygen (Fig. 2.3). Molecules are continually attracted to one another by intermolecular force that is electrical rather than gravitational. They are in constant motion, in a random jumpy dance. They are too small to be visible and the dance cannot be seen but can be felt as heat. When a substance is cold, e.g., ice, the dance is slow; when hot, e.g., boiling water, the dance is extraordinarily rapid. Turn an electrical current through a cold iron and the dance of the molecules is changed from the slow to rapid rate. The motion never stops. The lounger in Hydrogen H ooo MOLECULES Oxygen Water Fig. 2.3. Diagram of the formation of a molecule of water by the sharing of elec- trons between two atoms of hydrogen and one of oxygen. Electrons are the particles that take part in chemical reactions. 14 THE FOUNDATION Part I Boston Common and the dead bench on which he sits both abound in speed- ing molecules (Figs. 2.4 and 2.5). Characteristics of Atoms. Nobody has seen the atoms. Their existence was assumed by John Dalton (1766-1844) and it has been proved by patient, skillful experimentation with radioactivity and other means. Fig. 2.4. Molecules are continuously repelled and attracted in a random jumpy dance. Those in a thin gas move in free curves. Those in a fluid or a solid are packed together as if in a crowded hall. (Courtesy, Gerard: Unresting Cells. New York, Harper & Bros., 1940.) The relatively small center body or nucleus contains practically all of the atom's mass. Electrically negative particles rotate around it. In comparison with their size, they swing through space relatively as great as that in which planets rotate about the sun (Fig. 2.6). The nucleus is composed of protons carrying positive charges of electricity and neutrons that carry no charge. The sum of their masses is the weight of the atom. The electrical charge of the nucleus indirectly controls the nature and behavior of the atom. Atomic nuclei are bound together by a force that was unimagined until experimental splitting demonstrated its reality. As interdependence permeates living organisms, so interdependence of parts is the keystone of the atoms that are the foundation of living matter. Within the space around the nucleus are particles called electrons, so light that they are ignored in the computation of atomic weight. Each carries a negative charge of electricity and spins like a coin that is spun upon a table top. It is generally believed that electrons revolve around the nucleus, but their spinning is independent of it. The number of electrons in an atom governs its chemical properties. Electrons, for example, determine that one atom of oxygen will unite with two atoms of hydrogen to form water (H^O). Isotopes. Isotopes are different forms of atoms existing in the same element (Fig. 2.6). They have nearly the same chemical properties but differ in the number of neutrons in their nuclei. Since the weight of an atom is the sum of the numbers of its protons and neutrons, the isotopes of an atom have differ- ent atomic weights. For example, hydrogen has three known isotopes: hydro- gen, atomic weight 1; deuterium (heavy hydrogen), atomic weight 2; tritium, atomic weight 3. Isotopes that have few neutrons in their nuclei are called light isotopes and those with the most neutrons heavy isotopes. In general the heavy isotopes are less stable, since an excess of neutrons weakens the co- Chap. 2 LIFE IS A CONCERN OF MATTER AND ENERGY 15 hesion of the nucleus. Those that do not readily change are called stable isotopes; the radioactive isotopes give off nuclear energy. Isotopes have been detected in nature and many radioactive ones have been made in laboratories. The separation of isotopes is a means of exploring changes that take place within the nuclei of atoms. One of the problems in dealing with isotopes is to separate out the kind which is to be used. In some cases this is easy; in others it is extremely difficult. In the distillation of water the vapor which first con- denses is water containing the light isotope of hydrogen. Later the heavy water Oxygen Carbon Hydrogen Nitrogen Misc. Water Proteins Carbohydrates, Lipoids, Minerals Fig. 2.5. Top, Percentages of different kinds of atoms in the human body. In- cluded under miscellaneous are, in order of decreasing amounts, calcium, phos- phorus, potassium, sodium, sulfur, chlorine, magnesium, and iron. Bottom, Percent- ages of different kinds of molecules in the human body. (Modified from Moment: General Biology. New York, Appleton-Century-Crofts, 1950.) 16 THE FOUNDATION Part I containing a heavy isotope of hydrogen also distills. Isotopes of uranium are not procured by any of the easier methods; skill, persistence, and elaborate equipment are required. Isotopes are also put to various uses, in war, in biological investigation, and in medicine. The atomic bombs of the Second World War contained isotopes of heavy atoms with unstable nuclei that flew apart establishing chain ex- plosions of tremendous destruction. The political condition of the world has established an association of isotopes and war. There is hope that this may sometime give place to great constructive uses. To the world at large, atomic bombs have almost hidden the importance of the radioactive isotopes that are being used as tracers in living plants and animals. Hydrogen atom Deuterium atom Fig. 2.6. Diagrams of the structural plan of the atom. As they are at this date generally named the particles inside the nucleus are: the protons ( + ) that carry positive charges and the neu- trons (0) that carry no charges; the electrons outside the nucleus bear negative charges. Hydrogen atoms have one proton and one electron. Deuterium atom, an isotope of hydrogen (heavy hy- drogen) consists of a nucleus with one proton and one neutron, and a single electron moving around it. Helium atom, the nucleus consisting of two protons and two neutrons, has two electrons moving around it. Helium gas is used in dirigible bal- loons. Helium atom Ions. Atoms may gain or lose electrons and are then known as ions. If elec- trons are lost, the ion is positively charged; if they are gained, it is negatively charged. Ions combine more readily than electrically neutral atoms. Water facilitates the splitting of substances into ions. Living organisms are largely water and many substances are present in them chiefly in a dissolved state. In solution many of these dissolved substances split into simpler ones and ions are formed (Fig. 2.7). When crystals of common salt (sodium chloride, NaCl), a component of the blood of all animals, dissolve in water, the ions of the sodium (Na+) already present in the lattice of the crystal are separated by the attraction of the polar molecule of water. The crystal framework is thus broken and the ions are free in the solution. Their formation in salt solution is expressed by the formula, NaCl = Na+ -f CI-. Because of the positive and negative charges of ions, the living body can conduct electricity. When the opposite poles of a battery are placed in water, Chap. 2 LIFE IS A CONCERN OF MATTER AND ENERGY 17 the sodium ions (Na+) are attracted toward the negative pole where they acquire electrons and their positive charge is neutralized. The chlorine ions (CI") are attracted toward.the positive pole, give up an electron and become neutral atoms. The moving ions conduct an electrical current and thus estab- lish a complete circuit. Any substance which thus ionizes in water is called an electrolyte because of its ability to conduct electricity. Fig. 2.7. Diagram of the ionization or dissolv- ing of salt in water. When sodium chloride (salt) is put into water the atoms Na (sodium) and CI (chlorine) separate and become electrically charged wandering atoms or ions, Na+, Cl~. The movements of the sodium ions (-(-) and chlorine ions ( — ) conduct an electrical current in water. In general, water promotes the forma- tion of ions and ions promote chemical reactions. The properties of electrolytes depend upon the kind of ions which they produce in a solution. On the basis of the simpler theory of electrolytes there are three classes: acids, alkalis, and salts. The degree of acidity or alkalinity of a compound depends upon the degree to which it ionizes in water, that is, the degree to which the molecules yield positive hydrogen ions (H+) or nega- tive hydroxyl ions (OH~) in the solution. Acids are electrolytes that as a group form positively charged hydrogen ions, giving the acid its sour taste. Hydrochloric acid ionizes in water: HC1^H+ + C1-. The alkalis or bases form negatively charged combinations of oxygen and hydrogen, the hydroxyl ions, OH~, The alkali, sodium hydroxide, ionizes thus: NaOH^Na+ -f OR-. Some compounds of protoplasm yield both H+ and OH~ in solution. The third class of electrolytes is the salts whose ionization produces neither H+ nor OH~, Sodium chloride is an example: NaCFNa+ + C\-. Many of the important characteristics of cells, such as the permeability of their membranes, their irritability or response, are associated with the existence 18 THE FOUNDATION Part I of electrolytes either within or outside them. The sensitiveness of the ani- mal organism to hydrogen ions is apparent in scores of cases; in a large num- ber of animals the control of respiration is through the hydrogen-ion concen- tration of the blood. Hydrogen-ion concentration (symbol pH) of substances in their surroundings is also of greatest importance to living organisms; the range of many aquatic animals, certain protozoans, insects, and fishes is limited by it; so is the range of earthworms. Tracers. The use of radioactive isotopes as tracers for investigating life processes is probably one of the most significant developments in modern bio- logical work. Such a possibility had been recognized for some years but was limited by the fact that all the work had to be done with heavy elements such as lead, bismuth, and mercury. The isotopes chosen are labeled by exposure to radiations from a radioactive element. After this treatment they give off radiations for a longer or shorter period. The ease of this modern technique is comparable to locating a white penny among ordinary copper ones. They are introduced into plants and animals in various ways (Fig. 2.8). For exam- ple. 2.8. The presence of radioactive tracers shown by radioautographs in slices of tomato, especially in the seeds. The vine from which the tomatoes were taken was grown in a solution containing radioactive zinc (Zn^^). This was taken up throughout the plant and affected the photographic plates like light. (Courtesy, P. R. Stout, University of California.) Chap. 2 LIFE IS A CONCERN OF MATTER AND ENERGY 19 pie, in the body of a rabbit they may be carried in and out of organs, into cells and perhaps out again. The travels and destinations of such labeled isotopes are detected most com- monly by the now familiar Geiger-MuUer counter. This apparatus detects and amplifies each radioactive disintegration of an atom. The number and rate of disintegrations are a measure of the amount of labeled material present. In general the use of tracers is directed toward investigations of the constant buildup and breakdown, and the come and go of chemically active molecules in the living organism. In this way it has been learned that thyroxin, the iodine-containing amino acid that is so important in the functioning of the thyroid gland, is manufactured by muscle and in the intestine as well as in the thyroid gland. Recent studies on the metabolism of rabbits by means of radioactive isotopes have shown that radioactive phosphorus administered to adult animals enters their bones and the enamel and dentine of their teeth. This shows that such hard substances, deposited in early youth, do not stay unchanged for a lifetime, but are continually exchanging material with the cir- culating blood. States of Matter Molecules are continually affected by the attraction of their neighbor mole- cules. Their relative sizes and the distances between them determine the strength of their mutual attraction and the state of the substance in which they are contained whether gas, liquid, or soUd (Fig. 2.9). Changes of matter from one state to another involve a change in energy, usually the giving off or absorption of heat. In gas, the molecules are scattered away from each other; their movements are rapid and disorderly and they take zigzag turns into their surroundings. The volume of a gas is dependent upon temperature and pressure. The gas spreads through all available space but is compressible because it does not RELATIVE DENSITIES MIXTURES Gas Liquid Solid B Solution Suspension o;.oi.?jo Emulsion Fig. 2.9. A. Diagrams showing the relative densities of molecules in a gas, liquid, and solid. B. Diagrams of mixtures: solution thoroughly dissolved and homogeneous; suspension with particles of one substance undissolved; emulsion with very large undissolved droplets. 20 THE FOUNDATION Part I actually fill the space. Air is a gas and its density varies with the compression, with the pressure and temperature of the atmosphere. In high places where pressure is lessened, its molecules are relatively far apart and it may be too "thin" in oxygen to be adequate for respiration. In liquids, the molecules are closer together. In a solid, such as iron, the molecules are crowded together in patterns. Solids have fixed shape and volume. The behavior of water molecules is very exceptional. Down to 39° F. they draw closer together; between 39° F. and 32° F. they move apart. Thus, ice expands and floats, forming a protecting cover to the animals beneath it (Fig. 5.17). Surface film. Surface films are composed of molecules that are attracted only by those at and close to the area where one substance comes in contact with another, such as water and air (Fig. 2.10). Molecules below the surface are attracted equally from all directions. Surface film occurs on all bodies of water and forms the boundary of such units as soap bubbles and raindrops. It is important in the lives of many small aquatic animals. Certain insects, such as the water striders, forage on the upper face of surface films that bend but do not break with the pressure of their feet (Fig. 2.10). Snails glide over the underface of the film and hydras are often buoyed up against it. SURFACE FILM OF WATER A o-o-o-o-o / \ / \ / \ o o o o o o o o B Fig. 2.10. The surface film of water. A, In surface film molecules of water are attracted only by those at the surface or just below it. B, Molecules below the sur- face are attracted evenly from all directions by other molecules. C, Hydras rest against the surface film in the topmost water where oxygen is plentiful. D, Water- striders skim over the surface film of quiet water and their feet make the dimples that cast shadows on the brook bed. Chap. 2 LIFE IS A CONCERN OF MATTER AND ENERGY 21 Mixtures of Substances Mixtures of substances may be of different kinds and states, those of solids, liquids, gases, or a solid and a gas (Fig. 2.9). Solutions. These are homogeneous mixtures. We usually think of solutions as aqueous since natural water is a solution containing dissolved air. Bubbles of air leave water when it is heated, appearing just before it boils. When it is freezing bubbles of air appear and are caged in the ice. Glass is also a ho- mogeneous mixture, in spite of its hardness, a true solution. Suspensions. The particles of at least one of the substances in a suspension are larger than molecules and remain undissolved. One or several kinds of substances, or different states of one or more of them may be suspended in another substance. Suspensions include various types of colloids all of which consist of one or more substances dispersed in another. There is no escape from colloids. We consume them as food, breathe them as fog and smoke, and are composed of them. Colloids. These are gelatinous substances that include two or more com- ponents: (1) a solid in a solid — the ruby glass of cathedral windows usually containing metallic gold; (2) a solid in a liquid — sodium chloride (salt) in water; (3) solid particles in a gas — blue cigarette smoke; (4) a liquid in a solid — natural pearl, which is water in calcium carbonate (a secretion of Ooo 1 .X=k- >.^k> Sol Gel Movement of ameba accompanied by changes sol to gel and reverse Fig. 2.11. Diagrams of the colloidal states, sol and gel. In the sol state the par- ticles and droplets (white) move about freely in fluid. In the gel state the surfaces of the droplets are in contact and the substance is jellylike. The protoplasm of an active ameba constantly changes from sol to gel and reverse. 22 THE FOUNDATION Part I oysters), natural opal, water in silicates; (5) liquid in a liquid — gelatin in water (gelatin may be a liquid or solid); (6) liquid in a gas — fog. Fog and mist are actually solid particles in gas since the water molecules are gathered on solid particles. It has been noted that at 6 a.m. the air over London may be clear and at 9 a.m. there may be a dense fog. The onset of the fog is largely due to the smoke that has provided particles on which the water gathers. The most important of all mixed substances is protoplasm. It is a colloid, the most complicated, most studied, and still largely unknown one without which life does not occur. This colloid varies in consistency; when it thickens its droplets swell, come closer together, and become a gel; when it thins, the droplets do not absorb water, are smaller and farther apart, and form a sol (Fig. 2.11). Protoplasm is a reversible colloid that may change from sol to gel and return. Such changes may be seen through the microscope in any ameba. White of egg is a gel when heated but it will not return to a sol. Emulsions. Although containing larger droplets than most colloids, emul- sions are similar to them. Familiar emulsions are whole milk, egg yolk, and mayonnaise dressing. Diffusion and Osmosis Diffusion is the movement of a gas or liquid from points of greater to those of lesser concentration continued until an even distribution is achieved throughout the available space (Fig. 2.12). Mice find the cheese from the D Water Sugar Diffusion Osmosis Permeability Fig. 2.12. Diagrams illustrating diffusion. In simple diffusion (A and B), mole- cules of sugar without any barrier become evenly distributed among the molecules of water in consequence of the motion of both. In osmosis, the diffusion through a semipermeable membrane (C and D), the molecules of water can pass through the membrane in either direction. They continue to do so until their number is equal on each side of the membrane. Thus, the level of the sugar solution is raised. The mole- cules of sugar, imprisoned by their larger size, continue to hit against the membrane in their random movements exerting the force called osmotic pressure. In the com- plete permeability (E) both kinds of molecules pass through the membrane at the same rate and the solutions have uniform content on each side. Chap. 2 LIFE IS A CONCERN OF MATTER AND ENERGY 23 particles of it diffused in the air. Skunks have few enemies because of the diffusion of their scent. The success of the great perfume industry is dependent upon human responses to the diffusion of its products, the various perfumes. Osmosis. The diffusion of water or of certain gases through membranes that permit certain simpler molecules to pass, but not the more complex and larger ones, is osmosis. A membrane which does this is said to be semi- permeable. Living cells are enclosed by semipermeable membranes containing sub- microscopic pores through which certain molecules can pass and others cannot. The rate of passage varies with the kind of membrane and the material on the two sides of it. Such membranes regulate many functions of the body such as the exchange of oxygen and carbon dioxide, the absorption of food, and the constant come and go between cells and body fluids. Two liquids that contain equal concentrations of dissolved substances are called isotonic. When living mammalian blood cells are examined microscopically they are usually immersed in a solution of 0.9 per cent NaCl in imitation of the body fluids whose salt content is isotonic with the cell content. An example of osmotic diffusion or osmosis through an artificial mem- brane illustrates this principle (Fig. 2.12). The membrane is permeable to molecules of sugar as well as water, but so much more so to the latter that equal amounts of sugar and water on each side are never reached. Red blood cells puff out like pillows (called laking of blood) if the salt content of the plasma becomes too much reduced, that is, hypotonic. This is because mole- cules of water enter them, establishing an equal concentration with the too watery plasma (see Chap. 12). If the salt content of the plasma is too high, i.e., hypertonic, the water is drawn out and the cells wrinkle. Vacuole Vacuole Fig. 2.13. Brownian movement occurs in the contents of vacuoles of an ameba (right) and of the green alga Closterium (left). With the high power of a micro- scope the zigzag pathways of the larger particles can be traced. The Brownian movement is due to bombardments of usually invisible particles striking unevenly against the larger ones. 24 THE FOUNDATION Part 1 Brownian movement. This motion is an irregular agitation of particles ol difTerent sizes. The molecules constantly jostle against relatively huge par- ticles, striking them unevenly on one side or another. Many of them are very small molecules and others are large molecules. The molecular motion is invisible, but that of the larger particles is evident with the high power of the microscope. The motion occurs in gases, fluids, and especially in colloids including protoplasm. It is common in the vacuoles of algae and protozoans (Fig. 2.13). It was discovered in 1827 by Robert Brown, an English botanist, who saw the motion in a fluid in which pollen grains were suspended. Like other diffusions, Brownian movement is an example of kinetic energy. 3 Living Matter and CelL No one has ever found anything aHve apart from matter. We see the evidences of matter in the protoplasm of every plant and animal: sunflowers turn toward the sun; bees gather about nectar; the ticket line moves toward the show. All of these beings are composed of matter uniquely organized in protoplasm and active in an equally unique process of living. Protoplasm reproduces itself; like produces like but never duplicates itself. A cat has kittens, not squirrels. Her kittens grow and they have kittens, and so, on and on, cats and kittens. None of them repeats its mother or father or grand- parents but each one shows its origin. Protoplasm occurs in cells. The cell is a sample of the complete basic organization and activities of protoplasm. It becomes more and more evident that nonliving and living states blend together since the most complex protein molecules have certain characteristics of protoplasm. The submicroscopic gene that carries hereditary qualities is believed to be a protein molecule that, like a living organism, reproduces itself. Whether viruses are alive or not is still debated; it appears however that they have many of the properties of living matter and are very active. Protoplasm came into being in a very remote time but even now in the nucleoproteins there may still be a twilight zone of originating protoplasm. Protoplasm General Features. We seldom see naked protoplasm. Generally we see and touch the dead remains of cells in the outer layer of skin, scales, feathers, and hair. The softness of a kitten's fur is all due to dead cells. Most animals shed such dead cells seasonally; human molting or shedding goes on the year round. No plant or animal is entirely alive. Cells contain nonliving as well as living structures; freckles are groups of cells holding lifeless pigment that has been deposited within them. Protoplasm looks fundamentally similar wherever it occurs. A dozen cells flecked from the lining of one's own mouth 25 26 THE FOUNDATION Part I and a living ameba shifting its shape through the water on the same micro- scopic slide can be seen to have many differences. Their differences are not surprising, but that their respective protoplasm should look so much alike is unforgettable. Protoplasm is a glassy fluid jelly that suggests the white of an egg be- sprinkled with translucent particles and globules of liquid whose sizes and arrangement change, at one time forming an open network, at another crowded together (Fig. 2.11). Even through the microscope protoplasm often appears inert. It is never really so as long as it is alive and after that it ceases to exist. Dead protoplasm is only the somehow disorganized remains of protoplasm and a contradiction of its name. Structure. Protoplasm consists of a watery solution (hyaloplasm) in which salts and other substances are dissolved and in which solid and semisolid bodies are suspended. Many of these are molecules, mainly proteins that are invisible through ordinary microscopes; others are clearly visible droplets. Water may pass into protoplasm, making it more liquid, or out of it leaving it less so. Under osmotic pressure (Fig. 2.12) minute amounts of solution pass in or out of the droplets by way of their surface films which play the part of semipermeable membranes. The numbers and sizes of the suspended bodies constitute a relatively enormous surface, all of it inviting to chemical and physical changes (Fig. 3.1). Protoplasm is an exceedingly complex colloid. At one time it may be as fluid as water (sol state) and at another a jelly (gel state) depending upon Fig. 3.1. In keeping with their colloidal nature, even minute particles in proto- plasm present a relatively enormous surface to the molecules which continually jostle them. (Courtesy, Gerard: Unresting Cells. New York, Harper & Bros., 1940.) Chap. 3 LIVING MATTER AND CELLS 27 conditions around it such as the degree of temperature, its chemical environ- ment, and its age, or phase of life. The streams of protoplasm which pour like water into the forming pseudopodia .of an ameba are in the sol state; their borders are changeful, now sol, now gel. If the cell membrane is broken slightly, a little of the sol will flow out and "set," thus healing the wound. Chemical Characteristics. Protoplasm has substantially the same chemical content in all plants and in the great procession of animals whether jellyfish, redbird, or man. The four elements, oxygen, carbon, hydrogen, and nitrogen make up 96 per cent of living matter. No element occurs in protoplasm which is not also present in nonliving substance. It cannot be recalled too often that it is not the content of protoplasm but the way it is put together that is unique. Water. The most abundant compound in active protoplasm is water, in general terms of weight at least 75 per cent of it. A jellyfish may be 96 per cent water, a paramecium 80 per cent. The gray matter often called the "thinking part" of the adult human brain is at least 80 per cent water; in early youth the percentage of water is still greater. The water content of a cell is controlled by the living membrane which encloses it. Protoplasm has a water- regulatory power which resembles that of gelatin in that it takes in water and swells to a limited amount and no more. Water heats slowly and holds its heat. Thus the temperature of an animal with its high water content rises slowly and tends to hold its level. Water works toward a temperate climate for protoplasm, whether it is in the body cells of a fish or surrounding the fish in a stream. Certain very important changes in the water content of their protoplasm make animals of low metabolism relatively cold-hardy, such as the numberless cold-blooded ones, insects and others that withstand temperatures of zero (F.) and far below. As winter approaches their proto- plasm loses water, but this is only part of the cold hardening. The water which remains is not all in the same state; it may be free or bound, more of one than the other. Free water is water that contains truly dissolved materials and acts as a dispersion medium for them. In both plants and ani- mals it transports digested foods and waste products and forms a liquid base for secretions. Bound water is held in a loose chemical combination with other molecules. Ordinarily bound water does not freeze. Free water freezes readily forming ice crystals, which because of their size and pressure kill the protoplasm. Studies on the bound and free water in gelatin and egg- white show that part of the water freezes when the temperature reaches — 6°C. (21.2°F.) while what remained did not freeze even at — 50°C. Thus, for the beetle that must endure a northern winter there are striking advantages in having a content of bound water. Chemical Activity. Water is the closest approach that we have to some- thing which dissolves everything. This is the basis of its prominence in diverse 28 THE FOUNDATION Part I metabolic processes, of its power to shape the earth's surface, and its efficiency in the digestive tract, in the washtub, and in the factory. Chemical reactions are hastened by any agent that finely divides a solid, and this happens when water divides a lump of sugar. Living depends upon chemical reactions, both continual and intermittent, all of them together making up the grand process of metabolism, the chemical changes in which water is a constant attendant. Water conducts electricity; when salt is added it does so much more readily. Thus, protoplasm is an efficient conductor since a variety of salts occurs in it and especially in body fluids, the latter being similar to sea water in their salt content. Atmospheric Gases. The gases of the atmosphere are soluble in water and therefore in protoplasm. Nitrogen (No), abundant in the air (79%), is always present in living cells but is chemically inactive; in pure form it does not take part in metabolism although its compounds, e.g., proteins, do so. On the other hand, oxygen, varyingly abundant in the atmospheric air (about 21%), takes an essential part in oxidation in the cells. Carbon dioxide, usually 0.03 per cent in the air, is produced as a by-product of oxidation in protoplasm. Although a by-product in the respiration of both plants and animals, carbon dioxide is essential for photosynthesis in plants (Chap. 4), and in small amounts for important functions in the respiration of animals. Mineral Salts. Protoplasm doubtless came into existence in sea water and mineral salts must have been included in it from the beginning. It con- tains a variety of salts; sodium, potassium, calcium, and magnesium are the chief positively charged ions, and chloride, carbonate, phosphate, and sulfate are the common negatively charged ones. Mineral salts are important in maintaining the osmotic balance between protoplasm and its environment, in regulating the passage of water into and out of the cell. Calcium may take part in the change of protoplasm from a sol to a gel state. Organic Compounds. The most important difference between inorganic and organic compounds is in the carbon content of the latter. This is so universal that carbon is the one element with which organic chemistry deals. Carbon is present in some inorganic compounds, but it is present in all or- ganic ones. Virtually every organic substance will char if hot enough and yield charcoal, that is, carbon. Roast pork and apples can be burned to char- coal; chicken fat and chicken feathers make a lively fire. Protoplasm contains many organic compounds which continually shift through interactions with one another. The most abundant of these are car- bohydrates, lipids or fatty substances, and proteins. They constitute the main part of food and are included in the discussion of foods and digestion (Chap. 11), but their distribution and importance make many other allusions to them essential. Certain fundamental facts about them may be appropriately taken up here with protoplasm. Chap. 3 LIVING MATTER AND CELLS 29 Carbohydrates. All protoplasm is believed to contain carbohydrates. Those of one group (pentoses) are one of the main components of the chromatin in the nuclei of all cells. Qther than that important role, carbohydrates are not actually a part of protoplasm but are only contained in it. Their great function is the immediate supply of energy, of which they are the chief source for all living organisms. The familiar carbohydrates are sugars and starches, the cellulose in the walls of plant cells, pectin, and glycogen or animal starch stored in animal cells (Fig. 3.2). Cellulose gives stiflfness to plant stems and forms most of the fiber of cotton. Pectin, a carbohydrate of fruit, insures the stiffening of jelly. Starch in plants and glycogen in animals are the reserve food supply of the cells. They occur in the watery solution of protoplasm and the mole- cules come and go through cell membranes (Fig. 3.3). All carbohydrates contain only carbon, hydrogen, and oxygen. In forming them, untold numbers of green plants capture the energy of the sun, the source of energy for all living matter, and use this energy to combine carbon dioxide with water, thus creating the energy-packed food, glucose, and the by-product oxygen. The simplest of the carbohydrates are sugars, all of them more or less sweet. They include the simple sugars, pentoses with five and hexoses with six carbon atoms (CoHjoOe), the latter including glucose (also called dex- trose). This is an almost universal protoplasmic fuel. It is the form of sugar present in human blood in which the essential blood-sugar content is about 0.1 per cent. One of the compound sugars (polysaccharides) is table sugar (sucrose, Ci:.HooOii) from sugar cane and sugar beets. It is the commonest sugar in the nectaries of flowers, easily tasted in violets and columbines. Sucrose is produced by the union of a molecule of glucose with one of . .,^vV- Fig. 3.2. Glycogen (black) or animal starch in human liver cells. It is stored in many kinds of cells but is most abundant in the liver and muscles. Soluble in water and therefore in protoplasm it is a quickly available food. (Courtesy, Bremer and Weatherford: Textbook of Histology, 6th ed. Philadelphia, The Blakiston Com- pany, 1944.) 30 THE FOUNDATION Part I fructose and the loss of a molecule of water — glucose (CiHu-Oo) + fructose (C,iH,:.0(i) — HjO = sucrose (Ci^-H^i-On )• When it is hydrolyzed sucrose gives one molecule of glucose and one of fructose. Other compound sugars are starch, glycogen, and cellulose. These contain units of simple sugars combined into large molecules. Starch is the common storage form of carbohydrate in plant cells and glycogen or animal starch in animal cells. The molecules of both are too large to go through the cell mem- branes, but protoplasm can hydrolyze both and obtain glucose with its smaller molecules. Fats. Fatty substances take part in the composition of cell membranes and therefore in their selective permeability (Fig. 3.3). In animals they constitute the principal supply of food. They produce more energy per gram than carbohydrates but oxidize more slowly and are less quickly accessible. Fat persons get hungry just as soon as lean ones. Fats are the backlogs of the fire of which carbohydrates are the kindling. Fats are abundant in animals and by no means absent in plants. They may be in the cells, as in bacon, or in the secretions that cells produce, as in cream, or in the wax of honey- comb. Fig. 3.3. Diagram of a cell membrane where there is continuous activity, con- stant separation of what shall and shall not pass in and out of the cell. These processes are discovered by chemical analysis. Here, the cell membrane is shown cut so that its inner surface is at the left and its outer edge at the right. Lipoid (fatty) particles are shaded, protein particles are white. Water channels (arrows) permit water and other smaller molecules to pass. Larger molecules are blocked by the small pores but those that are soluble in fats may enter the lipoid (shaded) particles of the membrane, mix with their molecules and thus pass in or out of the cell. (Courtesy, Gerard: Unresting Cells. New York, Harper & Bros., 1940.) Chap. 3 LIVING MATTER AND CELLS 31 Fats resemble carbohydrates in being composed only of carbon, hydrogen, and oxygen but differ in the proportions of each of these, the hydrogen atoms being twice as numerous as. those of carbon and the amount of oxygen rela- tively small. Fats are colloids, relatively insoluble in water. They liquefy at various temperatures, oils at room temperature or lower, others near the body temperature of the animals in which they occur. Those of snakes and other cold-blooded animals liquefy at relatively low temperatures. The complex phosphorus-containing fats (phospholipids) include lecithin, abundant in egg yolk, in nerve tissue, in bile, and blood. The steroids, an- other group of fatty substances, include cholesterol, well known in the bile and gallstones. The male and female sex hormones are also related to these fats. Certain vitamins are associated with them; the growth vitamin A and vitamin D, which prevents rickets, occur especially in butter and cod-liver oil and in green vegetables; the fertility vitamin E is in butterfat and lettuce. Proteins. All protoplasm contains proteins. They are the keystones in its organization and next to water its most abundant compound. Different pro- teins occur in different kinds of cells. The proteins of every species of organ- ism evidently differ from those of every other. The kinship of animals is recorded in the proteins of their blood. Proteins in the blood of whales that have lived in the sea for countless generations are more like those of their relatives, the land mammals, than of their neighbor fishes. Proteins are prominent in the nuclei of all cells. Chromatin, the chief physical basis of heredity, is composed of nucleic acid and extraordinarily complex proteins. The nuclei of the male and female sex cells together contain most of what determines the inherited qualities of an offspring, maybe its chance to become a codfish or a senator. Proteins are the most complicated and various of all substances. They are composed not only of carbon, hydrogen, and oxygen, like the carbohydrates and fats, but include nitrogen, sulfur, phosphorus as well. Their molecules are very large, often containing thousands of atoms, and are complex, and variable like living matter itself. This means variety of structure and enables protein to interact with many other substances and to share continually in the metabolism without which life ceases. Proteins are constructed of chains or groups of smaller molecules called amino acids, the simplest of which is glycine (C1.H5O0N) which can be syn- thesized in the body. Molecules of proteins are too large to enter cell mem- branes, but those of amino acids go through them freely and form within the cell the kind of proteins which are characteristic of it (Fig. 3.3). By varied combinations of about thirty-odd amino acids, a variety of protein molecules enormous beyond imagination is achieved. They not only differ with every species but with every individual. This is shown in many ways, such as the usual difficulty in skin grafting, even between nearly related 32 THE FOUNDATION Part I persons as contrasted with its success between identical twins. The variety of proteins is no less remarkable than their constancy. One remembers the whales that after thousands of years in the ocean still have blood proteins similar to their near kin on land. In the inheritances of plants and animals proteins have not only kept their basic patterns for millions of generations, but countless variations have been added, making their constancy all the more remarkable. Enzymes, Vitamins, and Hormones. These are associated with other sub- jects that are discussed later, the first two with foods and digestion, the hormones with endocrine glands. All known enzymes and many of the vita- mins and hormones are proteins or intimately associated with proteins and all are catalysts. Enzymes are vital ^catalysts of living matter affecting the rate, and even initiating chemical reactions of all cells. Their importance is realized in light of the fact that they participate in the breaking down of proteins into amino acids, of starch molecules into simple sugars, and of fats into fatty acids and glycerol before any one of them can go through a cell membrane (Fig. 3.3). Characteristics of Protoplasm The physical basis of life is made of common materials largely composed of a few of the most abundant substances in the earth and atmosphere, all of them easily attainable. Its organization is in the highest degree complex, a continuous series of reactions which follows a permanent general pattern with details that are related to particular surroundings. It has its own char- acteristic organization and punctuality, precision of arrangement, and inter- dependence of parts. Plants and animals exist in multitudinous variety yet they are fundamentally similar. They all have the capacity for the composite of continual chemical changes called metabolism. Protoplasm has a capacity to change and yet hold its stability: in its con- tent of water, an almost universal solvent; in its abundance of proteins; in its colloid structure, with variability in size and shape of particles allowing large total areas of exposure to surrounding influences and subject to con- tinuous movement. It is susceptible to external and internal influences and consequent shifts in the phases sol and gel. It has rhythms and continuity of income and outgo of materials, resulting in a balance maintained between constructive and destructive changes. Cells Cells are the units of the architecture of plants and animals. A cell is a bit of protoplasm containing a nucleus without which it cannot grow or re- produce itself (Fig. 3.4). As long as it lives the cell constantly builds and burns in the unceasing chemical changes of metabolism. Chap. 3 LIVING MATTER AND CELLS 33 A cell is enclosed by thin protoplasmic layers forming a semipermeable membrane. This membrane is the lifeguard of the cell. It is permeable to certain dissolved substances but impermeable to others, a constant control over what may enter or leave the cell. The plant cell produces on its outer surface a definite wall that is not living, an important difference between it and the animal cell. Cells may live independently of others and if so each behaves like a com- plete organism, as an ameba does. In multicellular animals each cell is con- tinually affected by its relations with others, and by the behavior of the whole cellular community comprising the animal of which it is a part. A cat consists of billions of cells, yet when it springs on a mouse it moves as a single organ- ism. Origin and Importance of Cells. Every cell originates from a preexisting one and in no other way. This is a complex process during which the new cells receive equal amounts of this essential substance of a parent cell. Every 1P1 za,s-m.o^o-co.e Ciar ora&t l-rx- Cy to-piasin PlasUci- P'a.t ^loioTjules Ceil wall ■Ceil Vea.c-cLoie hA.itoc'hio-rxS-tPi.m Fig. 3.4. Diagram of body cell. Some of the parts are visible only after special preparation and very high magnification. Plasmosome is another term for nuclet)]us. The karyosome is a body of nuclear substance. Organoids such as the centrosome, chondriosomes (mitochondria), Golgi bodies, and fibrillae are parts of the cell that have particular functions. Plastids are characteristic of plant cells. There may be many nonliving inclusions, e.g.. droplets of water; and granules of yolk: the yolk of a hen's egg is loaded with these. The inner cell membrane, an extremely thin layer of protoplasm, is ordinarily invisible. It is in close contact with the porous outer cell membrane (or "wall"); m animal cells the inner and outer membranes are together and commonly called the cell membrane. (Courtesy, Stiles; Individual and Community Health. New York, The Blakiston Company, 1953.) 34 THE FOUNDATION Part I multicellular animal begins its existence as a single cell which soon divides into two. Each of these grows and divides into two, and thus in the majority of the cells the repeated growing and dividing go on as long as the animal increases in size, whether it is a flea or a cow. This reproduction of cells is entirely independent of sex. The characteristics of a many-celled animal are the expressions of its cells acting together. A bird flies and its sensory cells react to light, gravity, and air currents; its nerve cells carry messages to and from the brain; its muscle cells contract; its body consumes more oxygen and releases more energy as flight demands it. The responses of its cells are the links between the bird and the world about it. Structures and Functions. Interphase means that the cell is in a phase of life between divisions. In this phase, also called the resting stage, the cell is resting from division. It is not in any sense resting from respiration and other routine metabolic processes. Certain structures are typical of animal cells though all are not necessarily present in every kind (Fig. 3.4). Some plant cells do not have an organized nucleus and the chromatin is naked in the cytoplasm. Nucleus. The nucleus is essential to the growth and reproduction of the cell. It is usually clearly defined and sharply bounded by a thin, scarcely visible membrane. It contains a foundation of nuclear sap in which definite structures are suspended. In living cells the nuclear sap looks watery; in prepared cells it often shrinks away leaving open spaces. With rare exceptions, the nucleus alone contains chromatin, the physical basis of heredity and the most remarkable substance of protoplasm. The delicate, darkly staining threads, the chromonemata or color threads form a webby network in the nuclear sap. They represent the future chromosomes. One or more minute spherical bodies, the nucleoli, are often conspicuous during the interphase; their substance disappears during cell division, much of it being incorporated in one or more chromosomes. The importance of the nuclei has been shown by removing them from living cells and noting the results. An ameba can be cut in two so that only one part contains a nucleus. After such treatment the part without the nucleus will live for some days, will respire, digest its food, and move about but it doe» not grow or reproduce. On the other hand the part containing the nucleus grows, replaces the lost part, and finally divides as usual. All well- established cells have nuclei at some time during their life history. The red blood cells of man and other mammals have no nuclei when mature as they usually are when in circulating blood. However, nuclei are always present when the cells are first formed. Cytoplasm. As already defined, the cytoplasm is all of the cell except the nucleus. The ground substance of cytoplasm is a clear semifluid, the hyalo- Chap. 3 LIVING MATTER AND CELLS 35 plasm (Fig. 3.5). In living cells it looks like white of egg; in stained ones it is usually granular, sometimes with and sometimes without a delicate network running through it. The cytoplasm is enclosed by the protoplasmic semipermeable membrane, mentioned earlier in this chapter as the lifeguard of the cell. It controls the passage of everything that comes in or goes out of the cell, water, the respira- tory gases, digested food, and other materials. Likewise it regulates the dis- posal of waste substances from the cell. Fig. 3.5. A bit of seemingly homogeneous protoplasm in a clear space in the living cell. Very highly magnified it shows particles such as protein molecules and others that are jostled about by molecules of water and other smaller molecules. (Courtesy, Gerard: Unresting Cells. New York, Harper & Bros., 1940.) The semipermeable membrane has submicroscopic holes through which smaller molecules, such as those of water and amino acids can freely enter or leave the cell. The passages are too small for the larger molecules. How- ever, those that dissolve in fat merge with the fatty substances in the mem- brane and pass between their molecules and into the cell (Fig. 3.3). Such fat substances include alcohol, ether, and many organic compounds. Mole- cules of these, among them alcohol and anesthetics, may enter in such numbers that they clog the surfaces of the cells and slow down their normal activity. Brain cells are especially rich in fat and take in alcohol or an anesthetic and are strongly affected by them, while muscle and other kinds of cells may be undisturbed. Thus, the cell membranes figure at the cocktail party as well as in the hospital. In both animal and plant cells, but more commonly in the latter, there may be vacuoles, evidently surrounded by ultradelicate semipermeable membranes and usually containing liquid. The cytoplasm contains the organoids which reproduce themselves, thus exhibiting one of the fundamental characteristics of living matter. It also contains nonliving cell-inclusions (Fig. 3.4). Organoids. The centrosome consists of a spherical mass of specialized 36 THE FOUNDATION Part I protoplasm called the centrosphere and at its center are either one or two minute, deeply staining bodies, the centrioles. During the interphase of the cell the centrosome is almost always located just outside the nuclear mem- brane (Fig. 3.4). It plays an important part in cell division and at that time divides into two parts from each of which rays extend stimulating a star. Centrosomes have been found in practically all animal cells except nerve cells, but are not present in those of higher plants. Chondriosomes (mito- chondria) are threadlike or granular bodies (lipoproteins) scattered through the cytoplasm, visible in specially treated cells and sometimes in living ones (Fig. 3.4). It is generally agreed that they are physiologically important although details of their function are unknown; in actively secreting cells they increase in size and number. The Golgi substance is an irregular net- work located near the nucleus, first discovered by Golgi, an Italian physician (1898), in nerve cells and later found in almost all the cells of vertebrates and in many invertebrates, especially in glands. Its nature continues to be debated. Fibrillae are fine threads that extend in a definite direction in the cell and may have a supporting, conducting, or contractile function (Fig. 3.6). CiHa and flagella are thin cytoplasmic processes extending from the Fig. 3.6. Extremely minute fibrils stretched by a microdissecting needle (black spot) pulling out one side of the living cell (a malarial parasite, Plasmodium). (From Seifriz. Courtesy, de Robertis: General Cytol- ogy. Philadelphia, W. B. Saunders Co., 1949.) surface of the cell and are used in locomotion or to create currents of fluid. Flagella are relatively long; there are few of them to a cell and different ones lash independently. One group of protozoans, the flagellates, are so called because they swim by means of flagella. Cilia are short and there are many on one cell. They move in unison, rhythmically. Paramecium is the most familiar ciliated protozoan though there are many others. In multicellular animals surfaces are often covered with ciliated cells: the lining of the human trachea, the gills of clams, the gullet of a frog. Gills of fresh, as well as salt water clams, are good material for the study of ciliary movement. Nonliving Cell-inclusions. In animal cells the most abundant of these is stored food: yolk granules and oil globules in eggs, glycogen in other cells (Fig. 3.4). In gland cells the materials to be secreted are often held in the Chap. 3 LIVING MATTER AND CELLS 37 cells as droplets or granules. Crystals, pigment, and droplets of water and waste matter are common cell-inclusions. Shapes and Sizes of Cells. The shape of a cell depends upon the viscosity of its protoplasm, the pressure from other cells, and upon its function (Fig. 3.7). Most cells are microscopic, with dimensions of a few thousandths of a millimeter (1 mm. = 345 ot an inch). Certain nerve cells of man and other large mammals have processes that extend from the cell bodies in the nerve Cell wall Cytoplasm Nucleus Cells have thickness Cells are usually seen in slices B Columnar often with cilia at one end Thin plates of lining cells Cuboided for covering /^^^ Packed m cords .'. .. '.l.i.i.i.i_L'|l'i' il ' ■ t^»i.d*«g^i^ """'•"'••''"" 'flT'i'i ill" liti'i"''''"'"^ Elongated in the direction of the pull Fig. 3.7. Shapes of cells. In a multicellular organism most of the cells are pressed together, often flattened, or six- or eight-sided. It has been recently maintained that packed cells are actually 14-sided. This is apparent only under special conditions and observation. A. Diagram of a cell cut in section as cells are commonly studied. B. The shapes of these cells, muscle and others, are correlated with their special functions and also affected by crowding. 38 THE FOUNDATION Part I cord along the whole length of the leg. The largest single cell is the unfer- tilized egg, commonly called the yolk, of an ostrich's egg. The egg cells of birds, reptiles, and amphibians are all large because of the yolk stored in them. Relatively large or small body cells are characteristic of different groups of animals. Cold-blooded amphibians with low metabolism have larger body cells than warm-blooded birds and mammals whose body tem- perature and metabolism are high. A horse has smaller cells than a salamander and literally lives faster because it has a relatively greater cell surface exposed to body fluids bringing in oxygen and food and taking away waste. Differentiation of Cells. Diff'erentiation is a process of becoming different and specialized. The skin of an embryo fish seems to be all alike; then scales and glands develop in it. The possibility of difference was there, but it ap- peared only under certain conditions. The epitheliomuscle cell of hydra has become specialized for contractility at one end. Shapes and sizes of cells, already mentioned, are results of differentiation. They are inherited patterns brought out and also modified by the surroundings of successive generations through the ages. Polarity of Cells. Polarity of a cell is consistent — difference between opposite regions. It is a special kind of differentiation as in the epithelio- muscle cell of hydra, one end useful as lining or as a gland, the other end muscular. Polarity is almost universal in cells as it is in all living organisms. Among the diverse examples are nerve cells in which the impulse enters at one end and passes out the other, and gland cells in which the secretion collects and passes out through the membrane at one pole. The polarity of plants and animals is well known by the differences in the opposite ends as in a turnip, a rose bush, or a donkey. Phases in the Life of the Cell Every cell goes through two phases: the first includes its growth, metabo- lism, and characteristic activity, such as secretion; the second includes metab- olism and reproduction by division. Interphase. The individual lifetime of the cell is known as the interphase. It begins when the cell is produced by the division of a parent cell and lasts until the cell itself divides or dies. The structure and general characteristics of an animal cell have already been described and shown (Fig. 3.4). Further mention of conditions in the nucleus should now be made. The nucleus con- tains a tangle of threads of chromatin, the latter containing genes, the bear- ers of hereditary traits. The chromatin threads are double, made up of two slender strands, the chromonemata or colored threads in which lumps of chromatin, the chromomeres, the probable locations of groups of genes, are arranged irregularly. The two chromonemata are actually two future chromo- somes lying so close together that the doubleness is difficult to discover. Chap. 3 LIVING MATTER AND CELLS 39 Each pair of chromonemata was formerly a single thread (potential chromo- some) with genes arranged along its whole length. As a thread doubles, each gene makes a duplicate of itself out of materials lying close to it. As a result of this a new string of genes, forming a new thread, lies close to the old one and is identical with it, gene for gene, in every part (Fig. 3.8). This creation of new genes, as pointed out by H. J. Muller (1947), "should perhaps be regarded as the most remarkable process in nature; it consists of the simultaneous creation, under the guidance of each gene, of a new gene in its own image, lying next to itself and built out of materials lying around it" (Fig. 3.8). Now having the layout of its future chromosomes, each with its quota of genes, the nucleus is ready for reproduction. Reproduction of Body Cells — Mitosis. Cell division usually includes that of the nucleus and cell body. However, the nucleus may reproduce when the cell body does not and a multinucleate cell results. The cause of cell division is not understood. If it were, the cause of cancer would be known, since that is a disease of too rapid and usually abnormal cell division. Mitosis is the almost universal method of cellular reproduction. The only significant exception is the variation of it called meiosis which occurs regu- larly in the multiplication of sex cells. Mitosis is the precise rearrangement, doubling, and separation of nuclear material by which two new nuclei are formed that are quantitatively and qualitatively similar to each other and to the nucleus from which they came. By means of it each daughter nucleus receives an equal share of every substance which was in the parent nucleus. It is a continuous process having four main stages; each stage has its own characteristics but each merges into the one following (Fig. 3.8). Prophase (Preparation). Features of the interphase gradually change. The knotted chromonemata are more distinct with the members of each pair clinging together. At first each pair forms an irregular open spiral. Then the coil tightens, shortens, and is filled in with darkly staining sub- stance finally forming a chromosome. At the same time the centrosome just outside the nucleus is active. It divides, and, if the cell has two centrioles, they move toward opposite poles of the nucleus. If there was but one centriole during the interphase, it now divides and the two new ones move apart. In either case the area between them contains lines of protoplasmic particles. These form the mitotic spindle, a double cone that at first lies a little outside the nuclear field and later extends directly across it. This region is now occu- pied by the chromosomes among the lines of the spindle and directly between its dynamic poles. The nucleolus may still be visible, but it looks soft as its substance begins to diffuse, seeming to scatter. Metaphase (Midway). The chromosomes are balanced midway between the poles of the spindle (Fig. 3.8). Each one of the two chromonemata in a chromosome has at exactly the same level a special point (centromere) of 40 THE FOUNDATION Part I y egg (cell) membrane 3 sperm chromosomes 3 egg chromosomes I. Egg shortly offer fertilization oecomes the first cell of the embryo. 2. The nucleus formed by the coalescence of sperm and egg nuclei. Interphase. 3. Soon after duplication of the chromosome threads. Early prophase. 4. Chromosomes shorter and thicker. Aster dissolving the nuclear membrane. Later prophase. 5. Lines from centrosomes are attached to each chromosome at a given point. Early metaphase. 6. Lines of force from centrosomes exert a pull that separates "sister chromosomes'! Later metaphase. 7. Pulling apart of two identical groups of chromosomes. Body of cell dividing. Late anaphase. 8. Two separate cells, each with a nucleus of tangled threads as in 2. Interphase (Telophase omitted) Chap. 3 LIVING MATTER AND CELLS 41 attachment to the spindle. This "owes its existence to a particular gene lying at that point" (Muller, 1947). When it is at the center the chromosome is V-shaped with the tip of the V in contact with a line of the spindle. Some- times it is fairly near the end and the chromosome then hangs J -shaped on the spindle, or, if very close to the end, it is rodlike. During the early part of the metaphase the centromeres are apparently repelled from the poles of the spindle and moving toward the equator they draw their chromosomes with them. There, all the chromosomes become arranged exactly half-way between the poles of the spindle at the midplane in an equatorial plate. The chromosome and its duplicate are still in contact (Fig. 3.8), Anaphase (Separation). Each chromosome and its duplicate begin to separate always starting at the centromeres which are responsive to the forces of the attraction of the spindle. The members of each pair of chromosomes gradually draw apart until they become entirely separated and each one moves toward the nearer pole. During this journey the centromere is always in front, pointed toward the pole (Fig. 3.8). Late in the anaphase the chromosomes are in two identical groups, one at each end of the spindle. In each group the chromosomes of the parent cell with their genes are all represented. In animal cells the division of the cytoplasm starts from the outside and Fig. 3.8. Diagrams showing changes in the nucleus during the reproduction of a cell by mitotic division such as occurs in every cell of a growing body, or in parts of the adult, except in the later divisions of maturing sex cells. /. Part of an egg shortly after its fertilization. Three chromosomes (black) represent the inheritance from the male parent, and three (in outline) the inheritance from the female parent. The descendants of these six chromosomes occur in all the cells of the new individual. The star-shaped centrosome is a center of force. 2. Interphase or "rest- ing stage" with the chromosomes uncoiled in threads so ensnarled that individual chromosomes cannot be identified except with great difficulty. 3. Early prophase. Each thread has doubled and now consists of two identical strands, thickened by means of the ultra-fine coiling of the strands. The centrosome has divided and there are now two centers of force. 4. Late prophase. The chromosomes are mates, shortened and lying side by side. Every one of the thousands of genes contained in one is duplicated in the other. The centrosomes are moving to opposite sides of the nucleus and the nuclear membrane is dissolving. 5. Early metaphase. Lines of force from the centrosomes have become attached at given points (centromeres) to the respective mates, called identical chromosomes. This has forced them into positions on an equatorial plane half way between the centrosomes. 6 and 7. Late metaphase and late anaphase. The apparent lines of force exert a pull on the centromeres, thus separating the identical chromosomes, and drawing the respective mates toward the opposite ends of the spindle formed by the lines of force. 8. Division completed. Interphase. (Telophase omitted.) The two identical groups of chromosomes are pulled near to the centrosomes and ceil membranes separate the cell body into halves. The fine coils of the chromosomes unwind in threads similar to those in 2. This process occurs in the telophase stage not shown here. With the attainment of two new cells in the interphase stage, the reproduction is completed. 42 THE FOUNDATION Part I Fig. 3.9. Stages (metaphase and anaphase) in the mitosis of cells of a white fish embryo. Microphotographs of stained and sectioned cells at an enlargement of about 700 times. Note the lines of force that compose the spindle and radiate from the centrosome in the metaphase, and the dimming of the spindle and the new cell membranes in the anaphase. (Courtesy, General Biological Supply House, Inc., Chicago.) the membrane separating the two new cells extends inward in a plane at right angles to the spindle. In plant cells it starts from the center as a cell plate and extends outward. Telophase (Reconstruction). The chromosomes in each nuclear group uncoil and lengthen into knotted chromonemata. The spindle and at the same time the rays about the centriole disappear. If two centrioles are characteristic of the interphase each centriole now divides; if not, each one remains single. The nucleolus becomes visible again and the boundary of the nucleus regains Chap 3 LIVING MATTER AND CELLS 43 its sharpness. The daughter cells are now complete growing cells in the inter- phase stage. The time required for the complete process of cell division varies greatly with the kind of cell and the surrounding conditions, especially temperature. A cell of a salamander's heart observed living in tissue culture completed the process in two hours. The process may be much quicker. Results of Mitosis. Two cells are formed that are identical with one another in respect to every gene and every chromosome. This is accomplished first by the doubling of the genes in the chromosomes, and then by the sepa- ration of the chromosomes and their inclusion in the new nuclei. The re- mainder of the cell may or may not be equally divided. In the growth of a multicellular animal, whether hydra or man, mitosis is repeated thousands to billions of times, and each time hereditary qualities originally received from the parents and contained in the first cell are distributed equally to new cells. In amitosis the nucleus simply constricts into an hourglass shape and then separates into two parts without forming chromosomes. This is a very rare arrangement which occurs only under unusual conditions, especially in de- generating cells. Reproduction of Sex Cells — Mitosis and Meiosis. Body cells reproduce exclusively by mitosis. Germ or sex cells reproduce by mitosis and meiosis. The reproduction of sex or germ cells in males and females includes an in- crease in numbers from a few original germ cells, a reduction to half their number of chromosomes, i.e., from the diploid to the haploid number, and changes in the shape and size of the cells (Fig. 3.10). The all-important genes inherited from the parents of the individual and present in the chromosomes of his or her original germ cells are distributed so that each gamete (egg and sperm cell) has an inheritance from its ancestors, even remote ones. The process in the male is spermatogenesis, the history of the sperm cell from its earliest stage to maturity, and in the female, oogenesis, the history of the egg cell. There are differences in size and numbers of the mature sex cells in the male and female, but the changes in their nuclei are essentially similar. Spermatogenesis. The original primordial germ cells in the male divide repeatedly by mitosis, gradually producing great numbers of extremely minute, nearly spherical cells called spermatogonia. These have the diploid (or body) number of chromosomes; half of them were in the male cell or sperm and half in the female cell or egg when fertilization occurred. Suppose, for example, that a primordial germ cell has six chromosomes, three derived from each parent (Fig. 3.10). Such cells divide mitotically, producing several generations of cells called spermatogonia, each one of which contains six chromosomes. A change then occurs beginning with the maturation or meiotic divisions. First the cells become relatively larger and are called primary spermatocytes. In the prophase of the first meiotic division 44 THE FOUNDATION Part I Distribution of chromosomes in the developmeni of sperm cells. Dork chromosomes = mole inheritance. Light chromosomes = femole inheritance. Body cell of fother I.e. skin, muscle, etc Germ cell destined to divide and develop into sperm cells Spermatogonium Primary — spermatocyte A.B.MEIOTIC divisions Secondary spermatocyte MITOTIC divisions Cell enlarges Similar chromosomes pair (Synapsis) Eacn chromosome duplicates itself. Tetrads result. Tetrads separate into pairs. Cell divides. MEIOSIS Tetrads separate into pairs. Cell divides — Sister chromosomes separate. Spermatids Sperm cells Fig. 3.10. Diagrams showing the behavior of the chromosomes during (A) the development of the sperm cell (spermatogenesis) and (B) the similar features in the development of the egg cell (oogenesis). In each sex cell the process includes: Chap. 3 LIVING MATTER AND CELLS 45 Distribution of chromosomes m the development of egg cells. Light chromosomes = female inheritance. Dark chromosomes = mole inheritance. MITOTIC divisions Body cell of mother I.e. skin, muscle, etc. Germ cell destined to divide and develop into eggs. Oogonium Fertilization Cell enlarges Similar chromosomes pair (Synapsis) Each chromosome duplicates itself. Tetrads result. Tetrads separate into pairs A.B. MEIOTIC divisions Cell divides, 3 pairs of chromosomes in each Cell divides 3 chromosomes ./a V Mature egg Second polar body B Primory Oocyte Primary Oocyte Secondary Oocyte First polar body These cells die an increase in number of chromosomes by MITOSIS and a reduction in the number of chromosomes by MEIOSIS. For simphcity six chromosomes are used here for body cells. Cells of the human body have 48 chromosomes. 46 THE FOUNDATION Part I the two chromosomes of each similar or homologous pair, one derived from the male and one from the female parent, come together and lie parallel to one another. This is called synapsis. Soon each chromosome duplicates itself as in mitosis, so that there is a cluster of chromatids (potential chromosomes), a quartet or tetrad in which two chromatids are of male and two of female parental origin (Fig. 3.10). A spindle forms and in the metaphase the tetrads become arranged on its equator. In the anaphase, the two chromatids of female parental origin in the tetrad go to one pole of the spindle and the two chromatids of male parental origin go to the other. Each of the resulting cells is a secondary spermatocyte with three chromosomes, each of which contains two chromatids. In these secondary spermatocytes a spindle soon forms for the second meiotic division, and in the metaphase the two chromatids of each chromosome separate and one goes to each pole. Each of the cells (spermatids) that result contains three chromosomes. Some of the cells may hold chromosomes entirely of male or entirely of female parental origin; some may hold chromosomes of both origins. Meiosis is now completed, the chromosome number being reduced by half, i.e., to the haploid number. The rest of the process is a change in form. The nucleus becomes more compact and the cell body relatively minute with a slender cytoplasmic tail or flagellum that acts as a swimming organ. At its base is the bead-shaped middle piece that holds the centrioles (Fig. 3.10). Thus, from each primary spermatocyte four sperm cells (gametes) are formed. The foregoing process is usually com- pleted before the sperm cells leave the testis. Oogenesis. Fewer and larger sex cells (gametes) are produced in oogenesis. Great numbers of oogonia result from divisions in the period of multiplication (Fig. 3.10). Following this period certain of the oogonia become primary oocytes which grow to be larger than the spermatocytes, the comparable stage of the male germ cells. But they are similar to them in the behavior of the chromosomes, in synapsis, tetrad formation, and the reduction of the number of chromosomes in the first meiotic division. In this division, however, one secondary oocyte receives practically all of the cytoplasm along with its three chromosomes, while the other one, called the first polar body, has very little cytoplasm with the same number of chromosomes. Likewise in the second meiotic division, the large secondary oocyte divides unevenly. The bulk of the cytoplasm surrounds the nucleus of the incipient egg (ootid or ovum) with its three chromosomes. The little remaining cytoplasm and the nucleus contjain- ing three chromosomes compose the second polar body, actually a rudimentary egg. The first polar body goes through a division that parallels the second meiotic one. Thus there are three polar bodies and the egg, each with three chromosomes assorted as in spermatogenesis (Fig. 3.10). The polar bodies with their loads of precious hereditary substance eventually degenerate and come to nothing. The egg keeps its form and is enlarged by its supply of yolk. Chap. 3 LIVING MATTER AND CELLS 47 In different species of animals the production of polar bodies may occur inside or outside the ovary. With the fusion of the nuclei of sperm and egg that occurs at fertilization, the number of chromosomes is returned to six, that of the zygote, the first cell of the new individual. Part II Ecology 4 Plants Provide lor Tneniselves ana me Animals The existence of the living world depends upon green plants since they alone make the food that is essential both to themselves and the animals. Through the long past animals became agile of movement, swimming, running, or flying, developed keen senses, and became alert to their surroundings. Great numbers of them fed upon plants, and as time went on many became carni- vores and devoured their fellow animals. But none of them could make their own food from the chemical elements about them. Human beings are no better off than other animals. Although they have extraordinary capabilities, their existence finally depends upon the carbohydrate foods, the sugars and starches that green plants make by photosynthesis. After years of study it now seems that photosynthesis may be understood, but to furnish the world with food is another and probably much more difficult matter. The meals of Eskimos are far removed from the cabbage patch, yet they too originate in plants. Eskimos live on seal meat and fish and birds, but ulti- mately all these are fed by the microscopic plants which swarm in the arctic seas. The seals and the birds feed upon the fishes; big fishes eat little fishes and both devour little copepods by billions; and finally copepods feed ex- clusively upon microscopic plants, mainly diatoms (Fig. 4.1). Thus, the substance of the Eskimo's diet is in origin mainly digested diatoms. For the dweller farther south in America or Europe the food chain is different, usually beginning with grass and ending with beef, or starting with diatoms and ending with codfish. Grass can live without cattle and diatoms without codfishes but no animals can exist without plants somewhere in their food story. Plants and animals are fundamentally similar. A sunflower and a horse look strikingly different; yet they are both living organisms existing basically in the same way. 51 52 ECOLOGY Part II I diatoms ) > ( copepods | — ► (crustaceans] Fig. 4.1. In their own way of living Eskimos are finally dependent for food upon diatoms and other algae, the microscopic plants that crowd the surface waters of the arctic seas. The dependence is indirect but sure, just as farther south human dependence for beef steak is upon plants. (After Transeau and Tiffany: Textbook of Botany. New York, Harper & Bros., 1940.) Plant and Animal Relationships Building Materials and Protection. Plants furnish building materials for all animals from insects to man. Wasps bite off wood fibers for their paper nests, a host of insects lives within burrows in stems and tree trunks. The habits of land birds would be changed beyond recognition if those birds did not perch and nest in trees, or nest and feed in grass and mosses. There is scarcely a mammal, short of ocean-going whales and their kin, that does not at some time take to plants for shelter. Hundreds of field mice live among the grasses of empty-looking fields; the wildcat climbs a tree for a meal of young birds; in South America trees furnish the bandstands for the howling monkeys and the hammocks for sleeping sloths. In the noon heat of the tropics the silent forest is populous with hiding animals. With the main exceptions of beavers and man, mammals do not use wood for building. Man is the great builder with plant fiber. From the time human animals left their caves they began to make earthen and wooden houses and long before that they must have used windbreaks of wood. The prehistoric lake-dwellers lived in wooden houses raised on piles above the lakes, ideal for safety as well as for fishing at home. Throughout history plants have supplied humanity with wood for boats and wagons, and fibers for ropes and cloth. In recent years the elegant and versatile rayons and plastics have been produced mainly from plant products. The existence of all this outfit of civilization hinges upon a microscopic struc- Chap. 4 PLANTS PROVIDE FOR THEMSELVES AND THE ANIMALS 53 ture peculiar to plants, their strong cell walls composed of cellulose, or cellu- lose impregnated with lignin if the tissue is woody. The Plant Cell Wall. Plant cell walls have long provided heat and power for humanity (Fig. 4.2). Whether lignified or not, cellulose burns rapidly in combination with oxygen; its stored energy is released in the form of heat and it is converted back to carbon dioxide and water. When cellulose is subjected to heat and pressure for long periods of time it undergoes chemical changes; hydrogen and oxygen are removed and solid carbon remains. This is what happened in the ancient swamps and forests where peat, lignite, and coal were formed, one or another product depending upon the material and the stage of the carbonization. Coal exposed longer and under the right conditions becomes graphite; exposed still further and properly conditioned, it crystallizes as pure coal, or with extreme hardness as diamonds. The heated live coal of the open Fig. 4.2. Typical plant cell. In plant cells the cytoplasm occupies a relatively small space and the central part contains one or more large vacuoles filled with watery solution containing many substances related to the life processes of the plant. The vacuoles are separated from the protoplasm by an almost invisible semi- permeable membrane (or tonoplast), a lively and important region of exchange of substances. In contrast to animal cells those of plants have a prominent cell wall strengthened by cellulose, made woody by lignin. (Courtesy, Rogers, Hubbell, and Byers: Man and the Biological World, ed. 2. New York, McGraw-Hill Book Com- pany, 1952.) 54 ECOLOGY Part II fire is "alive" in so far as it is freeing energy gathered from the sun and stored in plant cells millions of years ago. Neither coal nor diamonds are modern upstarts: both have long been important to humanity, in fires for the tempering and molding of metals, in various techniques, and in tokens and jewelry. Distribution. There are many ways in which plants depend upon animals. Most animals can travel around freely; plants cannot. Plants are carried about by the natural forces of air and water and by animals. Thus insects carry pollen (male sex cells) and cross-pollinate the flowers as they seek nectar and pollen in one after another (Fig. 4.3). Birds carry seeds across land and water often to germinate safely in distant regions. Plants are directly dependent on the content of the soil and animals fertilize this with their excretions and disintegrating remains. Photosynthesis Green plants are, with exceptions such as nitrifying bacteria, the only self- supporting organisms on the earth. They accumulate energy from the sun and Pathway of male cell to the Pollen grains touch stigmatic surface Fig. 4.3. The parts of a typical flower. Insects visit flowers to gather nectar and pollen. The nectaries are at the bases of the petals and many flower-visiting insects must brush against the pollen-bearing anthers in order to reach the nectar. In mov- ing around they transfer the pollen containing the male cells to the stigma of the flower or other flowers of the same kind and thus bring about the fertilization of the ovules (eggs). Chap. 4 PLANTS PROVIDE FOR THEMSELVES AND THE ANIMALS r 55 Fig. 4.4. Carolus Linnaeus (1707-78), the Swedish botanist, at age 25, in Lap- land dress, holding his favorite, the twin-flower (Linnaea) and equipped with a col- lecting kit for his Lapland journey. Linnaeus made one of the great contributions to natural sciences, the two-word naming (binomial nomenclature) of plants in 1753 and of animals in 1758. His work made way for the natural arrangements of living organisms. (Courtesy, Greene: Carolus Linnaeus. Philadelphia, Christopher Sower Co., 1912.) store it as chemical energy in carbohydrates (starches, sugars). The process of photosynthesis or carbohydrate-making is the greatest chemical industry in the world with the widest importance of all biochemical reactions. It is carried on by all chlorophyll-bearing plants from microscopic algae to the largest trees. Red and brown seaweeds and plants of various other colors contain chlorophyll cloaked with pigments. Although the manufacture of food by land plants is enormous, it is estimated that 90 per cent of the total is produced by the large (seaweeds) and small algae of the ocean (Fig. 4.5). They constitute the basic food supply of the great animal populations of the seas. In general, the plants themselves use a good deal of the food which they produce. Much of it is decomposed into water, carbon dioxide, and mineral salts by the decay of leaves and plant bodies in water and on land, and is used over again by the plants. Materials and Conditions. The natural conditions for photosynthesis include the presence of chlorophyll, the energy of sunlight or artificial light, water, and 56 ECOLOGY Part II Fig. 4.5. Common brown seaweeds that are great food producers. From left to right, fan kelp, Laminaria: giant or vine kelp, Macrocystis; bladder wrack. Fucus; ribbon kelp, Nereocystis. (Not drawn to scale.) Seaweeds constitute a large percent- age of the basic food supply of the seas. On the rocks between the tides where they abound they furnish food and holdfast for hosts of small animals. carbon dioxide. The chlorophyll occurs in chloroplasts usually rounded green bodies in the tissues of leaf and stem. It is a complex protein, in higher plants consisting of two pigments, a blue-green one, chlorophyll a {Cr.-Mi-Or.NiMg) and the less abundant yellow-green, chlorophyll b (C55H7oOGN4Mg). The chemical content of chlorophyll is in many ways similar to that of the hemo- globin of blood except that iron occurs in the latter instead of magnesium. In the higher plants chlorophyll is almost always associated with yellow pigments, the carotenoids, and the various xanthophylls related to carotene. Their func- tion is not wholly known; if they are concerned with photosynthesis they are far less important than chlorophyll. Carotene and xanthophyll are much more stable; the rich yellow autumn colors of birch and elm leaves are exultant witnesses that these colors endure after chlorophyll has broken down. The Process. During photosynthesis the kinetic energy in light is changed to the potential chemical energy of food. Carbon dioxide is mainly absorbed from the atmosphere. It enters the leaf through the millions of pores or stomata, diffuses through cell membranes in a dissolved state, and goes into the chloroplasts (Fig. 4.8). Water enters chiefly through the roots. In the presence of chlorophyll and with the aid of the energy of light, the carbon Chap. 4 PLANTS PROVIDE FOR THEMSELVES AND THE ANIMALS 57 dioxide and water unite to form glucose (CoHn-Oe), the simple sugar from which all the organic compounds of plants and animals are eventually derived. The chlorophyll itself is not used up and is evidently a catalyzer that hastens other chemical processes. Green plants include the seed plants, and the mosses, ferns, the green algae, and the lichens, many first named by Carolus Linnaeus in his two-name system (Fig. 4.4) . As already noted, besides these there are other plants whose chloro- phyll is blanketed with various colors, as in the deep red, yellow, or variegated Coleiis often called foliage plants. The pigment of red and brown seaweeds also effectively clothes the chlorophyll as does the brown cloak of the microscopic diatoms of fresh and salt waters. Although the process of food-making in these plants is not clearly worked out, it is certain that pigments other than green ones take an important share in it. One investigator has observed that in red seaweeds the light absorbed by red pigments is more efficient in photosynthesis than that absorbed by the green of chlorophyll. The food product in blue-green algae, for example, is not glucose but glycogen which is also found in fungi (bacteria, molds, mushrooms, and rusts) and in the tissues of animals. The tons of rockweed washed by the breakers on many headlands press home the estimate that "90 per cent of the photosynthesis on earth is carried out, not by green land plants, but by the multicolored sea algae" (Fig. 4.5). Studies of Photosynthesis. In 1772 Joseph Priestley discovered that a plant produced oxygen. He piped air into a glass jar from another jar in which a mint plant was growing. Then he put a lighted candle in the empty jar and the candle, being well supplied with oxygen from the plant, went on burning. Later he took the candle out and put a mouse into the same jar. The mouse breathed comfortably and Priestley wrote of it, "nor was it at all inconvenient to a mouse which I put into it" (Fig. 4.6). In 1779 Jan Ingenhousz, a court phy- sician to Empress Maria of Austria, observed that plants "corrected the bad air" in which they were growing. He wrote of his observations, "I found that this operation of the plants is more or less brisk in proportion to the clearness of the day and the exposition of the plants." Julius R. von Mayer, who formu- lated the principle of conservation of energy, first stated in 1845 the physical function of photosynthesis as the conversion of light energy into chemical energy. Photosynthesis is a subject of joint chemical and biological inquiry in which new dscoveries are made from month to month, and sunlight has created sugar from carbon dioxide and water. Organization of a Green Plant Essential Needs. Plants are light-seeking, light-directed organisms. They have four essential needs, light, air, water, and certain minerals. The sun sheds its energy in light and heat upon the earth. It creates currents in the water, winds in the air, quickens the activity of water molecules that scatter as vapor. 58 ECOLOGY In sunlight a green water plant gave off bubbles (of oxygen). Part II Mouse could breathe in closed jar. (Oxygen supplied by plant ) PRIESTLEY'S DISCOVERY Fig. 4.6. The chemist (England, 1733-1804), Joseph Priestley kept a plant grow- ing within a glass jar connected with another jar in which he kept a mouse. The mouse breathed on comfortably because the plant provided it with oxygen, a product of its photosynthesis. (Data for figure from Memoirs of Joseph Priestley, 1:253. London, J. Johnson, 1806.) and activates the photosynthesis of green plants. Thus the sun surrounds plants with light and keeps air and water circulating about them. Plants may have all of this without going after it as the majority of animals do. Light bathes the whole plant from above or from one or more directions; the branches reach out for light and the leaves take positions to receive it. Light does not penetrate deeply into the tissues, but leaf surfaces are spread out and the chlorophyll is always near to them (Fig. 4.8). The spread of maple leaves to receive light is a marvel of efficient arrangement. The essentials for a green plant's existence are in two layers of its environment. Light and air are above; there the plant is green and its stem upstanding. Water and minerals are be- lov/; there the plant is colorless and its roots are pliant. The Individual. The plant has a particular form recognizable as character- Chap. 4 PLANTS PROVIDE FOR THEMSELVES AND THE ANIMALS 59 istic of its species and of itself — the barrel cactus of the southwestern desert, the American elm, the jack-in-the-pulpit. There is a strict division of labor in the plan of the body; different parts perform particular functions such as protection, support, and water transport (Fig. 4.7). The plant body has two main regions, the shoot system of stem and leaves which is intimate with the atmosphere and the root system which is correspondingly intimate with the soil. Stem. The stem or axis is a support and a highway. Its first function is the raising of leaves to the light, of flowers upward for light and pollination, of seeds in position for better dispersal. Its second function is the distribution of water and nutrient solutions and gases throughout the plant. In most plants, the stem is a cylinder that tapers at the top and gives off branches that are Absorption Water Salts-- Oxygen ^^ >Respiration Fig. 4.7. A diagram indicating the main structures and functions of a seed plant, the bean. The first leaves (cotyledons or seed leaves) are richly stored with protein and contribute only slightly to photosynthesis. (Courtesy, Woodruff and Baitsell: Foundations of Biology, ed. 7. New York, The Macmillan Co., 1951.) 60 ECOLOGY Part II ultimately continuous with the veins of the leaves. Stems vary in circumference: the stem of a California redwood is thick enough for a car to drive through; that of the young maidenhair fern has a hair's thickness. Stems are squat in turnips and tall in royal palms. The main layers of the stem are the cambium, and the phloem, and xylem, the latter two named from the Greek words for bark and wood. Cambium is the vital growing layer from which the other two layers originate, the xylem from its inner and the phloem from its outer side. In tree trunks the wood is composed of xylem and most of the bark of phloem. The xylem holds the supporting tissue and tubes through which water and dissolved substances are conducted from root to leaf. The phloem contains tubes through which manu- factured foods are distributed especially from the leaves to regions of the plant where they are stored or used. The epidermis covers the stem and is continuous over the leaves and roots. Tons of water mixed with mineral nutrients ascend from the soil and through the tubes of the xylem into the veins of the leaves. Great quantities of food made in the leaves pass through the veins and stem by way of the tubes of the phloem. The pattern of con- duction in xylem and phloem is essentially the same whether in a buttercup or an oak tree. Sugar cane, potatoes which are underground stems, and tree trunks are stems that have million-dollar values and high places in history. Except for the plant stems that made his ships, Columbus would not have crossed the ocean nor the Norsemen set foot upon American shores. A few plant stems made the raft Kon-Tiki on which six men crossed the Pacific Ocean. Leaf. A leaf is a thin blade, greener on the upper than the underside and freely exposed to light and air. Continuous with its petiole or stem is the stiffened vein or group of veins from which other more delicate ones branch olT and hold the leaf outspread. The unique function of green leaves is photo- synthesis. Water from the plant stem is conducted to the leaf, and carbohydrate food from the leaf to the plant stem. There is great variety in the shapes of leaves, but, whether they are simple or compound they all fit three types: the rounded leaf like that of the nasturtium, the linear leaf like the grass blade, and the cone-shaped one such as the elm leaf. Microscopic openings of stomata occur in the otherwise waterproof epi- dermis, especially on the lower side of the leaf (Fig. 4.8). Each opening is between two specialized cells of the epidermis, called guard cells because changes in their size and shape determine whether the stomata are open or closed. Water enters through the root hairs and passes out mainly through the open leaf-stomata and to some extent through the cuticle, in the process of transpiration. Of the total quantity of water absorbed by the roots, as much as 98 per cent escapes by transpiration. Stomata also regulate the exchange of gases between the air and leaf. If the leaf is well lighted they are open and Chap. 4 PLANTS PROVIDE FOR THEMSELVES AND THE ANIMALS 61 Sun's energy Palisade cell Chloroplost — Cuticle Upper epidernnis ?'l I* gases, diffuse toother cells. CO2 H2O C6H12O6 ^"^MIM spongy tissue •Vein Air and fluid spaces Lower epidermis COz enters with air Guard cells turgid Stonnates open in the normal daytime condition Excess O2 leaves during sugar making Excess water (HjO) goes out as vapor sroiAUR Fig. 4.8. The leaf blade. The essential structures are: the upper and lower cover- ing layers or epidermis; the cells of the palisade and spongy tissue containing the chlorophyll that carries on photosynthesis; the veins that are the highways of trans- portation between leaf and stem (the xylem ducts transport water and the phloem carries food) . Each stoma is a breathing pore leading to the air spaces in the spongy tissue. The guard cells on either side of the pore regulate its size according to the moisture and the amount of oxygen and carbon dioxide exchanged. photosynthesis is in full swing. The bean-shaped guard cells are then rotund with stored sugar and water which the sugar has attracted by osmosis. Their plumpness causes them to pull apart and thus to form an opening between them; when they collapse the opening closes. Other conditions within or with- out the leaf affect the guard cells, especially scarcity of water. The stomata are then closed and what water there may be left in the leaf is kept from passing out in transpiration. Respiration occurs in all cells of the leaf as it does in the root, the stem and other parts of the plant. Within the green leaf the upper layers of cells hold an 62 ECOLOGY Part II iff :■:} i1 ^ "te 'n 1 \B. f ^ ^ y B Central cylinder Cortex Tubes and growing cells Moturing zone Epidermis root tiairs Elongating zone Growing point Protective root cap Air space Soil particles rmal cell, comparable to outer skin layer of animals Chap. 4 PLANTS PROVIDE FOR THEMSELVES AND THE ANIMALS 63 abundance of chlorophyll (Fig. 4.8). Here the leaf is greenest and the light falling on it is strongest. These cells are the all important food-makers, the links between the energy of the sun and the living world. The lower layers contain spongy cells of odd shapes and hold less chlorophyll than those of the upper layers. They are loosely packed in clusters with air spaces in between. This region of the leaf provides for the income and circulation of gases and the outgo of water. Extra water is also eliminated in droplets (guttation) from openings at the tips of the veins of grasses, corn and many other plants. In early morning the droplets hang in beautiful symmetry on the edges of the leaves of strawberries and jewelweeds. During the day some water is lost from the leaf and at night moisture in the air condenses on its cool surface. The main supply of water is always from the root. Root. The main functions of the root are the anchorage of the plant, the absorption of water and mineral matter, the storage of manufactured food and sometimes of chemicals, e.g., nicotine is produced in the roots of tobacco plants and transported to the leaves. The spread of surface necessary for ab- sorption also makes it an efficient anchor in the soil (Fig. 4.9). The root is the extension of the stem and resembles it in having long tapering branches and an essentially similar structure, although the pattern of the conducting tubes is different. Although roots are various in size, form, and structure, they have no such diversity as the leaves and stems, for conditions in the soil are less variable than those in the air. Of all the material which the root absorbs the most important is water. It is a great part of the plant substance and as essential for the processes of living as it is in animals. Absorption occurs exclusively in the microscopic root hairs in the white terminal parts of roots, the ones whose injury in transplanting is followed by the familiar wilting of the plant. Near the tip of each new root, hairs are continually forming, a little farther back they are constantly dying. The root hair is a single cell of the epidermis. It grows outward in a hairlike projection that turns and twists about the particles which in any moist soil are clothed in a thin capillary film of water (Fig. 4.9). The root hair is an osmotic mechanism (Chap. 2, p. 22). Water and salts enter it but sugar does not pass out. Although each root hair is virtually microscopic, their total area is a marvel of expansiveness. In one species of grass the total length of root hairs Fig. 4.9. A. The root system of a corn plant. (After Weaver.) B. Diagram of a section of a root tip and its different zones. Cells of the root cap are worn off and replaced by new ones from the growing zone above it. The force that pushes the root through the soil is the lengthening of cells in the elongating zone. Epidermis, root hairs, and the ducts of the food-transporting phloem and the water-transporting xylem all develop in this zone. (After Woodruff & Baitsell: Foundations of Biology, ed. 7. New York, The Macmillan Co., 1951.) C. Root hairs are branches of epidermal cells. In every well-grown root billions of root hairs take in water from the films of it that surround particles of soil wherever there is moisture on the ground. 64 ECOLOGY Part II held within one cubic inch of soil has been estimated to be four-fifths of a mile. Root pressure pushes sap to the top of the tallest trees. It acts under various conditions, in trees of tropical rain forests where there is no evaporation from the leaves, and in trees of temperate climates before the leaves appear in spring. In some parts of our country the maple sugar season is the time of the first great lift of sap from its winter storage in the roots of sugar maples. Root pressure is all-important to plants. Details of the causes of it are complex and not completely understood. Root hairs are the first actors in root pressure because they carry on the absorption of water from the soil. About one-third of the pressure is believed to be osmotic and two-thirds metabolic, that is, due to respiration and other life processes. Reproduction Higher plants reproduce asexually and sexually. Some species reproduce more often or exclusively in one way, some in the other. Young strawberry plants develop from creeping stems which grow from the parent; grass plants spread out many sprouts from older plants. The white potato of the dinner table is a food-filled underground stem. When used for planting it is cut into pieces each containing an "eye" or bud from which a new plant grows. In most higher plants both methods of reproduction are common which is never the case in higher animals. A strawberry plant buds forth a new plant; a cat never buds off a kitten. The root, leaf, and stem are concerned with the vegetative functions, the intake of food and water, digestion, respiration, and asexual reproduction; the flower with sexual reproduction. In higher plants sexual reproduction is more important than asexual. Any bouquet of flowers — roses, orchids or butter- cups— is a cluster of reproductive organs. Although sexual reproduction differs greatly in detail in plants and animals, its essential features are the same. Flower. The flower is the reproductive organ of the plant. The more or less conspicuous parts are the sepals, petals, stamens, and pistil. The latter two are directly and the others only indirectly concerned with the formation of male and female sex cefls and their union in the process of fertilization. Flowers differ greatly in the position and form of the parts and whether male and female cells are borne on the same or different plants of a species. They are often in the same flower as in the diagram (Fig. 4.3). The stamen consists of the stalk supporting the anther and its pollen sacs. When it is mature, the pollen sacs break open and liberate the pollen grains within each of which there are two male sex cells. These are equivalent to the male sex cells (sperm) of animals. The pistil (or pistils) usually consists of a central stalk with a sticky tip, the stigma. At its base is the ovary containing the ovules, the female sex cells equivalent to eggs. The union of the sex cells is brought about in one way or another, such as by the locations of the parts, or by insects. The Chap. 4 PLANTS PROVIDE FOR THEMSELVES AND THE ANIMALS 65 male cells come in contact with the stigma and make their way down through the stalk of the pistil to the ovary. Finally one of them reaches the ovule and enters it. The subsequent fusion of the male and female cells within the ovule is fertilization. These are the essentials of the journey of the male cell and its union with the female, with many complexities omitted and numbers of irregu- larities unmentioned. The fact remains that the behavior and function of the primary sex cells are strikingly the same in plants and animals. Seed. The seed is an embryo plant which has developed from a fertilized ovule. A fruit is a growth around one or more embryos (seeds) which protects them and is a common means of their dispersal. Similarities of Plants and Animals 1. Cells. Their basic material is protoplasm organized in cells. 2. Food. Their main food and chief sources of energy are carbohydrates — starches and sugars. Amino acids, the "building blocks" of proteins, are essen- tial to them. Water is a vital need. 3. Metabolism. The basic processes of respiration and of digestion and assimilation are similar. Excess products of metabolism are mentioned below. In the respiration of plants and animals oxygen enters the cells and unites with carbohydrates, fats, and lastly with proteins. Oxidation, i.e., chemical burning occurs. Chemical energy is released as activity and heat. Carbon dioxide and water are formed. During digestion food is changed to simpler chemical compounds. During assimilation the digested food becomes part of a specific kind of protoplasm. For example, food assimilated by the chromosomes in certain cells of an oak tree acquires the characteristics of the appropriate substances in those chromo- somes; food assimilated in the chromosomes of certain cells in a goat does likewise. In both plants and animals, certain excess by-products may be stored. Ex- amples of these are digitalis in foxgloves, opium in poppies, calcium carbonate in earthworms. The use of these, if any, to the producing organisms is not clearly understood. Certain other by-products may be used; carbon dioxide by green plants in photosynthesis, and by animals in small amounts as a stimulus to breathing and as a control of the force of the heartbeat. Differences between Plants and Animals 1. Locomotion. The majority of plants do not move from place to place. The majority of animals move about freely. 2. Food. Green plants make carbohydrates by photosynthesis. Animals take carbohydrates from plants. Plants are the chief makers of proteins which they elaborate from amino acids. Animals take proteins from plants and other animals. 66 ECOLOGY Part II 3. Metabolism. Even in higher plants the rate of metabolism is low. In active respiration the temperature of plants may rise only slightly above their environment. In higher animals the rate of metabolism is high. The temper- ature of birds and mammals is usually much higher than that of their environ- ment. In the majority of animals, there are special organs of excretion by which nitrogenous waste products of metabolism are eliminated. In plants, there are no such organs. The only approach to an excretory product in plants is prob- ably the excess by-products of metabolism such as opium (see similarities of plants and animals). There are no excretory organs in plants. 4. Hormones. Plants produce relatively few hormones and these have general effects, such as, growth of stem and growth of root. Animals produce an elaborate and delicately adjusted series of interacting hormones which have specific effects, such as, thickness of skin. 5. Responsiveness. In plants, the ordinary cells are variously responsive, e.g., to light, to temperature, in some regions more than others. In animals, special sensory cells are highly responsive to one or another kind of stimulus, e.g., the rod cells and cone cells of the eye. 5 Animals and Tlieir Environments Animals abound in great numbers. Thrust a stick into a large ant nest on a July day and millions of ants pour out, many carrying white packages that taken altogether contain myriads of their eggs and young ones. Sea birds scarcely have room to sit on their eggs during the great gatherings of the breed- ing season (Fig. 5.1). Populations of animals, except the human ones, seem to stay about the same size, but those that have been carefully observed have proved quite the opposite. The dips and peaks in the populations of one kind of animal also affect others. In Labrador in a recent year the numbers of field mice ran up to a peak and the hawks and snowy owls grew fat; in another year they almost vanished becoming so scarce that the snowy owls flew down to New England for better eating. Animals enter every part of the earth except craters of active volcanoes and places poisoned by civilization. They abound in the damp tropics. Microscopic organisms crowd the surface waters of arctic seas, for cold water holds more oxygen than warm water and food is abundant. On their journey into the Antarctic members of the Robert Scott Expedition found emperor penguins incubating their eggs, holding them on the tops of their feet in the dark of the antarctic winter "with the temperature seventy degrees below frost and the blizzards blowing." The Numbers of Species. The term species is commonly used but difficult to define. Animals of one species resemble one another, interbreed with one another and do not usually interbreed with animals of other such groups. The number of described species is still growing. For birds and mammals it may for the present be nearly complete; for protozoans and insects it is far from that. Frequent estimates suggest that only ten per cent of all insects is yet accurately described. In 1946 the total number of known living species of ani- mals was figured at about one million (Fig. 5.2), Variety and Similarity. Large numbers of animals have basic similarities; they also have many less fundamental differences. Likenesses and differences 67 68 ECOLOGY Part II ->«. — r— v^' » mm ^^^E.f ^^ ^ '* ** '''. 1 ^* ,, ^ - m^^' ^ "_i ■ -a^^5^^^ - n M 4^ -'w ¥ V 1 4 ^;/4 I Fig. 5.1. Abundance. Gannets nesting on ledges of Bonaventure Island, off the coast of the Gaspe Peninsula, on rocks as high as a 20-story building. A gannet is about the size of a duck. (Courtesy, Allan D. Cruickshank, from National Audu- bon Society.) make classification possible. Animals may have two or four or more legs; insects have six; spiders have eight; there are a hundred or more in millipedes. The bones of the arms and legs of a man are arranged like the comparable bones in the legs of a horse (Fig. 9.13), but in other ways the legs are differ- ent. Such structures are correlated with the history of their environment, human arms and legs with ancestors that climbed trees and the horse's legs with ancestors that ranged the swamps and the plains. Various noses are adapted to various functions in addition to smell; an elephant can give itself a shower bath with its nose (Fig. 5.3). Sizes of Animals and the Environment. Animals of a given species vary relatively little in size. Size, proportions, and structure of the body and en- vironment are mutually related. Water lifts and supports weight as air does not; boats can anchor and float but airplanes cannot poise in the air without special devices. Many animals can swim, but few can fly. Aquatic animals are often larger than their terrestrial near-relatives and literally lean on the water Chap. 5 ANIMALS AND THEIR ENVIRONMENTS 69 Fig. 5.2. Diagram showing the approximate number of living species of animals. The grand total is often given as one million. Numbers differ greatly with the methods and time of counts. New species of insects are being discovered even in familiar places; probably only a fraction of all the tropical insects have been de- scribed. (Courtesy, Hunter and Hunter: College Zoology. Philadelphia, W. B. Saunders Co., 1949.) for support. Giant grasshoppers are small compared to the largest lobsters, their marine relatives. Blue whales, the largest living animals, are ten times as long as elephants, more than twenty-five times heavier, and, like large ships, are helpless when stranded (Fig. 5.4), Only the smallest mammals burrow or live in grassy runways. The pigmy shrews are very small, one of them, Microsorex hoyi winnemana, total length with tail, 3.12 inches, is the smallest mammal known in North America. Noc- turnal and mouse-like but more slender it travels comfortably in a runway half an inch wide. 70 ECOLOGY Part II Fig. 5.3. Noses are adapted to many uses in addition to smell and breathing. A ground mole bores its way wedging with its nose and digging with its feet; mice and other rodents use their noses as wedges; anteaters probe into anthills. The noses of elephants are general tools, for shower baths, hfting logs, and picking up nuts; a pig's snout is a living plow. Form, Symmetry and Segmentation. The symmetry of animals is the ar- rangement of structures with respect to a point, a line or a plane. In radial symmetry the structures are placed like the parts of a wheel in relation to its center. In bilateral symmetry the right and left sides correspond to one another. Symmetry is correlated with an animal's way of life, especially its lack of locomotion or kind of locomotion. Hydras, corals, jellyfishes, and others are radially symmetrical. Such animals move about slowly or are attached like the corals. In sea anemones and starfish and their kin bilateral symmetry appears within the radial; that is, the wheel or cylinder shows a division into two parts. This is a persistence of the bilateral symmetry of their free-swim- ming young. The majority of animals, and all the vertebrates, are bilaterally symmetrical (Figs. 5.5, 5.6). They move about freely, often with great speed, Fig. 5.4. The relative size of the blue whale (length, 90 to 100 feet), whale shark, and giant squid. All of them live surrounded by the lifting capacity of buoyant salt water. The ostrich and elephant receive no such support. Chap. 5 ANIMALS AND THEIR ENVIRONMENTS 71 ■^i-^llum SPHERICAL ^-tMi0^ RADIAL ASYMMETRICAL BILATERAL Fig. 5.5. Types of symmetry. Spherical, a protozoan (radiolarian) floats in water that presses against it equally on all sides; radial, a sea anemone, its shape common in animals that are attached for most of their lives; asymmetrical, in a snail that no plane will divide into halves; bilateral, in a salamander, in animals that move about freely, and are mainly symmetrical on each side of a plane extending the length of the body. and the brain and sense organs are always at the end that arrives first. Scarcely any animal is perfectly symmetrical, whatever the type; all tailors know that the human ones are a little one-sided. Segmentation. The bodies of all animals from earthworm to man are segmented, i.e., partitioned into sections that are joined together in a series. The segmentation may be conspicuous inside and out, as it is in the earth- worm; it may be mainly on the outside as in the abdomen of an insect; or prominent in certain structures such as vertebrae and ribs. The arrangement has the advantage of making parts of the body more independent of one another; it is an insurance lessening the disaster of injury to the whole body. If one or more segments are hurt, others can carry on. Segmentation gives flexibility to long slender bodies such as those of worms. It allows great variety by the modification of different segments for different functions, as in a lobster, in which some segments bear swimmerets while others bear mouthparts and eyes. 72 ECOLOGY Part II ORAL lone or section ABORAL RADIAL BILATERAL Fig. 5.6. Axes, planes and regions in animal bodies. Environments Rhythms of Sun and Moon. The lives of all plants and animals are inter- woven with the rhythms that originate outside the earth, their income of energy from the sun, the changes of the tides, and shifts of climate. Patterns of living change from hour to hour as the earth rotates on its axis in its journey round the sun. Evening with its own ways comes to a countryside as it is turned from the sun. If it is New England and early June, the wood thrushes sing through the sunset and afterglow; the whippoorwills begin calling when the hedges are black; the mosquitoes are enlivened by the subdued light and the dampness. From moment to moment animals as well as plants respond punctually and precisely to changes in light and atmosphere. The gravitational attraction between the sun and the earth and the moon and the earth constantly pulls upon these bodies, its strength varying with their respective positions in their orbits. On land its effect is relatively slight but upon the sea it is the basic cause of tides. Sun and moon both take part in the changes of the tides, but the moon, being much nearer the earth, has the stronger influence upon them. With many variations there are in general four tides on every seashore, two high and two low ones in each period of 24 hours. The tide rises and water that has swept the ocean bottom floods over the tide pools bringing additions to the already crowded communities of ani- mals, some of them to eat, others to be eaten. Each little group is continually changed by flooding and ebbing water. Everything that belongs to the sea waits on the tides. Fishermen in harbors put out their seines for the fishes that follow the rising tide. Great ocean steamers wait at their docks until the tide rises. The Sun, a Great Provider. The sun sustains life upon the earth, providing living organisms with heat, light, the energy stored in food, and indirectly with water. The sun is a great furnace of transmuting atoms, extraordinarily differ- ent from the earth yet with a similar chemical content. According to certain theories the earth originated from a torn-out piece of it. It is the source of Chap. 5 ANIMALS AND THEIR ENVIRONMENTS 73 Fig. 5.7. Types of marine plankton, the great population of minute plants and animals that live in the surface of the seas and includes the eggs and developing young of the majority of marine animals. Top, the larva of the porcelain crab like other plankton organisms is translucent and bears outgrowths that serve as floats characteristic of animals of the plankton. (Photograph by D. P. Wilson, Marine Biological Lab., Plymouth, England.) Bottom, the protozoan, Globigerina biilloides. Enormous numbers of these live among the plankton in the surface waters of the sea. Their chalky frames and fine spines dropping through the water for millions of years have formed the globigerina ooze of many parts of the ocean bottom. (After Murray and Hjort. Courtesy, Coker: This Great and Wide Sea. Chapel Hill, N.C., Univ. of N. Carolina Press, 1947.) practically all the energy on earth, excepting atomic energy. It is the prime mover of the winds because it heats different places unevenly and this sets currents of air in motion. As heat it lifts water by evaporation eventually to form clouds and be distributed in rain. With its energy plants make the food for which directly or indirectly all animals including man struggle unceasingly. 74 ECOLOGY Part II Types of Environment. The Land. Terrestrial animals of various groups are described briefly in Part 5. The Sea. The greatest numbers of living organisms in the world are the plankton that live in the surface waters of the sea. They are small, mainly minute and microscopic plants and animals that drift with the currents. No plants are so completely open to the energy of the sun. No mixed population of animals is more uniformly short-lived and prolific. In no other place are there, in season, such multitudes of floating eggs and swimming young (Figs. 5.7, 5.8). The richest population in numbers and kinds of animals visible to the naked eye lives between the tides and near the bottom out to depths of about 400 feet. Hosts of them are attached to rocks and seaweeds; or crawl and burrow on the bottom (Fig. 5.9 and 5.10). Farther from shore are the larger free- swimmers (nekton), the fishes; coastal waters are the main fishing grounds. The deep water of the open sea from the surface well into its depths is the home of the largest fishes, the giant squids, sea turtles, and the mammals, porpoises, dolphins, and great whales. Except for the whalebone whales all M^:: Fig. 5.8. Photograph of marine diatoms. Their beauty and variety are due to their silicious shells. Diatoms of fresh waters are less various but equally beautiful and important in the economy of their environments. (Courtesy, Paul B. Conger, United States Museum, Washington, D.C.) Chap. 5 ANIMALS AND THEIR ENVIRONMENTS 75 Fig. 5.9. Hosts of animals cling to the rocks and seaweeds between the tide lines. Common rock barnacles (Balaniis balanoides) {above), and edible periwinkles {Littorina litorea) {below). Periwinkles are about the size of cherries. In British shore resorts "winkles" are roasted and sold like peanuts in America. (Photograph by D. P. Wilson, Marine Biological Lab., Plymouth, England.) of these live upon one another and the oflspring of one another (Fig. 5.11). Salt water is a far better support and carrier than fresh water. The eggs of marine animals float easily; those of fresh-water animals often drop to the bottom, are attached to vegetation, or carried about by the parent. The young ones climb, creep, and hold onto whatever comes their way. Ponds and Lakes. Healthy ponds and the coves of lakes usually hold goodly populations; in midsummer they teem with them (Figs. 5.12, 5.13). Ponds are smaller than lakes. They are defined as bodies of water so shallow that green plants can grow attached to the bottom even at the center. Lakes are too broad and deep for this. Near the borders of ponds and the protected shores of lakes the plants are food depots and shelters for invertebrates, snails, climbing fingernail clams, innumerable crustaceans, and aquatic insects. There are a few resident vertebrates, chiefly frogs and turtles. The plants have partly or completely submerged stems — blue-blossomed pickerelweeds, arrowheads, rushes, and waterlilies. All of their stems are coated with green algae and bac- teria (Fig. 5.14). Yellow perch, bass, and pickerel come among them to forage. 76 ECOLOGY Part II Fig. 5.10. With every high tide the tide pools and surrounding rocks are flooded with water carrying milHons of little plants and animals that are fit for food. During low tide the pool dwellers are busy consuming the meal. They are attached and slow moving protozoans, bryozoans, barnacles, tunicates, and many mollusks often along with a few crabs, starfishes, brittle stars, and sea urchins. (Courtesy, the American Museum of Natural History.) Chemical Conditions Plants and animals are continually taking materials from their environments and making them into their own bodies. Certain substances and conditions must be present around them. Whether in arctic or tropic regions, in water or on land, these essentials are: sufficient energy from the sun for the plants to synthesize food, enough oxygen for respiration, enough water, the chemical elements which take part in protoplasmic activities, and certain physical con- ditions, such as temperature and pressure. Chap. 5 ANIMALS AND THEIR ENVIRONMENTS 77 Fig. 5.11. Larger free swimmers (nekton) of the open coastal waters. Upper left, dolphins, length up to 12 feet; North Atlantic sea turtle (loggerhead), 100 to 200 pounds. Center, swordfish, 250 to 400 pounds. Bottom, blue-fin tuna (or marlin), up to 600 pounds. Not drawn to scale. Carbon Cycle. Carbon, a main element in protoplasm and its products, is available only in small amounts. Ordinary air contains about 0.035 per cent of carbon dioxide by volume and only a quarter of this is carbon. From this small amount, plants obtain all they use and in turn become the source of carbon for all organisms. The sources of free carbon dioxide are plant and animal respiration, decay of the bodies of plants and animals, and the release from burning oil and coal. From all these sources it is automatically returned to the atmosphere. The only way that it gets back to protoplasm is by green plants. Plants take carbon dioxide {CO 2) from the air and with the help of energy from the sun during photosynthesis, produce the valuable food, carbohydrate. When a carbohydrate unites with oxygen, the energy of action and heat and carbon dioxide are set free, the latter in part a waste product respired into the air. One branch of the cycle is thus complete. In another branch of the circuit, carbon is built into the protoplasm. It is locked within the cells until they die, decompose, and free it into the air to unite with oxygen as carbon dioxide (Fig. 5.15). Oxygen Cycle. Plants and animals take oxygen (Oo) from air or water in 78 ECOLOGY Part II im Ibis ^m \%::v\ am BLUE-GREEN ALGAE DESMIDS DIATOMS Arcella Ceratium PROTOZOANS Cyclops ROTIFER young stage (Nauplius) Bosmina CRUSTACEANS Fig. 5.12. Important groups in fresh-water plankton. Blue-green algae, common in lakes especially in hot weather, sometimes turn color and create "red water"; green algae (desmids) and diatoms, present the year round with spring and other upswings of abundance; protozoans, few; rotifers, many; crustaceans, abundant, creating the basic fish food. respiration. They return it to the atmosphere in combination with carbon as carbon dioxide and with hydrogen as water. In addition green plants release oxygen in photosynthesis. In an aquarium properly arranged for plants and animals, the output of carbon dioxide from respiration and of oxygen from photosynthesis is balanced. Nitrogen Cycle. The great reservoir of nitrogen in the atmosphere (78.03 per cent of volume) is an inactive associate of oxygen and carbon dioxide. The nitrogen dissolved in bodies of water comes mainly from the atmosphere. Its cycle is more complex than that of carbon because living organisms do not release nitrogen in a form that green plants can use. It is released from animals as nitrogenous waste such as urea (CO(NH2)2) and from decaying tissues after death (Fig. 5.15). Saprophytic bacteria attack these and produce ammonia. Other bacteria feed upon the ammonia, combine oxygen with it, derive energy from the oxidation, and produce nitrites (NOo) — upon which they feed. Still other bacteria (Nitrobacter) attack the nitrites and, through anaerobic (without free oxygen) respiration, derive energy from them and Chap. 5 ANIMALS AND THEIR ENVIRONMENTS 79 SR6LASER Fig. 5.13. Stems and leaves of pond lilies are nurseries for hatching eggs and young animals, mainly invertebrates. A, strings of jelly that shelter minute eggs of midges. B, eggs: on the under side of a lily leaf: 1 , snail; 2, water mite; 3, caddis fly; 4, whirligig beetle; 5, beetle (Donacia); 6, beetle, the waterpenny (Psephenus). convert them into nitrates (NO3) — that are taken up by green plants, and finally converted into the amino acids and proteins of green plants. Blue-green algae are now known to fix nitrogen and the process may be even more general than this. Many commercial fertilizers contain nitrates. Nitrogen-fixing bacteria are able to fix free atmospheric nitrogen in nitrog- enous compounds which can be used by green plants. Some of these bacteria live in the soil, estimated at least two billion to a teaspoonful in garden soil; others live in nodules on the roots of clover, peas, and beans. The value of these plants in building up the nitrogen supply in the soil is recognized by farmers who rotate crops of clover with corn in order to supply the soil with nitrogen which corn exhausts. Denitrifying bacteria occur in some soils. These reverse the nitrifying process and reduce nitrites to free nitrogen which is then released into the atmosphere. This is the nitrogen that is compounded with water and brought to the earth in an electrical storm. The bolts of lightning fix the nitrogen as nitrites and nitrates that are brought to the earth by the rain. Mineral Cycles. These include the time in which iron, phosphorus, or other minerals are in the crust of the earth and in the body of a hving organism. Calcium carbonate (CaCO:0 or lime is a good example for it is widely dis- tributed in nature and an important component of bone. The developing embryo of a mouse receives lime from its mother and after birth from its food, notably milk. Lime is maintained in the body of the mouse, chiefly in its bones, as long as it lives. Exactly the same storage of lime occurs in an elephant ex- 80 ECOLOGY Part II :S.P. 6WSER :■•..• ;.'.:;.*.•:■. •.^. .•;■>/ :»/.-. Fig. 5.14. The web of feeding habits among the animals of pond and lake bor- ders: frogs on immature insects, snails, small fishes, crustaceans; pickerel on insects, fishes; turtle on tadpoles, frogs. cept that a larger amount is involved and for a much longer time. Large amounts of lime and other minerals are temporarily stored away in plants and animals. Water Cycle. The internal environment of the body is completely dependent upon the come and go of water. It enters the body bearing traces of iron, iodine, sulfur, or salt from the external environment. It leaves the body carry- ing the wastes of metabolism that are records of protoplasmic activity. Water rises in vapor from the sea and land, floats in the atmosphere as clouds, con- denses, falls as rain, and runs down from the highlands to the sea again. Water is a traveler. Like mineral matter it is taken into plants and animals but it Chap 5 ANIMALS AND THEIR ENVIRONMENTS 81 never remains in them. Whether they are pine trees or cattle, living organisms take in relatively large amounts of water that gradually filters completely through their bodies. Physical Environment The chief physical influences upon plants and animals are gravity, pressure, temperature, and light. Gravity. Its weight, actually the earth's pull, greatly affects an animal. The bridge-type of four-legged animal is a four-cornered support of the body against the pull of the earth (Fig. 9.11). Birds are the master adjusters to the force of gravity. No other animals approach them in lightness and strength, due to the air-filled outpocketings of their lungs that extend into the bones, their rapid elimination of waste products, and the lightness of feathers (Chap. 36). Pressure. The medium in which animals live presses upon them continually from every point, upon their forms, actions, and the amount of gases which they hold. The atmosphere of the earth is like a haystack (Fig. 5.16). At the bottom or sea level its content is closely packed; the atoms of oxygen are near to- gether. At sea level an animal, like every other object, carries 14.7 pounds of atmospheric pressure on each square inch of the surface of its body, and this pressure so evenly permeates its body that none is felt. At 20,000 feet (300 feet lower than Mt. McKinley) the same animal would be exposed to pressure less than half that of sea level. In spite of their high oxygen demand in breath- ing, birds fly through air of low oxygen content probably securing an adequate supply because of the speed with which they drive into it. At 18,000 feet mules in South America carry riders without great difficulty, and this is said to be due to their frequent stops during which oxygen accumulates in their blood. Anyone acquainted with them knows that mules have the same sagacity at sea level where they also make frequent stops. Water is about 775 times more dense than air and consequently heavier. It is peculiar in that it becomes denser and heavier as it cools to a temperature of 39.2° F. (4° C). When colder than that it is less dense and lighter, finally floating as ice. Because of this the pond is covered with a blanket of ice below which fishes can disport themselves in safety (Fig. 5.17). The pressure upon an animal in water is the weight of a column of water extending above a given area of its body plus the atmospheric pressure above. The pressure on a fish in Lake Tahoe in California, over 6,000 feet above sea level, is far less than that on a codfish in the Atlantic Ocean. At great depths of the ocean the pressure is several tons per square inch. It does not crush the animal because the fluids in its body are under the same pressure as the water surrounding it. Pressure compresses gas which expands when deep-sea fish are 82 COz ECOLOGY Atmosphere Part II CO2 CO2 CO2 f CO2 Photosynthesis C O2 Bacterial action Decomposition Respiration of plants \ Bacterial action Decomposition /Respii of an ration imals CO2 Air, water, rocks, soil CARBON CYCLE Free nitrogen Ng is mode available to plants and animals ifixed) by certain bacteria. Also fixed by lightning and washed to earth. N2 fixing soil bacteria The processes from free nitrogen to protein are carried on mainly in the ground NITROGEN CYCLE Chap. 5 ANIMALS AND THEIR ENVIRONMENTS 83 60,000 Ft THE ATMOSPHERIC HAYSTACK Fig. 5.16. Atmospheric pressure illustrated by stacked hay showing the weight it would carry at various heights to 60,000 feet. The proportions of the gases in the atmosphere do not change at different heights but their total amount does. This is why the air is thin in high places. brought to the surface, just as gas expands when a bottle of compressed fluid pops. When deep-sea divers rise to the surface rapidly the pressure on the nitrogen in the blood is released too quickly; it gathers in bubbles in their muscles and joints producing a condition known as the bends (Fig. 5.18). Temperature. Except for those that live in hot springs, plants and animals can live only within a narrow range of temperature and can endure relatively low temperatures better than high ones. Many tropical animals cannot bear extreme exposure to the sun's heat. In zoological gardens ostriches, croco- diles, and snakes have often been killed by heat. Birds have the advantage in their cooling devices of air sacs and mammals of panting and sweating. Wherever there are severe winters, animals resort to various ways of avoid- ing or meeting them. Birds go to warmer regions or remain in the cold and depend on heavy feeding to keep up their metabolism; many mammals, rab- bits, foxes, and others are active but must have abundant food; other mam- mals hibernate, put on layers of fat in the fall, and live at a kind of physio- FiG. 5.15. Chemical cycles. The carbon cycle. Respiration of plants and animals returns most of the carbon to the air as carbon dioxide. The storage of carbon in coal and oil is an important exception to the general rule that the carbon used by the green plant in photosynthesis returns to the air. Coal is largely carbon derived from the cellulose of the trees about 250 or more million years ago. Carbon is also captured in the calcium carbonate (CaCO^) of clams, crabs and others. The nitrogen cycle is much more complex than that of carbon. The main reason is that many organisms do not release nitrogen in a form that can be immediately used by green plants. They can use it when it appears as certain inorganic salts, particularly nitrates. 84 ECOLOGY Part II MIDSUMMER TEMPERATURE A TYPICAL DEEP LAKE OF A TEMPERATE ZONE Water surface (Ti.e'F) 2 re (69.8''F) io'c (SCF) Epilimnion 29 Ft. Wind-stirred, air- mixed water. Plenty of light. Abundant plankton Thermocline Transition area, 65% fall in temperature here. 45 Ft. Hypolimnion S-S^C .1 128 Ft. Bottom Still water. Little or no light. Maximum range of temperature for year about 40° F. ■ 1 1MB "■•'ill" r--' " ■•''■'•■■•■■•''■■•'-•-■-'••'■• •.'■■.:■••■••.••• •:■ i: :■.':■■■. .•...••- ■ . ■■. ■■ .. .-. t.- :•■■.:.■■ ■ '■ ■-•■. B V -*^ -«— ■ ;|kN. Epilimnion :| Thermocline :'\ Hypolimnion 69.0°F SO.O'F 41. CF SUMMER Layers as in A Wind blows surface waters. Temperature shift in thermocline. v/yy/y/A .ce ^//////////a 32.2" f 37.2° F 39.2° F TrWTTy^TTr' 39. 2 F WINTER Ice cover is a boon to population beneath it. Plankton sinks with heavy water. 39.2° F •::':y^^T?.Ui.i.i!.t|ii|giii 39.2° F 39. 2° F AUTUMN OVERTURN Cold winds blow and chill surface waters. Their temperature changes fo 39.2° F. They fall and mix. " ^ if ' "' ic ' '' '' ^j/^ 39. 2° F 39. 2° F 39. 2° F 39. 2° F SPRING OVERTURN Ice melts. Surface water changes to 39.2°F, IS heavier and falls, mixes ond displaces lower waters. Stir brings plankton to surface. Chap. 5 ANIMALS AND THEIR ENVIRONMENTS 85 logical low gear for which little or no food is needed. In winter the water is warmer than the air; frogs stay in muddy pond bottoms but do not drown because they take in enough, oxygen through the skin for their lowered metab- FiG. 5.18. Bubbles of nitrogen gas (black) collect at the joints when a person, e.g., a deep- sea diver, rises suddenly into greatly lowered pressure. olism. Insects go through a special cold-hardening, partly by loss of water and the production of bound water which does not freeze except in extraordinarily low temperatures. Earthworms burrow below the frost line and gather in clus- ters conserving heat and moisture. Lady beetles spend the winter in companies Fig. 5.17. A, midsummer temperature of a lake. Water contracts with cooling and becomes heavier but only to 4° C. (39.2° P.). When warmer or colder than this it becomes lighter. Water takes its place in layers according to its weight which is dependent on temperature. B, sections of a lake showing the seasonal changes in temperature. Summer. The light is stronger but the diatoms decrease probably because of inadequate nourishment and perhaps of silica since the thermocline seems to bar the way to chemical substances that might otherwise well up from the bottom. Autumn. With the mixing of the water and disappearance of the thermocline there is an upward diffusion of nutrient salts. Another increase of diatoms occurs, not so great as in spring since the sunlight is weaker. Winter. The lake is covered with ice which is water at its lightest and coldest. Spring. Light increases and with it an increase of diatoms called the spring pulse, of great importance in the food supply of all young animals. 86 ECOLOGY Part II Fig. 5.19. Social hibernation of ladybird beetles. With the first frosts the beetles fly to the ground and then to trees searching for holes in which they gather by hundreds. Animals that are solitary in summer may be social in winter. (Photo- graph by Carl Welty.) though they are solitary at other seasons (Fig. 5.19). Cold as well as sex encourages sociability. Light. Light is necessary for vision but there are other ways in which it con- cerns animals. Like plants they are deeply affected by longer or shorter days. This shows in their breeding seasons, in the migrations and seasonal changes of color in birds, and in the color changes of snowshoe rabbits, and other northern animals. In general, animals are responsive to light whether they have light-perceptive organs or not, but lenses are present even in certain proto- zoans. The majority of higher animals probably find their way chiefly by vision, but by no means entirely. The amount of Hght that enters water depends upon the direction of the rays, which differs with the time of day and year, the amount and clearness of the water through which the rays pass, and the intensity of the light. In rela- tively clear water, one-third of the light is generally lost in about three feet and three-quarters of it in 16 feet. At depths of 2,000 feet or more the ocean is completely dark except for the luminescent animals, mainly fishes. Biological Environment The neighboring plants and animals compose an organism's biological envi- ronment. Whether the organism is a crocus in a mountain meadow, a parasite Chap. 5 ANIMALS AND THEIR ENVIRONMENTS 87 in human blood, a squash bug on the vine, or a citizen in the town, it is con- cerned with a biological environment, human or otherwise. The animals of an environment are roughly divided into producers of food and competitors in the consumption of food. Some of the consumers are predators that rob and kill. Search for Food. Numerous and widely distributed animals are apt to live on common foods. Rodents — squirrels, field mice, and rabbits — all abound in great numbers; so do the shrubs, grasses, and clover which they eat. Grass- hoppers and crickets live surrounded by grass and grain. At the height of their season the only grass-eaters that compete with them in open fields are cattle and sheep. During the great migrations of grasshoppers nothing stands in their way (Chap. 30). Birds, small mammals — shrews, ground moles, and chipmunks — commonly prey upon them. But their reproductive capacity is so high that these predators do them the good turn of keeping the population to a size which the space and food can support. Animals multiply greatly in regions where they have few or no competitors for the particular food on which they live. This is strikingly true of penguins in the Antarctic. The same principle applies to nocturnal animals such as owls and skunks that hunt by night when there is less competition. Biological environments obviously depend on the chemical and physical ones. Plant populations rely particularly upon water and temperature and animals follow the plants. Animals abound at river mouths to which the river brings rich organic deposits. Rivers and their valleys have always determined the location of animals just as they have always determined locations for man- kind. Size of Food. Man is the only animal that can catch all sizes of animals, from frogs to cattle, oysters to whales, and use them for food. He can eat small, large, and medium-sized animals indiscriminately: an important control to have over the environment. The scavengers — vultures (turkey buzzards), lobsters, pigs, and chickens — approach mankind in the variety and sizes of food which they appropriate. With the exception of parasites and scavengers, other meat-eaters must deal with food that is adequate but not too large to be manipulated. Fierceness and skill may take the place of size in capturing prey, so may social behavior. Packs of wolves will attack a moose but a solitary wolf seldom does so. Millions of South American army ants will set upon and kill small mammals but no one of them could do it alone. Food Relations. The food relations of a community are exceedingly com- plex, changeful, and affected by factors in the immediate environment as well as others far outside it. The complexity of the human food market is an exam- ple with its many and remote causes of undersupply and oversupply and resulting prices. The food relations between animals are expressed as food chains, food webs, and pyramids of numbers (Fig. 5.20, 5.21 ). A food web is 88 ECOLOGY Part II dead animals Food- web on Bear Island in the Arctic zone. (Sinriplified from Elton.) The arrows are read as "eaten by/ e.g./bacteria — ► protozoa" means bacteria are eaten by protozoa. Fig. 5.20. In food webs the successive eaters are usually larger, e.g., insect, ptarmigan, fox, but fierceness, cunning or group action may take the place of size, e.g., in army ants, wolves, and wild dogs. (Reprinted from Readings In Ecology by Ralph Buchsbaum, by permission of The University of Chicago Press. Copy- right 1937.) literally what eats which in a community of animals or of animals and plants. Plant-eating animals are the basis of any community; they serve as food for the small carnivores which are in turn eaten by the larger ones. Such a series of food links is a food chain. In a pond bacteria and unicellular plants are the Chap. 5 ANIMALS AND THEIR ENVIRONMENTS 89 basic supply. Beginning with them, smaller animals are eaten by larger ones, protozoans by minute crustaceans and the fry of fishes, and these by aquatic insects and so on to the large, fishes and turtles. If they die in the lake their bodies are returned to the bacteria; if they are caught and taken elsewhere they may become part of another food chain. In any long food chain, the successive eaters are not only larger in size but fewer in number. There are few sparrow hawks compared to the number of sparrows, few owls to the number of field mice, one fox to dozens of rabbits. In communities of animals there are many more small adults than there are large ones (Fig. 5.21). What seems obvious is borne out, in broad fines, by analyzing a definite area of a community, counting the animals of various sizes and measuring the totals by bulk or weight. The result is a pyramid of num- bers. Such a pyramid applies particularly to predatory animals. It shows that smaller animals have a higher reproductive capacity than large ones and are Hawk A A Fish CarnivA /warblers orous \ /Thrushes Beetles \ / + + -t- + + + + +\ / Spiders \ / Carnivorous Daphnia \ / Beetles Cyclops \ / + + + + + + + + 4- + 4- + + + + \ / Aphids Protozoans \ / ++++++++++ + + + + + + + + + + + + + + + \ Open woods MILLIONS OF INSECTS TO ONE HAWK Pond BILLIONS OF PROTOZOANS TO ONE FISH Fig. 5.21. A pyramid of free living animals in one area. Plus signs express abundance of types of animals. The smallest ones are most abundant. They supply food to carnivores that are larger in size and fewer in number and these in turn supply other carnivores that are still larger and fewer. 90 ECOLOGY Part II generally the prey of larger ones, that there are great numbers of small animals and relatively few large ones. This food situation is very complex. It clearly involves sizes of food; it also includes feeding equipment such as cilia, teeth, and claws, all sorts of locomotion, and kinds and extent of territory covered in hunting food, as well as shifts in population due to cataclysms from the action of weather and humanity. The food relations of animals, actually the connections between the soil and the beefsteak, are exceedingly important to human economy. Protecrive Resemblance and Mimicry. Protective resemblances are charac- teristics that seem to make life safer for animals in their own environments. Such protection is a debatable subject which has much to be shown for it and considerable against it. It is a pattern of colors that makes an animal unrecog- nizable against its home background. A brown streaked sparrow is lost among the twigs of a brush pile; katydids are as green as the leaves beneath them; ground squirrels (gophers) and prairie chickens are streaked Hke prairie grass; fishes that swim in and out between bright-colored corals are also brightly colored. Polar bears are white. Snowshoe rabbits and weasels (white phase is ermine) are brown during the short northern summer and white in winter. There are vast numbers of animals whose coloration does conceal; there are also many in which it does not. There are animals whose coloration seems to have no significance in their survival. Throughout the Arctic there are two color phases of arctic foxes, one of them is brown in summer and white in winter; the other is grey or black in summer and blue or black in winter. Both the blue and white phases interbreed and are common and successful in the same areas of Greenland and Alaska. Camouflage is the painting or screening of boats, buildings, other objects, or persons so that they are lost to view in the background. It was first widely used in World War I. Its principles were based upon those of protective coloration suggested by a British zoologist, E. B. Poulton, and later developed by an American artist, G. H. Thayer, and published in his finely illustrated book. Concealing Coloration in the Animal Kingdom. The first of the princi- ples is counter-shading, a generalization of the fact that in the great majority of animals the back is dark and the underparts are pale. By painted models Thayer showed that any object so colored is less conspicuous on being strongly lighted from above and with dark reflection from below. Another principle is related to the break-up of a familiar form such as that of a dark-colored bird whose head is separated from its body by a white ring around the neck. Colors of animals are often strikingly different in the two sexes, the males usually the more brightly colored, especially in birds, fishes, and insects. Sexual coloration is often associated with endocrine secretions and is men- tioned further in connection with them (Chap. 15). 6 Mutual Relations nips or Animals Whirligig beetles spin and turn in companies on the pond surface; a hundred starlings swing into a treetop; swarms of gnats rise and fall in quiet air; men and women join in a folk dance. These are all social beings, those of each group sharing particular surroundings. Animals express their sociability by being in the same place at the same time. Two kinds of behavior, competition and natural cooperation, are character- istic of sociability. Competition and Cooperation Competition occurs when there is a common demand on a limited supply. A certain amount of it is stimulating and healthy. An unlimited competition is dangerous to individuals and communities. Its basic cause is the overpro- duction of animals, human or otherwise. During the spring breeding season many small ponds are populated with toads and each female lays about 15,000 eggs in a clutch. Presently the water swarms with toad tadpoles. All these tad- poles have insistent appetites for the algae of the green pond scum that over- spreads the water. At the start there is an abundance of algae as well as tad- poles but it thins out as the eating goes on. Then competition begins. Some of the tadpoles manage to get food, but many of them starve. If they were fighting animals, there would be conflicts along with the starvation. In all communities plants and animals compete for such essentials as earth, water, food, warmth, and light as well as for less necessary things. Competition is commonly accom- panied by a struggle for power and dominance usually gained by one or a few individuals. Competition is usually keenest between those of the same species since they have the same wants; two rabbits go for clover, but a sheep eats grass and a cat eats birds. The overpopulation, sparsity of food, and starvation of individ- uals that occur in nonhuman animals have been matched in human ones throughout history. Competition is reduced by differences of diet: among 91 92 ECOLOGY Part II *^j>.~'V.*«««» jS^ Fig. 6.1. The overpopulation of rabbits in Australia, too many for the space and food available, a prime cause of competition and ultimate destruction. This tele- photo lens picture shows how rabbits denude the pastures and drink the water holes dry. (Courtesy, Australian News and Information Bureau, New York.) birds, as in seed-eaters and insect-eaters, among larvae of insects, e.g., tomato worms and cabbage worms. The rabbits of Australia, a country almost without predators, have repeatedly overpopulated the land, devastated vegetation, and brought themselves to starvation (Fig. 6.1). Cooperation, conscious or unconscious, is the behavior of plants or animals which benefits the lives of those about them. Animals may produce a flourish- ing population beneficial to all concerned. They easily pass this point however, by multiplying to such an extent that they are hungry and sick for want of food and space. Thus their cooperation may be turned to disoperation. Exam- ples of cooperation are plentiful. In winter bees crowd together in clusters within the hive and thus conserve the heat in their bodies. Northern musk oxen stand close together, heads down, against attacking wolves; geese band to- gether with outstretched necks to hiss their disturber. People join in applause by clapping their hands together; tent caterpillars join in making their web and mending it when it is torn; beavers work together on their winter lodge and their food stores (Fig. 6.3). Competition and cooperation are fundamental biological principles. Com- petition has long been recognized as such, especially since Darwin based his Theory of Natural Selection upon it. Although the importance of cooperation had been suggested by certain European workers, its prevalence and the J Chap. 6 MUTUAL RELATIONSHIPS OF ANIMALS 93 Fig. 6.2. Cooperation. Tent caterpillars and their community web. The young caterpillars spin a dragline of silk from the time they hatch. After a few days of feeding and trial spinning they begin to work together constructing the nest, at first a small night tent, then a larger one a foot and a half or more long. They leave the tent in the day time and creep in single file to a feeding place leaving a trail of silk behind them. (Photograph by Lynwood Chace. Courtesy, National Audubon Society.) soundness of the principle have been demonstrated in recent years by the observation, experiments, and conclusions of the eminent American ecologist, W. C. Allee and his co-workers (Suggested Reading, Chap. 6). Varieties of Partnerships Partnerships may occur between plants, between animals, or between plants and animals. Symbiosis. Living together is known as symbiosis. This is a general term that includes all aspects of physiological and ecological association (Fig. 6.4). It is often difficult to determine the exact nature of the relation between two organisms that live together, whether it is a neutral aflfair or an advantage to both partners. In either case, symbiosis would describe it. Commensalism, mutualism, and parasitism are types of symbiosis. 94 ECOLOGY Part II Fig. 6.3. Cooperation. Beavers' lodge and winter food storage — a community project. The lodge and passageway to the pond bottom are shown as if cut open and the ice bound pond as if in section. The two beavers working below the water line must frequently come up for air. (Courtesy, Hamilton: American Mammals. New York, McGraw-Hill Book Co., Inc., 1939.) CoMMENSALiSM. Meaning at the same table, commensalism was originally applied only to sharing the same food. It is now used for neutral associations which do not seem to affect either partner. A classic example is the sea anemone that rides about on the shell of the hermit crab and thereby gains wider range for forage, but does not eat the same kind of food as its host. Less familiar is the mahout beetle that rides on the head of a worker termite and takes bits of food as it is passed from one termite to another (Fig. 6.4). Mutualism. A symbiosis that benefits each partner is mutualism. Honey- bees and many flowering plants aid one another to the point of dependence. Honeybees eat nothing but flower products. And as they collect the nectar and pollen they distribute the latter, usually to flowers of the same kind be- cause they grow together. Thus the bees cross-pollinate them. Many flowers are so formed that they can be pollinated only by insects. In nature the yucca lily (Spanish bayonet) and the yucca moth (Pronuba) are entirely dependent upon each other (Fig. 6.5). The lily is pollinated by the moth, which thrusts a blob of pollen onto the pistil. Thus she effects the fertilization of the ovules and then lays her eggs in the ovary where the larvae can feed on the ovules. The plant does not suffer, for more seeds develop than are eaten by the larvae of the Pronuba. Yucca lilies are native to southern North America but are cultivated farther north, since they are easily pollinated by hand. Chap. 6 MUTUAL RELATIONSHIPS OF ANIMALS 95 Fig. 6.4. A minute beetle, Termitonicus mahout, that rides on the heads of the workers of the termite, Velocitermes beebei, and takes bits of the food as it is passed from one worker to another. An example of symbiosis, a general term that includes a variety of partnerships. (Redrawn after Allee et ah: Principles of Ani- mal Ecology. Philadelphia, W. B. Sanders Co., 1949.) One of the most remarkable examples of mutualism is that between wood- eating termites and certain species of protozoans. The protozoans live pro- tected within the intestines of the termites and in turn actually digest their food for them. Bits of the cellulose food are taken in by the protozoans and changed to sugar (dextrose) which is squeezed back into the intestine and absorbed by the tissues of the termite. Experiments have shown that termites cannot sur- vive long without the protozoans unless they are given a diet other than cellu- lose. On the basis of the evolutionary history of termites it is estimated that these intestinal intimacies have existed for 150 million to 250 million years. Parasitism. Another form of symbiosis in which an organism lives on or in and at the expense of a larger plant or animal, called the host, is parasitism. The parasitic mistletoe grows on a tree, commonly an oak. Animal parasites are always small in comparison with their host and usually numerous. The parasite obtains food, protection, or transport from its host, often all three of these. Parasitic animals are discussed in the chapters dealing with the groups to which they belong. These are especially: Chapters 21, Protozoa (sporozoans, e.g., malaria); 25, Flatworms (tapeworms, et al.); 26, Roundworms (trichi- nae, hookworms, et al.); 28, Annelids (leeches, et al). The relationship of parasitism costs the host its substance and the parasite its independence. People who must have special food are restricted in their travels; so are fleas and bedbugs. The Host, a Living Habitat. Plants and animals have three major dwelling places: terrestrial — on or in the earth's crust, aquatic — in fresh or salt water, and on or in living organisms. Parasites occupy living habitats. In them there are special places in which various parasites thrive, such as the skin or the liver, just as different seashore animals thrive in tide pools or in mucky sand. Living habitats offer ready food and protection, within limits. Parasites must 96 ECOLOGY Part II anther Yucca flower natural size Sickle shaped jaw, a pollen collector Moth gathering pollen from anther Yucca lily MOTH AND LILY, MUTUAL BENEFACTORS Fig. 6.5. Mutualism, a partnership that benefits each member. The yucca lily, Yucca filamentosa, whose stalks of white flowers grow four to six feet high in the eastern and much higher in the western United States. When the female moth visits a flower she thrusts her long ovipositor into the ovary and deposits an egg beside each of the several ovules (eggs). Then she climbs to the tip of the pistil and carrying pollen that she has collected from some other flower she pushes it into the stalk incidentally making it possible for the transported male cells to fertilize the ovules of the flower she is visiting. After fertilization the ovules develop into seeds; some of them are eaten by the larvae of the moth but others that are untouched propagate the plant. hold their places, often against pressure, lack of oxygen, and the defenses of their host. If parasites of digestive tracts did not have a protective immunity to digestive fluids they themselves would be digested. Parasites must reproduce and be distributed in such a way that the young ones can enter into new hosts of the right type and at the right time. Trichinae, the minute worms resting in the pig's muscle, must arrive still alive in a human stomach by way of a sand- wich or a sausage. It is a great gamble, but not a rare feat for trichinae in the United States (Fig. 6.7 and 26.5). Development of a Food Habit. Parasitism is primarily a food habit and parasites are mainly chronic predators. Typical free-living predators are larger than their prey, kill it quickly, and devour it soon. A cat pounces upon a mouse, and if hungry, kills and eats it at once. Cats, foxes, and hawks are Chap. 6 MUTUAL RELATIONSHIPS OF ANIMALS 97 typical predators. Parasites are smaller than their host, feed upon its substance persistently, and chronically weaken or gradually kill it. A field mouse can supply blood to a moderate -population of lice without great injury. But an excessively large population results in great competition among the lice and the death of the mouse from loss of blood. Like tax collectors after more in- come the lice must then find another mouse. Development of Parasitic Living. In the early stages of parasitism the in- cipient parasites visit their hosts only for meals. Blood-sucking leeches clamp their suckers to the flesh, insert their jaws, suck blood until they are satiated, and then drop off into the water. Such a meal supplies a leech with food for several weeks. The blood-sucking mosquitoes, always female, spend even less time on their hosts and simply take a firm stand on the skin while they suck up the blood (Chap. 30). In certain species mosquitoes do not lay their eggs until after they have had a blood meal. In laboratories where they are reared they are allowed to bite a human victim whenever eggs are needed for experiment. Such mosquitoes have taken a long step into parasitism -is. ^ LARVA ^.j ^/ (CHieeiR) EGG o ADULT Fig. 6.6. Ectoparasites; examples of parasitic life on the external surface of the body: fleas, lice, chiggers. Left, the common rat flea, Nosopsyllus fasciatus (after Bishopp) : upper, the nonparasitic larva and pupa that live near the host, not upon it; lower, the blood-sucking parasitic adult (female) that stays much of the time feeding on its host, slips easily between the hairs, has great ability to spring on and off its host, and is able to adopt a human one temporarily. Center, common chig- ger, or jigger mite, harvest mite, Entrobicula alfredugesi. The exceedingly minute six-legged parasitic larva that bores into the skin, liquefies the local tissue and sucks up the fluid. After feeding the mite is no longer parasitic but drops to the ground and develops the free living eight-legged stage, Chiggers are distributed from New York to Minnesota and are pests in the southern states attacking all land vertebrates including man. Right, human head louse, Pediculus humaniis. var. capitis. Adult showing the claw and thumb that lock around the hairs. Lice are highly adapted for clinging and blood sucking and do both throughout their life history. {Left, courtesy, Matheson: Medical Entomology. Ithaca, N.Y., Comstock Publishing Co., 1950. Center, courtesy, Stiles: Individual and Community Health. New York, The Blakiston Company, 1953. Right, courtesy, Herrick: Household Insects. New York, The Macmillan Co., 1916.) 98 ECOLOGY Part II in being so seriously dependent upon the special diet of warm blood that the species will die without it. Male mosquitoes do not show any such trend to- ward the habit; they still drink fruit juices. Fleas and sucking lice represent steps in increasing parasitism in the persistence with which they stay on their host. Fleas stay on a dog most of the time; they also frequently jump off. Lice stay on except by accident. Their claws lock onto the hairs of the mouse or other host and they cling fast as fleas never do. Chiggers go still further. They are the parasitic larvae of certain kinds of mites that actually burrow into the skin (Fig. 6.6). The parasites so far mentioned are a few of the great host of ectoparasites that attack the outsides of animals and represent the earlier stages of para- sitism. Endoparasites spend most of their lives inside the bodies of animals and represent the extremes of adjustments to parasitic living (Fig. 6.7). The easiest way for an endoparasite to enter an animal is by way of the mouth along with food or drink. Other possible entrances are into the breathing organs, the excretory ones, the reproductive organs, and through the skin. Life Histories. Whatever their habit, animals go through various phases during their life spans. The embryo of any animal is very different from the adult; young animals may live in one environment and later move to a very different one. Parasites often change from one host to another while in their egg or larval phase of life. This is especially difficult for endoparasites which have to take advantage of the habits as well as the structure of their second hosts in order to enter them. A parasitic animal may pass directly from one host to another of the same Fig. 6.7. Endoparasites; phases in the life of two endoparasites in which parasit- ism is highly developed. Left, trichina worms: Trichinella spirella, coiled and dormant among muscle cells, an example of the phase of waiting, characteristic of many endoparasites. Right, trypanosomes: Trypanosoma gambiense, a proto- zoan blood parasite. (Fig. 21.10, trypanosomes and West African sleeping sick- ness.) They reproduce in enormous numbers in the blood of man and in the big game of Africa and are transmitted by the tsetse fly. The multiplicity of their populations and dependence upon a second transmitting host are characteristic of many endoparasites. (Courtesy, General Biological Supply House, Inc., Chi- cago, 111.) Chap. 6 MUTUAL RELATIONSHIPS OF ANIMALS 99 species, in which case it has a direct life history. Such parasites may live through their entire lives in one host, producing eggs and larvae which in turn live and reproduce in the sarne place. Many of them are usually carried out of the body with waste from the intestine. They then await the chance of getting into the mouth of another individual; this is the usual history of pinworms {Enterobius vermicularis) , common parasites of children (Chap. 26). In con- trast to such direct life cycles are the indirect ones of parasites with hosts that belong to two or more different species. Larvae of these parasites develop to a certain stage in one host, such as a sheep. But they cannot develop further unless they are cast out of the sheep's intestine at the edge of a pond where they can enter certain pond snails, their intermediate hosts. In the snails they develop to a particular stage in which they leave the snails, swim about in the water, and finally onto the wet grass around it. In this stage and in no other are they able to infect another sheep when swallowed (Fig. 25.11). These para- sites, called flukes, prove that gambling is a very ancient and enduring practice. Certain important variations apply to both direct and indirect life histories of parasites. Some species with direct life histories can live parasitically in several related animals, such as sheep, cattle, and others that chew the cud; while other species, such as the human hookworm, can live only in one type of host. Parasites with indirect life histories spend part of their lives in an intermediate host before they can pass to the definitive host in which they re- produce. An example of indirect life history is that of the liver fluke of sheep; the intermediate host is a snail in which the parasite is immature; the definitive hosts are sheep in which the flukes reproduce. Effects of Parasitic Life on Parasites. Parasitic animals have to contend with many difficulties and risks. Such gamblers stake their lives on finding their hosts and maintaining themselves upon or within them. They accomplish this by an enormous production of sex cells, by the development, in many species, of male and female organs in the same individual, making fertilization of the eggs more certain, and by parthenogenesis, the production of young from un- fertilized eggs. It has been calculated that the beef tapeworm of man produces between 50 and 150 millions of eggs a year. American hookworms probably release about 6 to 20 thousand eggs per day. Numbers are also increased by asexual reproduction. In certain parasitic wasps one egg divides so as to produce several embryos. The single-celled malarial parasite produces many new in- dividuals by division. It has been estimated that these parasites (Plasmodium vivax) can produce about 40 thousand parasites to every cubic millimeter of the host's blood. Eventually parasites kill their host and destroy their own welfare by overpopulation, just as too many gasoline stations kill a business. Some Important Parasites of Man. Parasites occur in all the main groups (phyla) of animals. Parasitic members of the phylum Chordata are extremely rare, e.g., hagfishes and a few blood-sucking bats. Of the invertebrates, the 100 ECOLOGY Part II protozoans, roundworms, and flatworms are deeply committed to parasitism. Among parasitic arthropods the insects are best known, such as fleas, lice, bed- bugs. They are transmitters of disease-producing parasites and are themselves in the earlier stages of parashism. The life cycles of various parasites are described and figured in Part IV with the groups to which they belong. The accompanying list shows the occurrence of some important animal parasites of man (Table 6.1). Table 6.1 Some Important Parasites of Man Parasite Means of Disease in Human Host Transmission or Other Mammal Spirochaetes Ticks Tick-borne relapsing fever Spirochaetes Bodily contact Syphilis Protozoans Eiidamoeba histolytica Water Amebic dysentery {Chapter 21 ) Plasmodium Anopheline Malaria (various species) mosquitoes (female) (various types) Trypanosoma gambiense Tsetse fly (Glossina) African sleeping sickness Schistosoma mansoni Water, snails Bilharzia in about 50 et al. per cent of popula- Blood flukes tion of Egypt, also in Flatworms other tropical coun- tries Inhabits intestine, {Chapter 25) Taenia saginata Cattle, Beef tapeworm beef muscle muscles Taenia solium Pork muscle Inhabits intestine. Pork tapeworm muscles A scaris Soil, food, clothing Ascariasis A scar is lumbricoides Enterobius vermicularis Clothing Enterobiasis Pinworms Necator americanus Water, soil Hookworm disease Roundworms {Chapter 26) American hookworms Wuchereria Mosquitoes Filariasis (several genera) Trichinella spiralis Hogs, rats, et al. Trichinosis Trichinae (in pork, sausage, etc.) Animal Communities Organization of Groups. Aggregation is a general term for a group of organisms of the same or different species, associated but not organized into societies. Many of them are examples of natural cooperation and as such were cited at the beginning of this chapter in connection with cooperation. Un- organized groups like these were doubtless the beginnings of complex societies such as those of ants and termites. Animals congregate because their environ- Chap. 6 MUTUAL RELATIONSHIPS OF ANIMALS 101 t I ^ ^ - V 1-^ > • ^^H t-^ : •~>.' -ft. IT* EmL' k..^' ^-^El^ . 'mt .^<« *; C'' *^^--'-A^'- . .Ifca.^ . -t. . .. •t" •^ Fig. 6.8. Hundreds of white pelicans rising and thousands still to rise from a preserve on Lake Washington, State of Washington. A typical aggregation of ani- mals associated because something in the environment has beckoned them. (Photo- graph by Hugh M. Halliday. Courtesy, National Audubon Society.) ment drives or beckons: the cold of winter, the heat of summer, the dark that starts the crows crowding into their roosts, the low tide that leaves new forage for the gulls, the lakes kept as safe stopping places for migrating waterfowl (Fig. 6.8). Animals are brought together by accident; starfishes, snails, and others thrown on the beach enmeshed in seaweed. The spring gatherings of frogs and toads and the shoals of spawning fish are aggregations stimulated by climatic conditions and breeding habits. Social Organization of Animals. Among invertebrates social organization 102 ECOLOGY Part II reaches its highest development in the insects — termites, wasps, bees, and ants. Their organization has a complexity comparable to that attained by vertebrates but of an entirely dift'erent character. It is a strictly defined and inflexible division of labor in which the various needs of the community are attended to by individuals whose structures and functions mark them, with rare exceptions, inescapably as members of particular castes with special work to do. Among bees such specialists are the queen, the workers always females, and the male drones (Chap. 30). Organization of Vertebrate Groups. This is based upon three general prin- ciples: the holding of territory, social hierarchies in which dominance and power exist in a graded order from highest to lowest, and leadership-follower- ship. Territorial Rights. Birds take possession of a parcel of good habitat, sing loudly from a prominent perch and defend it against trespass, driving off members of their own or other species. American song sparrows sing special proclamations of their ownership of territory and defend the mating and nest- ing grounds by fighting. The willow wrens that migrate into England every spring have a regular system of dividing up their usual territory into roughly equal parts, and the males fight among themselves for their respective rights. Social Hierarchies. Groups in which one individual dominates all the others have been observed in birds, rats, cats, dogs, apes, and human groups. A dominance known as peck right, observed in small flocks of domestic hens, has been investigated mainly by Alice and his co-workers. In these flocks one particular hen pecks any other hen without being pecked in return, that is, she is dominant with peck right over the whole flock. Below her a small group of hens peck those of lower social levels than themselves without receiving return pecks. Below them again, similar levels occur down to the lowest level, the hen which is pecked by all others yet does not peck back. During observations each hen was tagged for identification by colored leg bands and other mark- ings. Observations were taken with great care and repeated many times. The dominance of a hen was generally first established by fights. Ailing hens and those newly installed were in the low levels, and regular members which were taken from the flock lost their positions by being absent. Similar social hier- archies or grades of power exist among flocks of male fowls. Flocks of white- throat sparrows represent social hierarchies similar to those of domestic fowls but are less fixed. Leadership and Followership. The leader of a group may or may not be its dominant member. The leader is the individual that wiU not desert the group in any emergency and that its members will follow. It is the experienced "knowing" animal, not necessarily the largest or fastest. In herds of Scottish red deer a stag is ordinarily the dominant member, but in crises the males leave the group and a female assumes leadership. With real leadership the Chap. 6 MUTUAL RELATIONSHIPS OF ANIMALS 103 Fig. 6.9. Relations of parents and young. Top left, termite, Hodotennes turke- staniciis, king and queen beginning to dig the burrow that will lead to an elaborate underground nest with thousands of occupants of which they will be the parents (After G. Jacobson). Top right, male water bug, Pelostoma fiiimineum, with de- veloping eggs glued to his back by the female. Such nursery-bearing males can be found commonly in ponds during the summer. Bottom left, Koala, Australian teddy bear. Female first carries the young one in a pouch like that of the kangaroo, then on her back. Adults are about two feet long. Bottom right, male sea horse (Hippocampus) with brood pouch in which the developing young are carried (After Boulenger). (Termites courtesy, Wheeler: Social Life in the Insect World. New York, Harcourt. Brace & Co., 1923. Waterbug courtesy, Morgan: Fieldbook of Ponds and Streams. New York, G. P. Putnam's Sons, 1930. Seahorse courtesy, Rand: The Chordates. Philadelphia, The Blakiston Co., 1950. Koala courtesy, Young: The Life of Vertebrates. Oxford, Clarendon Press, 1950.) 104 ECOLOGY Part II followers are dependent upon the leader, and the leader upon the followers in a way which is not the case with the dominant animal or with the pseudo leader which chance may place temporarily at the top. Interdependence be- tween leader and followers is complete in the queen honeybee and the workers, and is very marked in other social insects. Male bees are the least social mem- bers of the hive. After the mating season, male deer separate from the rest of the herd and forage for themselves. On the other hand, the females are accom- panied by the young ones wherever they go. Many similar habits point to the female as the deeply social influence in groups of animals. The Family. Both parents may take part in rearing the young. The male water bug carries the eggs stuck to his back until they hatch; the male sea- horse has a brood pouch where the female deposits the eggs which he carries until the young ones swim out into open water; male birds usually take their turn at bringing food to the nest (Fig. 6.9). In general, however, the mother and young relations are more stable and intimate, more truly social. Mother and young have a comparatively long association in widely different types of animals. Female spiders carry nurseries of spiderlings on their backs; cray- fishes and lobsters swim about for many weeks with eggs and then young ones hanging on their swimmerets; for days the female robin keeps close company with her young ones, showing them what it is to listen for earthworms and how to tackle them. A great company of young mammals are carried or trail beside their mothers, young kangaroos or joeys, skunklets, bear cubs, and fawns. They explore the surroundings from their shelter of maternal care. They imitate the turns of their parents and gradually take part in the customs of their kind. They are products of family associations, mothers, and some times both parents, and young. Thus the family constitutes one of the bases, though not the only one, from which society has sprung. Competition and co- operation exist in the family as they do in other groups. PART III Tne Internal Environment or tlie Body 7 Ti issues In multicellular animals, cells live crowded together and constantly affected by one another. Whether similar or different they cooperate closely in the organization of the animal. Differentiation, the modification of certain parts for certain functions, and cooperation are fundamental properties of their structure and activity. The body of a flying bird and the body of the pilot of an airplane are both great companies of cooperating cells. Cells are associated in groups, the tissues and organs, and these in turn in systems. The study of groups of cells is histology or microscopic anatomy. Tissues, Organs, and Systems. Tissues are groups of similar cells with the intercellular substances which they may produce. The substances may be of hardly noticeable amount as in epithelium, or conspicuous as in bone, or fluid as in blood. An organ is an association of tissues all of which cooperate toward the per- formance of one or more particular functions. The heart is an organ that con- sists largely of muscle; it is covered and lined with epithelium; nervous tissue acts in the control of its pulsation; and connective tissue holds the other tissues together. A system is a group of organs which collectively perform certain related functions. The digestive system is concerned with intake of food, its prepara- tion for absorption, and elimination of undigested waste substances. The animal body, like the plant, is built of groups of cells that form tissues, of tissues that form organs, of organs forming systems, and of systems that compose the whole body. The tissues are discussed in this chapter. The organs are included with their respective systems. Classes of Tissue There are four types of tissues, grouped according to their structure and function: epithelial, connective tissue including blood and supporting tissues, muscular, and nervous tissue. 107 108 THE INTERNAL ENVIRONMENT OF THE BODY Part III Epithelial Tissues Epithelium covers the outer surfaces of the body, Hnes its cavities such as lungs, alimentary canal, the coelom or body cavity, and the blood and lymph vessels (Fig. 7.1). It forms glands and the essential parts of the sense organs — sensory cells in the eye and nose. Epithelium is an essential guardian of the integrity of the body. It has a general and vital part in metabolism since all substances which take part in metabolic activity must go through epithelial cells. All digested food is ab- sorbed through epithelium, mostly in the small intestine. The amount of water contained in an animal is controlled through epithelium, in the skin, alimen- tary canal, and kidneys. In the liver and kidneys it takes part in excreting waste substances. It secretes such varied products as oyster shells and pearls, the chitinous cover of insects, the digestive fluids of all multicellular animals, and the hormones of glands such as the thyroid and the pancreas. It is directly the protection against all manner of mechanical and chemical injuries. It was Fig. 7.1. Epithelial tissues through which all substances that take part in the metabolism of multicellular animals must pass. A, simple flattened or squamus epithelium from the surface of the mesentery of a guinea pig; B, lining of a small vein of mesentery. Intercellular cement is darkened by the preparation. xl200. (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York, McGraw-HiU Book Co., Inc., 1953.) Chap. 7 TISSUES 109 Fig. 7.2. Cuboidal or low simple columnar epithelium: A, lining of a collecting tubule in the kidney of a monkey; B, in the thyroid gland of a monkey. These cells produce the thyroid secretion. xl200. (Courtesy. Nonidez and Windle: Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1953.) due to the epithelium on their bodies and in their kidneys that the animals of ancient time could leave the sea and gradually become adjusted to living in fresh water or on land. The epidermis or outer layers of human skin which is formed of epithelial cells is in general about as thick as tissue paper. Yet a bit of vinegar dropped on broken and unbroken skin are vividly different expe- riences. In certain regions, the epidermis is many-layered, as on the palms of m. lil Sgi^^ligg:^^^ Fig. 7.3. Columnar epithelium with motile cilia (c) lining the trachea of a monkey. Mucous or goblet cells (g) secrete the mucus (m) that passes through the membrane at one end of the cell and spreads over the inner surface of the trachea. The delicate non-cellular basement membrane (b) separates the epi- thelium from the loose connective tissue beneath. A lymphocyte ( 1 ) is migrating through the epithelium. X1200. (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York, McGraw-HillBook Co., Inc., 1953.) no THE INTERNAL ENVIRONMENT OF THE BODY Part III the hands, and thick and tough in the footpads of cats and dogs; the cells are heavily cornified in fingernails, in the horns of cattle, in the hoofs of horses, the outer shell of turtles, the hair of mammals, and feathers of birds. New epitnelial cells are formed as others are worn out or injured. Otherwise we should be walking records of our encounters — scrapes, burns, and pinches. Regeneration is constantly going on in skin and its outgrowths of feathers and hairs. The snake's skin comes off in one piece, the human skin in little frag- ments; feathers are shed in late summer, human skin at any time. Different Kinds of Epithelial Cells. These are classified according to their shapes — flattened, cuboidal, columnar, and arrangement in single or multiple layers, simple or stratified (Table 7.1). A single layer of simple flattened (or squamous) epithelium lines blood and lymph vessels including the heart (Fig. 7.1). Cuboidal epithelium lines the ducts of glands (Fig. 7.2). The cells of columnar epithelium are tall prisms or cylinders (Figs. 7.3, 7.4). They form the lining of the small intestine where they secrete digestive juices and absorb the digested food. All columnar cells have polarity, that is, are different at their two ends. In ciliated columnar cells the polarity is conspicuous since they bear a large number of cilia only on their free surfaces. Cilia beat with rapid effective and slower recovery strokes, always bending in one direction. The movements travel over the surface in waves which rapidly succeed each other at regular intervals. This occurs in the lining of the human trachea with the stronger stroke toward the mouth. Cilia on the gills and lips of clams wave particles of food toward the mouth. In the oviducts of mammals they create currents which move the eggs toward the uterine cavity (Fig. 18.13). Stratified flattened epithelium of the skin is several layers thick; the outer Table 7.1 Forms and Functions of Epithelium Name Form Examples 1 Flattened Mesentery of frog; in man, lining of capillary \ Cuboidal Lining salivary gland in insect; lining normal Simple ( thyroid of frog / Columnar Lining food cavity of hydra; small intestine of cat Pharynx of frog; human trachea; gill of clam Ciliated Columnar Stratified Cells in layers, outer ones flattened Skin of frog and man Name Function Examples Glandular Digestive secretion Small intestine of mammals Sensory Response to vibration. Lateral line organ in fishes, tadpoles; rod and light, chemicals cone cells of human eye; chemoreceptors in jellyfishes Germinal Origin of sex cells Seminiferous tubules in testes of frog, cat, man; ovary in hydra, grasshopper Chap. 7 TISSUES 111 Fig. 7.4. Simple columnar cells in a gland in the human uterus. Droplets of the secretion have collected at the ends of the cells and are about to pass through the membranes; other droplets are free of the cells in the cavity of the uterus. All the epithelial cells have polarity, most striking in the ciliated and glandular ones. Preparation by Dr. G. N. Papanicolaou. xl200. (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1953.) ones are dead and horny (Fig. 8.2). They are constantly being worn away at the surface and replaced in the deeper layers. Stratified epithelium is extremely thick in the footpads of large carnivores — tigers, lions. Connective Tissue, Including Blood and Supporting Tissues Connective tissue contains a large amount of nonliving intercellular sub- stance, fibers in connective tissues, tough resilient chondrin in cartilage, hard rigid substance in bone, and the fluid plasma in blood (Fig. 7.5). Connective tissue connects and binds together the tissues and organs of the body. It seems ever present, penetrating into glands and muscles along with the blood vessels, and binding nerve and muscle fibers into compact bundles. If all other tissues were destroyed, the body with its organs would keep its shape because of connective tissue. During dissection its whitish sticky strands have to be pushed aside and torn. In beefsteak and roast beef such strands display their tough and threadlike quality. Surface wounds are closed mainly by connective tissue and scars of all kinds are chiefly composed of it. Loose areolar or open tissue is the papery fastening which must be torn as one skins any animal, especially birds and mammals. This most generalized connective tissue supports and surrounds other tissues and is a living pack- ing material in the body. The substances which other tissues receive from the blood and lymph — 112 THE INTERNAL ENVIRONMENT OF THE BODY Part III {Legend on facing page) i Chap. 7 TISSUES 113 oxygen, food, water — and the metabolic products which tissues pass on to the blood and lymph must all go through connective tissues. Like epithelium it is a screen through which substances pass to and fro. The characteristic of the body known as its constitution is probably connected with properties of the loose connective tissue. Abnormal growths such as tumors persist or fail to develop, depending to some extent on the reactions of this tissue. In its defensive response the phagocytic cells called macrophages (large eaters) which originate in it are the main actors. These cells are scattered throughout the body and are ordinarily quiet, but if properly stimulated, as by infection, they become mobilized like an army, enlarged, and active. The structure of loose areolar connective tissue is typical of all connective tissue (Fig. 7.5). It is composed of: (1 ) cells, such as macrophages, fibroblasts (associated with the formation of fibers); (2) nonliving collagenous white and elastic yellow fibers; (3) a thin jellylike ground substance. Collagenous fibers are so-called because they contain a protein, collagen, which on boiling yields glue and gelatin. In areolar tissue they run in all directions, are very flexible and resistant, but are not elastic. They are really bundles of very, very fine cross-striated fibrils, but these are invisible except by special tech- niques. Elastic fibers appear as single strands, branched and like rubber bands; when a pull is released they return to their original length. Areolar tissue pulls the skin into place after it has been pinched up from the back of the hand, more quickly in a younger than an older person; it also surrounds organs. Dense areolar tissue, the dermis of the skin, is the fibrous part of leather. In many ligaments and tendons collagenous fibers are predominant and compactly arranged according to the strains put on them. They are densely woven like felt in the sclerotic coat commonly called the white of the eye. Connective tissues often contain very few collagenous white fibers and many yellow elastic ones, the latter so abundant that the whole tissue is elastic. This is the case in the nuchal ligament of grazing animals: a strap of ex- FiG. 7.5. Connective and supporting tissues. Top, cross section through the human tailor's or sartorius muscle showing how muscle cells are held together by a web of interlacing strands of connective tissue, the white lines in most cross cuts of meat. This muscle is the longest in the body originating on the hip, crossing the thigh obliquely, extending down the leg, and attached to the inner side of the shin bone. Bottom, microscopic structure of the loose areolar connective tissue of a kitten, spread out and stained to show its parts. This tissue tears like paper as one skins an animal, a tissue with many open spaces, c, non-living collagen (or pro- tein) white; e, elastic yellow fibers; /, fibroblasts, the cells associated with produc- tion of the fibers; /, lymphocytes; m, macrophages, the cells that consume bacteria and foreign particles; m^, mast cells, function unknown. [Top, courtesy, Maximow and Bloom: Textbook of Histology, ed. 6. Philadelphia, W. B. Sanders Co., 1952. Bottom, courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1953.) 114 THE INTERNAL ENVIRONMENT OF THE BODY Part III Atlas Nuchal ligament Stieet like extension of ligament Scapuio Cartilage of scapula Humerus Elbow Radius Fig. 7.6. Nuchal ligament of the horse, a strap of tough, yellow elastic fibers, often called whetleather, highly developed in grazing animals. tremely tough yellowish tissue, sometimes called whetleather, which extends along the back of the neck (Fig. 7.6). In the larger arteries these nonliving elastic fibers form a large part of the wall. In older animals they lose their elasticity. Certain connective tissue cells are storage places for fat. In adipose tissue or fat each cell is so filled with the fat globule that the nucleus and cytoplasm are pushed into a thin rim around it (Fig. 7.7). Fat enters and leaves the cell in soluble form. Fat cells border the blood vessels, often great masses of them in the mesentery of the human abdomen constituting the so-called fatty apron. Blubber, the fat of whales, has long been a valuable source of oil; for the whale it is a great insulation against cold as well as a store of food. All insects contain more or less fat, especially caterpillars and various pupae. The weight of full-grown larvae of honeybees is 65 per cent fat, due to rich diet and no exercise. Supporting Tissues Cartilage and bone are living tissues with cells that produce the substances giving these tissues strength and rigidity. Chap. 7 TISSUES 115 Fig. 7.7. Development of adipose (fatty) tissue in the larynx of a newborn kitten; c, blood capillaries; /, nucleus of developing fibers (cells); s, signet fat cell. A, in a region in which fat droplets (white spots) have appeared in only one cell; B, another region in which fat droplets almost fill the cells crowding the cytoplasm and nucleus against the cell membrane so that the shape is like a signet ring. Cells containing large amounts of fat are found in connective tissue almost everywhere throughout the body. (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1953.) Cartilage. The intercellular substance of cartilage is firm and gumlike. Normally it contains no lime but with age may gather deposits of it. Hyaline, glassy cartilage or gristle, occurs in the higher vertebrates in many regions, such as the ventral ends of the ribs, the joints, end of the nose, the rings of the trachea (Fig. 7.8). The cells are surrounded by their semitransparent secretion in which there are no blood vessels. Yellow elastic cartilage contains a network of elastic fibers and is more flexible and elastic than the hyaline type (Fig. 7.9). It constitutes much of the external ear of mammals, such as man, bats, donkeys. White fibrous cartilage composes the intervertebral discs which act as cushions between the vertebrae (Fig. 7.10). Those of the human body are subject to various disarrangements especially in the lumbar region where there is most pressure upon them. Bone. This is a supporting tissue composed of bone cells surrounded by organic material, collagenous (protein) fibers, and inorganic salts (Fig. 7.11). 116 THE INTERNAL ENVIRONMENT OF THE BODY Part III Fig. 7.8. Hyaline cartilage from the head of the thigh bone (femur) of a puppy. The cells (chondrocytes) secrete the glassy substance surrounding them and from which they have shrunken away. Nourishment filters through the cartilage to the cells. (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1953.) Fig. 7.9. Yellow elastic cartilage from a pig's ear; groups of hyaline cartilage cells are isolated by the hyaline substance which holds a meshwork of the elastic fibers of connective tissue. The springback of the human ear when pulled is due to these fibers. (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1953.) Chap. 7 TISSUES 117 These salts are largely calcium phosphate and calcium carbonate. Two types of structure are found in most bones, compact bone and latticed or spongy Fig. 7.10. The intervetebral disk or cushion be- tween the vertebrae mainly composed of white fibrous cartilage. A human vertebra seen from above with part of the intervertebral disk adhering to it. The outer side of the vertebra is down; in life the hole contains the nerve cord. 1, rings of fibers arranged in layers; 2, a small central body of cartilage (nucleus pulposus). (Courtesy, 2"^'"' •5^ Elements of Anatomy, ed. 11. New York, Long- mans, Green & Co., 1915.) bone. The Haversian system is the unit of bony structure (Fig. 7.12). Its odd name comes from that of Clapton Havers, an English anatomist, who de- scribed the system in the 17th century. The unit is an irregularly cylindrical structure with a central or Haversian canal containing nerves and blood Fig. 7.11. Bone cells in a thin section of human thigh bone (femur) with bone cells and their processes highly magnified; the naturally colorless nuclei have been deeply stained. The bone cell lies in a minute cavity (lacuna) with its living processes extending into extremely fine canals (canaliculi) which branch out in all directions through the intercellular substance often connecting with those of other cells. Materials pass through these to and from the cells, ultimately to blood ves- sels. (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1953.) 118 THE INTERNAL ENVIRONMENT OF THE BODY Part III Fig. 7.12. Microscopic structure shown in a cross section of human dried com- pact bone, one complete unit of bony structure (Haversian system) and parts of others. In life the central or Haversian canal (black) contains nerves, blood and lymph vessels; the lacunae, also black, contain the bone cells. All nourishment and oxygen come to the bone cells by way of the canals. Layers of bone surround each canal like successive coverings of a cylinder. (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1953.) and lymph vessels. The blood vessels of the Haversian canals are connected with those in the marrow or with larger vessels entering and leaving the bone. Thus, when young, even compact bone proves to be a living tissue through which body fluids can circulate. Respiration occurs in bone cells and conse- quently metabolism does also, the latter at a lower rate than in other tissues. In the finest structure of the bone around the Haversian canal the fibers are wound spirally and are thus made stronger as the fibers of rope are strengthened by twisting. In spongy bone the "lattices" are like bridges which increase the strength against blows and breakage. The intercellular substance of bone acts as a storage for calcium and phosphorus. There is continual inter- change of calcium between the blood and bones which keeps the calcium content of the blood constant. Insufficient calcium and phosphorus cause rickets, a softening of the bones. In small children this may be a cause of bow-legs. Bone marrow is a soft cellular tissue in the central cavity of long bones and the spaces of spongy bones. There are two closely related kinds, the yellow and red. The yellow marrow that fills the central cavity of long bones I Chap. 7 TISSUES 119 is chiefly fat. Red marrow occurs mainly at the ends of long bones. It con- tains fewer fat cells and is characterized by the development of red blood cells and granular white ones. Great numbers of these are continually passing into the blood and a comparable number of worn-out cells is withdrawn. This is an instance of the regulated economy of the body which breaks down comparatively seldom. Blood and Lymph Blood and lymph, its supplemental fluid, are tissues comparable to connec- tive tissue and the skeletal tissues, bone and cartilage, to which they are related. As here described, there are four types of connective tissue in each of which the cells are surrounded by abundant intercellular substance. In ordinary connective tissue the substance is gelatinous; in cartilage, it is tough and jellylike; in bone, hard; in blood and lymph, a liquid in which the cells float freely. As far as its origin and related tissues are concerned, the discussion of blood should be included at this point. Instead it is given in Chapter 12, Blood and Circulation, and is thus placed with the vessels that carry it through the body. Muscular Tissue Muscle cells are so elongated that they are commonly called muscle fibers; thus, the terms muscle cell and muscle fiber are used interchangeably. A mus- cle fiber is living matter; a connective-tissue fiber is not. Muscle fibers, that is, muscle cells, contain fibrils (myofibrils) within their cytoplasm; the shorten- ing of these is the contraction or muscular action. Muscle cells are usually in bundles held together by connective tissue. Muscle has a high degree of contractility. This fundamental character of protoplasm is evident in the movements of an ameba and the action of its contractile vacuole, as well as in the movements of all other animals. Contraction of protoplasm is accom- panied by chemical and physical changes. Chemical Composition of Muscle. About three-fourths of muscle is water. Of the remainder about four-fifths is protein; the other one-fifth includes car- bohydrates and fats, nitrogenous substances (urea, creatine), lactic acid, pigments, enzymes, and inorganic salts. The most abundant protein is myosin which makes up most of the contractile myofibrils. The carbohydrate is largely glycogen, the ready-to-use food stored in many tissues. When a muscle has been excited and fatigued its store of glycogen disappears and an equivalent amount of lactic acid takes its place. When the oxygen supply is renewed and after oxidation occurs the lactic acid is reduced and a proportional amount of heat results. Muscles contain a red pigment, muscle hemoglobin or myoglobin, which has an even greater affinity for oxygen than has the 120 THE INTERNAL ENVIRONMENT OF THE BODY Part III hemoglobin of blood cells. It is abundant in the "red" muscle of birds and mammals and the heart muscle of all vertebrates. Types of Muscle. There are two main types: smooth, unstriated, or invol- untary; and striated, skeletal or voluntary. Cardiac (heart) muscle, although striated, is involuntary and contracts rhythmically. Smooth Muscle Cells. These spindle-shaped cells occur in sheets held together by connective tissue (Fig. 7.13). They include muscles in blood vessels, in the urinary bladder, in the bronchial tubes of the lungs, in the alimentary canal, and in other structures not under voluntary control. The contraction of the iris of the eye in bright light is due to the contraction of smooth muscle. The contraction of smooth muscle causes goose flesh, the erection of hairs on the arms resulting from fear or cold, and the vivid lift of hairs on a cat's tail. Striated or Skeletal Muscle. This is the muscle attached to the skele- ton, the voluntary type that comprises the bulk of muscle in the body. Most of the meat that we eat is voluntary muscle, cut in slices, actually cross- sections, taken at right angles to the length of the muscle cells (Fig. 7.14). Striated muscle differs from the smooth type in the size and shape of its cells. The most conspicuous microscopic structures are the alternating light and dark crossbands of the cells. Striated muscle fibers are regarded as giant multinucleated cells. Some very long ones have about 100 peripheral nuclei. Each muscle cell contains a bundle of contractile fibrillae. In insects probably all muscle is more or less striated. Striations are prominent in the flight muscles of the honeybee when spread thinly on a slide in their fresh condition. ii^^^^^;^:;^mimMi:mm Fig. 7.13. Smooth muscle. A, fibers (cells) from a frog's bladder; B, cross sec- tion of smooth muscle from the bladder of a kitten; the muscle cells are held together by connective tissue; the section misses the nuclei of many cells; C, branching smooth muscle cells in the aorta of a dog. x 900. (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1953.) Chap. 7 TISSUES 121 Fig. 7.14. Skeletal or striated muscle cells. A and B, in long section; C and D, in cross section. Note the nuclei with large nucleoli. The differences in appear- ance are due to different methods of preparation, an example of what often happens to preserved material. (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc. 1953.) Red and White Muscle. The cells of dark red muscle (dark meat) contain an extra amount of muscle hemoglobin (myoglobin), and abundant cytoplasm. This muscle also has a large blood supply and is usually active for long periods of time. Pale muscle fibers (white meat) contain less cyto- plasm, less myoglobin, and have a smaller blood supply. The color of muscle also varies with the animal; in birds, red and white; in rabbits, red and white; in nearly all human muscles, a mixture of both types. Cardiac Muscle. In all vertebrates the heart is composed of a network of striated muscle fibers. They are unique in being branched and having centrally placed nuclei and intercalated, or literally, inserted discs, that is, dark bands that cross the fibers at irregular intervals whose function is not known (Fig. 7.15). Nervous Tissue The functioning of nervous tissue is due to two properties of protoplasm: irritability, the power to react to various chemical and physical stimuli, and 122 THE INTERNAL ENVIRONMENT OF THE BODY Part III Fig. 7.15. Cardiac muscle. A and B, ventricle of a monkey's heart; C, from a human heart. /, intercalated disks, the cross bands that are characteristic of heart muscle; p, granules of pigment; v, blood capillaries carrying rich supply of blood. (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1953.) conductivity, the ability to transmit the reactions from one place to another. The nerve cell or neuron is the structural unit of the nervous system. Its striking feature is the extension of the cell body into processes. These in- clude two types: the relatively short dendrites through which the changes known as nerve impulses move toward the cell body, and a single process, the axon, through which nerve impulses move away from the cell body (Fig. 7.16). In different parts of the nervous system the cell bodies vary widely in size and shape but all of them have certain characteristics in com- mon. They have prominent nuclei, no centrosomes, fine fibrils which become visible in the cytoplasm with special stains, the neurofibrils, and irregularly shaped bodies, the Nissl or tigroid bodies. The state of the Nissl substance is a sensitive indicator of the condition of the nerve cell. It is depleted in infec- tions such as poliomyelitis, in intoxications, and exhaustion, and is reformed during recovery from illness or during sleep. In all but the simplest animals, such as hydra, the nerve-cell bodies exist only in ganglia and in the gray matter of the brain and spinal cord. Chap. 7 TISSUES 123 Nervous tissue is mentioned here because it is one of the four main types of tissues. Since nerve cells are peculiarly related and interdependent as a whole system, the general discussion of them is given with The Nervous Sys- tem, Chapter 16. Fig. 7.16. Nerve cell from the cerebral cortex or gray matter of a rabbit. The axon gives off numerous branches and then enters the white substance, within which it extends a long distance. Only a small part of the axon is shown in the drawing, a, axon; b, white substance; c, collateral branches of axon; d, descending or apical dendrite; p, its terminal branches at the outer surface of the brain (After Ramon y Cajal. Courtesy, Maximow and Bloom: Textbook of Histology, ed. 6. Philadelphia, W. B. Saunders Co., 1952.) Important Reactions in Tissues Inflammation. The defense reaction of living tissues to an unfavorable condition such as an infection is evidenced by inflammation. Its general results are redness, swelling, heat, and pain at or near the site of the injury. The region becomes congested and swollen by an accumulation of body fluids and their associated cells. There is increased activity of these cells; this and the greater supply of blood produce a local heat rise. The congestion with pressure on the nerve endings results in soreness and pain. There is an efficient cellular defense against inflammation. Cells which produce antibodies or antitoxins and may be phagocytic are scattered every- where in loose connective tissue and in the blood and lymph. In the loose connective tissue there are many capillaries from which increased numbers of leucocytes migrate to the inflamed areas (Fig. 7.18). The neutrophils move in first and act quickly; monocytes enlarge and, along with the now 124 THE INTERNAL ENVIRONMENT OF THE BODY Part III ••;*• >••« H? Fig. 7.17. Drawing of a leucocyte (neutrophil) at half-minute intervals showing its ameboid movement, and the intake of bacteria (black dots). The nucleus (black) is many-shaped. (From Best and Taylor: The Living Body, ed. 3. Copy- righted by Henry Holt and Co. Reprinted with their permission.) active macrophages (connective tissue), attack and take in the poisonous alien matter. The ability of these cells to adjust themselves to a different situa- tion is characteristic of protoplasm and a keystone in the body's defense against injury. As the inflammation decreases, healing begins. Scar tissue forms with new connective cells and white collagenous fibers. Some of the macrophages remain in resting condition among the new connective tissue Tilpty ipUnten Dilated, congested capUlarle* rnake ^*su.'pface pedL, escaping ploLSma. capillaries and - veriules cau.^e» sv^ttUing. Fig. 7.18. Diagrams to show how leucocytes (neutrophils) migrate from small congested blood vessels to combat bacteria introduced into the tissues by an injury. (Courtesy, Ham, Histology, ed. 2. Philadelphia, J, B. Lippincott Co., 1953.) Chap. 7 TISSUES 125 cells. In the walls of adjoining blood vessels, cells (endothelial) multiply and form branches which extend into the scar tissue, their presence account- ing for the "red scar." By this time the surface of the scar is covered by epithelium. Contraction of the white fibers reduces the capillaries and the "white scar" results. Bruises. Such bruises as a black eye are produced by blunt objects which crush blood capillaries and other tissues. The capillaries bleed; the hemoglobin of the accumulated blood breaks down, causes the black and blue and later the greenish colors. Fever. There may be a general response to injury in a fever involving the whole body. It results in an increase of metabolic activity and a consequent rise in temperature. High temperature is a dependable sign that something unusual is going on in the cells of the body. Hypertrophy. The enlargement or hypertrophy of a particular region or organ may be due to enlargement, i.e., hypertrophy of individual cells and/or increased number of cells, i.e., hyperplasia. If one kidney has been removed, the other usually enlarges with more cells and does extra work. Atrophy. This is a degenerative process in which cells diminish in size and number. It is sometimes due to lack of blood or nervous control. A com- mon example is the degeneration in leg muscles following the destruction of parts of the nerve cord in infantile paralysis. 8 An Agent or Evolution— T lie Body Covering Skin is a meeting place, the frontier between an animal and its surround- ings, a region of come and go, of shutting in and out. The body coverings of animals are strikingly different: tenuously delicate in a jellyfish, tough enough to stop bullets in a rhinoceros. They include such contrasts as the ectoplasm of an ameba, the ciliated pellicle of paramecium, the simple slimy skin of earthworms, the thin skin of birds, the leathery skin of mammals. The multiplicity of structures that have developed from skin is a record of its many functions that usually help and sometimes hinder animals that live surrounded by shifting climates and shifty neighbors. Skin glands secrete the shells of oysters, the chitinous exoskeletons of grasshop- pers, the scales of butterflies, the slippery mucus of fishes and frogs, the watery sweat of mammals, and the oil that waterproofs the feathers of birds. Cellular outgrowths of skin form the claws of owls and tigers, horns of cattle, beaks of birds and turtles, and hair — bent and crinkled in the wool of sheep and straight on a monkey. Although less significant than the kidneys, the sweat glands are also excretory organs. Sweat is similar to very dilute urine; in man it contains about 99 per cent water, about 0.08 per cent urea and some other salts. Skin is more or less resistant to disease and to the entrance of bacteria and parasites. The mucus secreted from the skin glands of fishes and the cornified layers in the skin of land animals are among its defenses. Pigment is deposited in skin cells making patterns — the spots on leopard frogs, the stripes of zebras, which disguise their owners against the back- ground of their homes. Certain cells of the skin are sensitive to touch, others to temperature, to chemicals, some of them to light. Animals, human and nonhuman, learn much about their surroundings through their skins. 126 i Chap. 8 AN AGENT OF EVOLUTION THE BODY COVERING 127 General Structure of Skin Skin consists of one or more layers of cells which cover the outside of the body and make a sheath over the delicate tissues beneath. Thus the outer layer of protoplasm that covers unicellular protozoans is not related to skin except in function. In all multicellular animals the outermost covering is a layer of epithelial cells, the epidermis. This is the only layer present in the invertebrates, except the starfishes and their near kin (Fig. 8.1). In the vertebrates there is also an underlying connective tissue layer, the dermis, sometimes called leather skin, because when properly prepared it is leather (Fig. 8.2). Epidermis. The epidermis is composed of several layers of epithelial cells. The inner ones next to the dermis form a growing zone (malpighian layer) where new cells are constantly being formed and pushed outward by the pressure for space. As this occurs they are gradually flattened and outspread (Fig. 8.2). In fishes and other moist-skinned animals even the outermost cells stay alive for considerable time, but in land animals they become dry and lifeless. Amphibians and reptiles molt the old epidermis in one piece; birds lose their old feathers; and mammals continually shed little fragments of skin. The constant flecking off of the human scalp in dandruff must be familiar to everybody, in advertisements if not otherwise. Epidermal cells become horny by deposits of the protein called keratin (horn). Keratin is prominent in land dwelling vertebrates, in hair and feathers, horns of cattle, footpads of dogs, and hoofs of horses. The "horny hands of toil" are actual facts. Many glands originate in the epidermis although they usually enlarge and Fig. 8.1. A section of the epidermis and cuticle of an earthworm highly magni- fied. It shows four mucous cells in different stages of secretion, all swollen with the mucus which has pushed the nuclei to the bottom of the cells. It finally pours out through microscopic pores, one at the end of each cell, and spreads over the cuticle (cm). Mucus keeps the surface of the body moist, makes skin respiration possible, lubricates the skin and lines the burrow in which the worm lives. (Cour- tesy, Dahlgren and Kepner: Principles of Histology. New York, The Macmillan Co., 1908.) 128 THE INTERNAL ENVIRONMENT OF THE BODY Part III Opening of duct of sweat gland Corneal layer •Malpighian layer -Hair follicle Sebaceous gland Erector - muscle of hair —Hair papilla Nerve "> Sections of coiled tubules of sweat gland Fig. 8.2. Section of human skin showing the two layers, epidermis and dermis, characteristic of all vertebrates. The outermost corneal layer of the epidermis com- posed of the horny remains of cells is gradually shed in small bits and replaced by new cells from the growing (Malpighian) layer beneath. Cells of this layer contain the pigment that is responsible for dark complexion. As shown in this figure a hair is a shaft of cells that arises from a layer of epidermal cells that form a narrow pocket in the dermis from the bottom of which a core of cells grows upward and forms the hair shaft. Sensory cells, nerves, and the erector muscle provide for the sensitivity and movement of the hair, and sebaceous glands for the oil. (Courtesy, Gardiner: Principles of General Biology. New York. The Macmillan Co., 1952.) Chap. 8 AN AGENT OF EVOLUTION THE BODY COVERING 129 push down into the dermis (Fig. 8.2). Their great variety includes the stinging cells of hydra, wax glands of honeybees, the mucous glands whose secretion earthworms leave, behind them in shiny trails, and the mucous glands that make the slipperiness of fishes. More familiar are the oil glands of hair and the sweat glands whose products have become the symbol of human toil, the lacrimal or tear glands, and the mammary glands which pro- duce food for all young mammals. The activity of these glands is deeply associated with human experiences. The epidermis has earned a high place in human history; Sir Winston Churchill gave it two-thirds of Blood, Sweat and Tears, Dennis. The dermis is the inner and thicker layer of the skin, the one where the prick of a needle first hurts (Fig. 8.2). The bulk of it is composed of the crisscrossing fibers of connective tissues familiar in leather. Dermis is a nutrient layer containing lymph and blood capillaries and fat cells, the latter often extremely abundant. There are many nerve endings in it; the autonomic (involuntary) nerves control the contraction and dilatation of the capillaries and consequent paling or flushing of the skin. The dermis is the scene of blushing. Heat regulations also occur there; blood may be spread out and cooled in the dilated surface capillaries or driven into the warm deeper parts of the body when they are contracted. The colors of frogs and other lower vertebrates are mainly due to pigment-bearing cells (chromato- phores) in the dermis. Epidermal structures, glands, and feather and hair follicles project into the dermis where dermal structures such as blood ves- sels, nerves, and smooth muscle are associated with them (Fig. 8.2). Skin Derivatives Such notable developments from the skin layers as horns, claws, nails, and hoofs should be added to the scales, feathers, and hair already mentioned. Teeth have a history of close association with the skin and in certain sharks there are rows of them just outside as well as inside the mouth cavity. The plates of whalebone that hang from the upper jaw of toothless whales are composed of cornified epidermal cells. Epidermal Glands. The epidermis contains glands. Lobsters, grasshoppers, and every other arthropod are completely clothed in the secretion of their epidermal glands. Natural pearls are epidermal secretions as are shells of the giant clam {Tridacna gigas) weighing 300 pounds or more, often used as basins for holy water. Scales. The scale of an insect, a butterfly, or moth is a minute plate of cuticle secreted by one or more epidermal cells. It is solely a secretion and does not contain any cells. The "hairs" and spines of other invertebrates are similar. In contrast to these, the scales of bony fishes and other vertebrates are composed of cells that originate from groups of skin cells. 130 THE INTERNAL ENVIRONMENT OF THE BODY Part III Certain members of each class of vertebrates bear scales except the amphibians, and in them scales are unknown. Most fishes and all reptiles are more or less covered with scales; birds have them on their legs; many mammals bear them on their tails — mice, rats, ground moles, opossums, beavers; and armadillos have them on their bodies and tails (Fig. 8.3). Fishes and reptiles are the typically scaly animals. In the yellow perch, sal- mon, and other bony fishes, the scales grow out from pockets of connective tissue in the dermis and overlap one another like shingles. Fishes do not molt and scales keep growing and wearing off as long as the animals live. The scales of reptiles are formed by the thickening and hardening of the cornified epidermis. Those of turdes lie flat over the bony plates beneath; those of snakes partly overlap one another. Turtles never shed their scales but each one increases in size as the animal grows. The cornified scaly epi- dermis of snakes and lizards forms a complete armor that is shed in early summer. It is then that reptiles appear most sleek and burnished in their new skins. Feathers. These are slender upgrowths from the dermis. A feather carries the epidermis with it and at its base sinks into a depression or pit in the skin. Feathers are cellular structures but only near the level of the skin do they remain alive as the feather grows. Nearly all of the feather consists of cornified walls of microscopic air spaces that once were living cells. Thus each feather is an extraordinarily complex horny air trap, an insulation, whose light weight is only a part of its great efficiency. The habits and successes of birds are peculiarly bound up with their feathers. (See also Chap. 36.) Hair. The most striking development of mammalian skin is hair, an in- sulation as characteristic of mammals as the feathers of birds. Among the very few almost hairless mammals are the armadillo, the hippopotamus with a few bristles around the snout, elephants, and whales that are covered with hair before birth but afterward have only a few bristles about the lips. A hair is a shaft of purely epidermal cells which projects outward obliquely from its bulb-shaped root that extends down into the dermis (Fig. 8.2), Below the surface of the skin a hair is a column of rapidly multiplying cells; Fig. 8.3. Hairs and overlapping scales on the tail of a rat, section of it magnified. Chap. 8 AN AGENT OF EVOLUTION THE BODY COVERING 131 the outer ones form a pit or follicle sunk in the dermis; the inner ones de- velop into the homy shaft which extends out as the hair. A minute papilla of dermal cells containing blood capillaries and nerve endings projects into a cup in the root and furnishes nourishment in this spot where growth is very rapid. Sebaceous glands feed oil onto the hair, sometimes in super- abundance. An involuntary muscle extends from near the base of the hair to the epidermis. When this muscle contracts it pulls on the base of the hair and makes it "stand up." In thickly furred animals this increases the insulating power of the coat. Standing hair on the back of a dog's neck is a warning; on human skin it is only "goose flesh," and no indication of danger to others, meaning only that its owner is scared or chilly. It is too sparse to create any insulation from the cold and is a sign of kinship to furred animals rather than a protection. Above the skin a hair is composed of the dead and horny re- mains of cells (Fig. 8.4). Pigment, most commonly black, is distributed along the rod in varying degrees of abundance, causing the different shades of brown and black hair. When the papilla of the hair does not supply materials for pigment, the hairs are gray or white. Air vesicles are frequent in white hair; it is an air trap, in a feeble way, like a white feather. Hairs are also like feathers in being shed at regular intervals. Human hairs are among the ex- ceptions in being shed irregularly; healthy human hairs of the head are esti- mated to live a few years, eyelashes only a few months. A curly hair is slightly flattened and shorter on one side than the other like a shaving; a straight hair is a perfect cylinder. Claws, Nails, and Hoofs. These are all structures of cornified skin (epi- dermis) (Fig. 8.5). Their development is similar to that of hairs; they are •■ \ . 1 : ; i i j' i ': ] I ■ i J -,» f !■ ■ 1 ^ Fig. 8.4. Left, diagram of a human hair showing the characteristic shape of the cuticular scales (F), colorless in all animals unless the hair has been dyed. Scales composed of dead or cornified epithelial cells are arranged like shingles with their free margins always directed toward the end of the hair. The main thread of the hair (medulla, C, and cortex, D) consists of compressed remains of cells, through which pigment is distributed. A, fusi or air vesicles; B, pigment granules; £, cu- ticle. Center, sections of hairs from the human head showing the distribution of pigment granules in hair of different colors. The color or absence of color depends upon the hair's content of pigment and air. Loss of pigment makes the hair look gray; when it contains much air, it is silvery white. A, cream buff; B, befza brown; C, black; D white. Right, hairs from various mammals have characteristic scales; hair of a star-nosed mole, percheron horse, sheep, and other. (Courtesy, Hausman, Sclent. Monthly 59:195-202, 1944.) 132 THE INTERNAL ENVIRONMENT OF THE BODY Unguis ,Subunguis Unguis (noil) /^^^^>>^^ A. CARNIVORAN CLAW (Cat) Pad- Subunguis B. HUMAN NAIL Unguis Pad Unguis /Subunguis C. HORSES HOOF Part III -Pad Subunguis Unguis C^ — Pad Subunguis Unguis /Subunguis Pod Fig. 8.5. Diagrams of claws, nails and hoofs seen in section and from beneath. All of these are modified scales, an unguis or scale above and a subunguis or cushion below. Thus, the front of a horse's hoof is a modified nail essentially similar to the claw of a lizard or a human fingernail. ( Redrawn after Walter and Sayles: Biology of the Vertebrates, ed. 2. New York, The Macmillan Co., 1949.) actually fused hairs. Lizards, turtles, and birds have claws as do many mam- mals, but nails belong solely to a few mammals. A claw fits like a hood over a terminal joint and beneath it is a pad of softer tissue. A nail is a thin horny plate growing on the upper side of the end of a finger or toe. The human fingernail is like a broad flattened claw on the upper surface of the fingertip. None of these structures is molted but broken nails are regenerated. The hoof of a horse is a claw which has become a greatly thickened sheath for the toe-tip. Horns and Antlers. The horns of cattle, sheep, goats, and Old World ante- lopes are outgrowths of bone covered by thick layers of cornified epidermis and, like claws and nails, are tough and resistant to chemicals. Horns are not shed and are never branched. The antlers of deer, reindeer, moose, and elk are annual growths of bone. Deer shed their antlers when they are about two years old and every year after that. At first the bony outgrowth is covered with hairy skin, later the skin is resorbed and the spike of bone breaks off. In the second year the antler de- velops in the same way, is shed, and in each following year the process is repeated with new branches added (Fig. 8.6). Growing antlers are said to be "in the velvet" because their skin is thickly covered with short hairs. They are hot and feverish to the touch due to the large blood supply and the almost explosive expenditure of heat in their rapid growth. Giraffes, which are close relatives of the deer family, do not shed their stubby antlers, that remain in the AN AGENT OF EVOLUTION THE BODY COVERING 133 Feb. 2 %^^ March 20 June 22 Fig. 8.6. Antlers of male mule deer. A, usual annual growth: Feb. 2, March 20, June 22. B, structure and shedding; diagrams of sections. 1, growing prong in the velvet, i.e., covered with hairy skin; 2 and 3, skin worn off and antler shed; 4, 5, 6, regrowth and mature condition in which the bone is bare. Each successive breed- ing season is marked by new antlers; to a certain limit older animals have more prongs. (A, redrawn from Hamilton: American Mammals. New York, McGraw- Hill Book Co., 1939. B, redrawn from Walter and Sayles: Biology of the Verte- brates, ed. 3. New York, The Macmillan Co., 1949.) velvet stage throughout life (Fig. 8.7). Antlers of deer, reindeer, moose, and elk are not composed of horn at any time. Functions o£ Skin Skin is a protection from heat and cold: by pigment in cells (frog); by coverings of feathers (birds) and hair (mammals), with few apparent excep- tions— whale, armadillo, et al.; by erection of feathers and hairs securing greater insulation from cold because of the increase of air space between them; by fat associated with the deep layer (dermis) — the blubber of whales and other marine animals. The amount of water in the body is regulated by the control of its entrance through the skin (frog), resistance to its passage through the skin by chitinous coverings (many insects) and by cornified layers and fat (mammals), by scales (fishes and reptiles), by feathers and hair, by oil or wax glands (in birds especially water birds, cockroaches, certain beetles, bees, ants, and aphids). Skin resists the entrance of parasites and diseases by special thickened areas, 134 THE INTERNAL ENVIRONMENT OF THE BODY Part III Fig. 8.7. Masai giraffe. Although giraffes belong to the deer family they never shed their stubby antlers which are knobs of bone permanently in the velvet. (Courtesy, New York Zoological Society.) e.g., soles of the feet, pads, hoofs (man, elephant, horse), by scales (fishes, reptiles, feet of birds), by feathers and hair, by secretions (mucus in fishes, frogs and toads, mild poisons of hydras, caterpillars of gypsy moths). The skin is a receptor of stimuli through sensory cells and nerve endings, sensitive to touch, heat, cold, and pain. Skin takes part in the heat regulation of the body: in mammals through control of surface blood vessels, through evaporation of sweat from the body surface (man, horse), by coverings of the body, i.e., by hair, or feathers. Vitamin D is produced through irradiation or the exposure to sunshine of oils in skin and on feathers and hairs. In licking their fur mammals secure irradiated oil containing vitamin D involved in the metabolism of calcium and phosphorus. Sweat glands located in the skin excrete products of metabolism, such as water, small amounts of urea, and certain salts. In certain invertebrates (earthworms, planarians, et al.) the respiratory gases pass through the skin. 9 Protection, Support, and Movement — Skeletons Skeletons provide protection and support. The advantage of having a skele- ton is made most vivid by the animal which does not have one. Jellyfishes drift and in calm seas can even swim. But let them be thrown on a sandy beach and, having neither protection nor support, they flatten against the sand and dry to papery wisps. All vertebrate animals have skeletons and the character of their existence is inseparable from skeletons. Imagine a spirited horse without bones! In their relations to their environments and their achievements of speed, strength, and grace animals are greatly dependent upon an outer or an inner frame. General Functions The skeleton determines the form of an animal. Contrast the long leg bones of an ostrich and the lack of them in a snake; or the seven long vertebrae in the neck of a girafi'e and the seven short ones in the neck of a man. Bones are the living tools of the muscles. Watch the fingers striking piano keys, or the legs taking part in defense when a donkey kicks, and in offense when a cat springs upon a mouse. The skeleton's oldest and most general function is protection. The shell is a complete armor around a lobster; the boxlike cranium encases the human brain. The red marrow that produces the vital blood cells of vertebrates throughout adult life is housed within bones. Skeletons are old in animal history. Even in early times the yielding proto- plasm of the smallest animals was doubtless protected by shells and rodlets of hardened secretion as radiolarians are now (Fig. 9.1). Tons of fossil deposits that have been dredged from the sea bottoms testify to the abundance of such microscopic skeletons in primeval seas. Fossil animals of other groups show 135 136 THE INTERNAL ENVIRONMENT OF THE BODY Part III that there were successive ages when skeletons were enormously large and heavy. Those of reptiles commonly weighed many tons. Even modern alli- gators have such heavy ones that they can scarcely lift their bodies from the ground. During their evolution vertebrate skeletons have changed from ponderous burdens to light jointed bones, adapted to muscular control. Of all the land vertebrates, birds have the lightest skeletons, for their tubular bones contain air cavities connected with the lungs. The frigate bird, a famous flier, has a wing expanse of seven feet and weighs two pounds, but its skeleton weighs only four ounces, less than its feathers. Types of Skeletons Skeletons are either exoskeletons, on the outside of the body, or endoskele- tons, within the body. Exoskeletons of invertebrates are composed entirely of nonliving material, the secretion of cells usually deposited in layers (Fig. 9.2). The majority are light in weight, except the shells of mollusks that are often heavy. The muscles are attached on the inner surfaces of the shells (Fig. 9.5, crayfish). Endoskeletons are composed of living cells with their products, such as the limy substance of bone. They are located between muscles and connective tissues, and the muscles are attached to their outer surfaces. Such skeletons are unique to the great group of chordates presently described. Skeletons of Invertebrates In the vast assemblage of invertebrates there is an unending variety of skeletons that fit their owners to live in thousands of niches, in water, on land, Fig. 9.1. Skeletons of representative radiolarians of crystal transparency, beauty and precision of pattern. A vast area of the ocean bottom is covered with ooze mainly composed of these skeletons that have dropped downward and accumulated through the ages. (Courtesy, Kudo: Protozoology, ed. 3. Springfield, 111., C. C Thomas, 1947.) Chap. 9 PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 137 or in the air. These skeletons are calcareous (limy), silicious (glassy), and chitinous (horny), or are combinations of these. Those of aquatic animals often have flotation devices, cavities that contain air or gas, fat, and oil droplets. In the larger groups of multicellular invertebrates there are three general types of skeletons. Permanent Skeletons. Clams, snails, and other mollusks have but one skele- ton throughout life enlarging it as their bodies grow. Although the molluscan shell is not called a skeleton it has the requirements of one. In clams the oldest part of the shell is the hinge region from which larger and larger concentric ridges show where new secretion has been added (Fig. 9.2). The swiftest mollusks are the squids whose skeletons are completely hidden by a fleshy mantle. An exoskeleton may be a network of minute units, or a mosaic of closely fitted plates. As the animal grows, the units are enlarged or new ones added. Clam Starfish , ossicles (black) Sponge, spicules Lobster SKELETONS OF INVERTEBRATES Fig. 9.2. Skeletons of invertebrates. Permanent: clam with lines showing the additions to the shell throughout life; cut across the arm of a starfish showing the limy ossicles (shaded) embedded in the flesh of the body wall. Left lower: spicules of fresh-water sponge that form a net-like support in the body wall. Temporary: lobster whose skeleton is periodically replaced by a new one as long as the animal's growth continues. 138 THE INTERNAL ENVIRONMENT OF THE BODY Part III These skeletal units are strikingly different, white limy ossicles in starfishes, glassy spicules in fantastic shapes and netted fibers in sponges. Temporary Skeletons. Such skeletons are shed and replaced throughout the growing period of the animal. The peak achievements in invertebrate skeletons are the jointed ones of insects and other arthropods that are shed and replaced by larger ones as their owners grow (Fig. 9.2). A new shell is formed before the old one is shed and while the new cover is still soft and pliable it stretches enough to allow for another interval of growth (Fig. 9.3). Most insect skele- tons are delicately wrought; those of moths and butterflies are covered with scales many of these lined with extraordinarily fine grooves. At the other extreme is that of the male Hercules beetle of tropical America, nearly five inches long, with heavy headgear that occupies a third the length of its body. Aquatic species are larger than the related land forms; crabs and lobsters have the heaviest skeletons of the arthropods. Yet when lobsters are submerged in Fig. 9.3. Dorsal shells (carapace) of the same crab before and after molting. A, hard shell that was recently shed; B, larger new shell that stretched and is still soft. Crab, Loxorhynchus grandis, Pacific Coast. (Courtesy, MacGinitie and Mac- Ginitie: Natural History of Marine Animals. New York, McGraw-Hill Book Co., Inc., 1949.) their native sea water they are so buoyed up by it that the tips of their claws touch the rocks as lightly as if they were engaged in a ballet. Joints. Joints are the places where adjacent parts of a skeleton join, often closely fitted together. In lobsters and other arthropods the outer covering or exoskeleton is continuous over them, yet it is so thin and pliable that the joint bends easily. Joints are highly developed in the skeletons of insects and vertebrates, two dominant groups of animals. Those of invertebrates began as creases in the epidermis and cuticle such as are so clearly visible in earth- worms. As an insect breathes, its abdomen rhythmically lengthens and shortens at the telescopic joints. When air enters the body, the plates of the skeleton move apart, stretching the soft membrane between them (Fig. 9.4). Alter- Chap. 9 PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 139 nately, as the muscles of the abdomen contract and air leaves the body, the plates are drawn together with the edge of one overlapping the one behind it. Insects and other arthropods also have hinge joints. The leg of a lobster or an insect bends like a jackknife. Changing Content of Skeletons The content of skeletons is in part changeable, in part permanent. Their composition depends upon the material brought by the blood to the cells which produce the more rigid substance. What is brought depends upon the materials Fig. 9.4. Joints of the arthropod skeleton. A, telescopic joints in the abdomen of an insect when outstretched; pieces of skeleton held to- gether by muscles and skin; B, insect's leg held straight and flexed showing the stretching and folding of the soft skin around the joints. (A, re- drawn after Guyer: Animal Biology. New York, Harper & Bros., 1936. B, redrawn after Ross: A Textbook of Entomology. New York, John Wiley & Sons, 1948.) in the animal's environment and the physiological pattern that the animal inherits. Calcium, occurring in limestone, soil, and water, is continually passed in and out of animals, but during its sojourn in an animal's body it is mainly located in the skeleton. Striking exceptions are horny structures and the chitinous skeletons of insects. In its usual state, 16 per cent of a crab's shell is calcium; when it is "soft," such a shell is but one per cent calcium. This is the only time when the shell stretches. The skeletons of primitive vertebrates are more or less cartilaginous; those of vertebrate embryos are at first composed of cartilage, later mainly replaced by bone. Cartilage is composed of connective tissue cells which produce a more or less resilient gel. The connective tissue cells which produce bone form two different materials: minerals, chiefly calcium and phosphorus, and collagen, a protein. The colla- gen fibers are arranged spirally in the mineral matter, binding it like wires in 140 THE INTERNAL ENVIRONMENT OF THE BODY Part III concrete. The combination of the materials makes bone hard and resistant to strain. Bone can support a greater weight than granite without being crushed. Despite its great firmness, it is moderately flexible especially in young animals. The flexibility of the human skull at birth is well known; even in an adult the skull can stand some compression before it cracks. Bone may be deprived of either mineral matter or collagen and yet keep its shape. Soaking in dilute hydrochloric acid will remove the minerals; burning will remove the animal substance (mainly protein) (Fig. 9.10). The proportion of calcium to living matter varies with age, with the amount of vitamin D in the diet, and other factors. The body's calcium supply is regulated by the parathyroid glands that are located on either side of the thyroid gland (Fig. 15.1). Calcium also indirectly controls the coordinated activity of muscles by slowing down the transmission of nerve impulses to them. When there is an excess of impulses, the secretion of the parathyroids circulating in the blood extracts calcium from the supply in the bones. This, in turn, circulated in the blood, slows the activity of nerves and muscles. On the other hand, if the body becomes sluggish, the parathyroid secretion is diminished and less calcium is called forth from the bones. Again, the parathyroids may be too active and may rob the bones of their calcium and produce abnormal formations. Sometimes this is deposited as kidney stones. Discoveries by Tracers. The behavior of calcium and phosphorus in the tissue of living bone has been observed by means of their isotopes used as tracer substances. The movements of radioactive calcium and phosphorus are detected by a sensitive instrument (Geiger counter) placed on the outside of the body (Chap. 2). Radioactive calcium has been demonstrated in the bones of mice 24 hours after its injection into the veins. Radioactive phosphorus was immediately deposited in the teeth, in the ends of bones, and in the ring of healing (callus) in a bone which had been fractured. Radioactive phosphorus in the form of a solution of sodium phosphate has also been given to human patients either by mouth or by injection into the veins and its movement in the body and its behavior in the bone followed by the Geiger counter. Such explorations are more and more frequently made in the treatment of broken and diseased bones. Skeletons of Vertebrates and Their Ancestors Notochord and Vertebral Column. Vertebrates are named from the chain of bones which composes the vertebral column, the oldest part of the skeleton and the support to which their development and dominance are supremely indebted (Fig. 9.6). "Having backbone" has long come to mean having strength and resolution. With a flexible, dorsal, median backbone, and the bilaterally symmetrical appendages which developed later, the vertebrates gained agility first in water and then on land. They moved about more. Chap. 9 PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 141 traveled in different ways and to different places, and made all manner of new relationships. Long before any of this occurred, the ancestors of vertebrates had an in- ternal axial support, the notochord, on the dorsal side of the body below the nerve cord and above the digestive tube (Fig. 9.5). Following their ancestors of millions of years past, every individual vertebrate, including man, has a complete notochord at some time during its embryonic life. In amphioxus the notochord persists through life; in the vertebrates it is replaced by cartilaginous or bony vertebrae. The presence of the notochord at some period of life in all vertebrates as well as in their nearer ancestors is the reason for the name of the phylum Chordata, the group to which they all belong. The more limited subphylum Vertebrata includes only the chordates that have vertebrae, lam- preys, fishes, amphibians, reptiles, birds, and mammals, including man. The notochord is a slender rod of turgid vacuolated cells held together so tightly within two sheaths that the whole structure is stiffened like a sausage and the substance itself resembles condensed jelly (Fig. 9.5). In mammals, it is soon replaced by bone and cartilage except possibly for a small part of the cartilaginous cushion (intervertebral disc) that persists between the verte- brae. In fishes, remains of it persist through adult life. The conical cavity at each end of a vertebra, familiar to us especially in salmon and tuna fish, was once filled with notochordal cells. Vertebrae. A vertebra is a ring of cartilage, in sharks and other lower fishes, or of bone surrounding the nerve cord in higher vertebrates (Fig. 9.6). The Dorsal Dorsal Ventral CRAYFISH exoskeleton (shell) — muscle nerve cord endoskeleton ( notochord ) Ventral AMPHIOXUS Fig. 9.5. A characteristic and important difference. Cross sections of an inverte- brate (crayfish) with exoskeleton and ventral nerve cord; and a chordate (amphi- oxus) with endoskeleton and dorsal hollow nerve cord. 142 THE INTERNAL ENVIRONMENT OF THE BODY Part III body or centrum occupies the space previously filled by notochordal cells and is so shaped that it fits closely to its neighboring centra or to the intervertebral discs. Dorsal to the centrum is the neural arch; fitted closely together the neural arches form the bony canal in which the nerve cord is enclosed. Each vertebra has particular areas, knobs, and edges, the attachment places of ligaments and tendons of muscles that bind one vertebra to another, as well as surfaces where the centra are pressed against the intervertebral discs. The thoracic vertebrae have special hollows where the ribs articulate. The joints between the vertebrae have only limited freedom of motion, yet Fig, 9.6. A section through articulated human vertebrae, showing one of the intervertebral disks that separate the suc- cessive vertebrae; / and 2, ends of circu- lar fibers; 3, central cushion of cartilage (nucleus pulposus). (Courtesy, Quain's Elements of Anatomy, ed. 1 1. New York, Longmans, Green & Co., 1915.) the backbone, like the spring from a curtain roll, can be bent backward, for- ward, or sideways and swung back into place (Fig. 9.7). A cat's back can take a high curve in a split second, and that of a bucking bronco outdoes the cat in curves; it lifts a cowboy and is just as fast. A snake coils and twists; a kitten sleeps in a ball; an owl rotates its head until it looks directly behind itself; and human acrobats are close competitors, yet the vertebrae stay in their places. Joints. In endoskeletons the muscles and ligaments are fastened to the outer surfaces of the cartilages and bones. Some joints are immovable, such as those in the cranium, little noticed except in very young infants in which they have not grown together. Among the familiar types of movable joints are (Fig. 9.8) : (1) hinge joints, such as those that are worked hard in typewriting; (2) ball- and-socket such as the hip joint in which the head of the femur fits into the pelvic girdle, a joint that is highly important in tap dancing, as well as in walking and sitting and rising; (3) rotating joints in which the radius of the human forearm shifts on its axis across the ulna as when the hand turns a door- knob; and (4) pivotal joints that rock one upon another, such as the im- portant "yes and no" joints, in action as the skull rocks upon the first vertebra (atlas) when we nod "yes"; the atlas revolves upon the vertebra behind it (axis) when we shake our heads "no." In every typical free-moving joint the ends of the bones are held together by sheets of tough connective tissue, the ligaments that enclose the joint in a 1 Chap. 9 PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 143 /"X.X'^^" Fig. 9.7. The flexibility of the vertebral column: in a walking salamander which swings from side to side like a fish; in a fighting cat that arches its back as easily as a bucking bronco. A human "backbone" bends forward, backward, and side- wise. capsule (Fig. 9.8). The end of each bone is capped with cartilage and folds of thin synovial membrane project into the capsule of the joint from the sides. This membrane secretes the synovial fluid, a lubricator that is transparent and viscid like the white of egg. When the synovial membrane of the knee becomes inflamed, its excess secretion often accumulates as "water on the knee." Long Bones. The humerus of the arm or femur of the leg may be taken as an example of the general structure of long bones (Fig. 9.9). The cellular structure of bone is described in Chapter 7. The tubular plan of long bones makes them much stronger than rods of the same size and weight. Two arrangements of their bony tissue, the compact bone mostly surrounding the hollow shaft and the spongy (cancellous) bone at the ends, create strength and lightness at the same time. Spongy bone is a network of plates laid down in lines running in the directions which best meet the stress that falls upon the particular part, such as the weight borne by the head of the femur (Fig. 9.8). It contains spaces filled with red bone marrow in which the red and some of the white blood cells are formed (Chap. 7). An important layer of connective tissue, the periosteum, surrounds all bones. It 144 THE INTERNAL ENVIRONMENT OF THE BODY Part III Finger bone Pholonx Bock of hand Metocorpus Fingers benf Typewriting Clinging lUMSAR ^iUt lUNATE A9T1CUU" CARTILAC- iOlNt CAVITY FC'/fA rttAP UG/v-WENT '- ACeTAfiULAR FAI PAD KANj/iSSE UG OF ACflABUtUM (NOTE. ARTERY OF HEAD IIO- MAY it DERIVED FROM MED. FEM CIRCUMFLEX) Fig. 9.8. Two important types of joints in the human body. Top, hinge joints: finger flexed as in striking typewriter, in clinging. Bottom left, ball and socket joint: the hip joint in which the head of the femur fits into a cup in the pelvic girdle. Bottom right, a section through the hip joint showing the capsule and the ligaments holding the head of the femur in place. The ligament that binds the head of the femur in place is the strongest in the body and rarely is torn even when the joint is dislocated. The section of the femur shows the smooth, very hard compact bone (whitish band) and outside it except at the joints the thin perio- stracum (black line) layer which is the growing zone of the bone. The network of bony tissue called spongy bone because of the many holes is well developed at the ends of long bones and its lines of strength here suggest the braces of a sus- pension bridge. It contains the red marrow in which red blood cells and granular leucocytes (white blood cells) originate. In life the center of the bone is occupied by the fatty marrow, here a black space. (Hip joint drawings courtesy, Ciba Clini- cal Symposia, Vol. 5, No. 2, 1953.) Chap. 9 PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 145 receives abundant nourishment through a network of blood vessels and is the region that provides for increase in diameter in growing animals. Arteries enter and veins leave the bones in an oblique direction and are /synovial fluid -Articular cartilage ^Jblood vessel yrOMPACT BONE