Outlines of biology

Front matter

Hi II II IftJ HU I

22102045194

Med

Preface

THE formal contents of this book have been determined by the syllabus of the conjoint examining board of the Royal Colleges of Physicians and Surgeons of England, issued for the guidance of candidates preparing for the examination in Elementary Biology. I have endeavoured to set forth the necessary facts plainly, but in such fashion that the relations they bear to each other, and their places in the science of Biology, shall be apparent. The actual facts can be learned only with the microscope and the scalpel. I have tried to make this book serve as a guide in the laboratory, and also to supply the necessary connecting links between the isolated facts presented by the seven or eight plants and animals selected out of the multitude of living organisms. I hope, therefore, that this book may supply outlines of Biology to those who will afterwards proceed to that special branch of Biology known as the study of medicine, and that it may also serve those who propose to devote themselves afterwards to more detailed study of zoology, with the aid of more advanced text-books.

In following so well-beaten a track, I am indebted to many predecessors, to whom I wish to make grateful acknowledgment. I have drawn all the figures specially for this book ; in two or three cases directly from figures already published, in several cases from the specimens with the aid of published figures. These are acknowledged in their places ; the others are original.

P. CHALMERS MITCHELL

London, 1894

vii

Contents

CHAPTER

I. Protoplasm ....••

II. The Building up of Protoplasm

I. 18. 19

TRADESCANTIA, MOVEMENTS OF PROTOPLASM. WHITE BLOOD CORPUSCLES OF FROG. YEAST CELLS BUDDING

AN ACINETAN, SHOWING CONTRACTILE VACUOLE

PROTOPLASM OF AMCEBA AND ACTINOPHORA , SHOWING NETWORK

PROTOPLASM FROM THE NUCLEUS OF A GANGLION CELL, SHOWING NETWORK .......

ARTIFICIAL FOAM MAGNIFIED TO SHOW RESEMBLANCE TO PROTOPLASM .......

FORMS OF BACTERIA ......

PLEUROCOCCUS AND PROTOCOCCUS . . . .

SPIROGYRA .......

AMCEBA PROTEUS .......

AN ACINETAN DIVIDING ......

LIVING VORTICELLA IN THE ACT OF EXPANDING .

DIAGRAM OF VORTICELLA ......

BOTRYDIUM

* • • • • •

MERISTEM TISSUE FROM MARSILIA UNCINATA . . .

EPIDERMIS WITH WAXY SECRETION, FROM SUGAR-CANE . STOMATA FROM LILAC-LEAF .....

VERTICAL SECTION THROUGH A STOMA

PARENCHYMA WITH STARCH GRANULES ....

COLLENCHYMATOUS AND SCLERENCHYMATOUS CELLS

STINGING-HAIR OF NETTLE . . ,

CORK FROM AILANTHUS GLANDULOSA

xi

PAGE xii

FIG.

35- 36. 37- 38. 39- page

DIAGRAM OF THE COURSE OF VASCULAR BUNDLES . . 109

ISOLATED CELLS FROM A FIBRO- VASCULAR BUNDLE . .110

LONGITUDINAL SECTION THROUGH A SIEVE-TUBE . . Ill

SPIRAL AND ANNULAR VESSELS . . . . .112

DIAGRAM OF BORDERED PITS AND SCALARIFORM VESSEL . 113

FIBRO-VASCULAR BUNDLES IN TRANSVERSE SECTION THROUGH

THE STEM OF A DAHLIA . . . . . . Il6

TRANSVERSE SECTION THROUGH A SINGLE FIBRO-VASCULAR

BUNDLE, MORE MAGNIFIED ....

CROSS-SECTION THROUGH LEAF OF A BEECH CROSS-SECTION THROUGH LEAF OF A SBLA GINBLLA TRANSVERSE SECTION OF A ROOT .

TWO CILIATED CELLS FROM ROOF OF MOUTH OF FROG HYDRA VULGARIS .....

NEMATOCYST OF HYDRA .....

OVUM OF HYDRA ......

hydra: longitudinal section

HYDRA : TRANSVERSE SECTION

DEVELOPMENT OF HYDRA ....

DIAGRAM OF GASTRULA ....

DIAGRAM OF CCELENTERATA AND CCELOMATA .

EARTHWORMS : EXTERNAL CHARACTERS

EARTHWORM : ALIMENTARY CANAL AND NERVOUS SYSTEM EARTHWORM : GENITAL SEGMENTS .

DIAGRAM OF NEPHRIDIUM ....

DIAGRAM OF NOTOCHORD AND VERTEBRA .

CAUDAL VERTEBRAS .....

SKULL OF DOG-FISH .....

PECTORAL AND PELVIC GIRDLES OF DOG-FISH .

HEART OF DOG-FISH .....

Il8

124 131 135 139 141

200 206 214

LIST OF ILLUSTRATIONS IN THE TEXT xiii

FIG.

50. VEINS OF DOG-FISH

PAGE

. 218 221

. 222 229

. 242 244

• 245 248

• 251 253

• 257 260

• 293

51. GILLS AND ARTERIES OF DOG-FISH .

52. DIAGRAM OF PRIMITIVE KIDNEY .

53. EXCRETORY AND GENITAL ORGANS OF DOG-FISH .

54. BRAIN OF DOG-FISH : DORSAL AND VENTRAL VIEWS

55. ANTERIOR VERTEBRAE OF FROG. HYOID OF FROG .

56. SKULL OF FROG: VENTRAL VIEW

57. SKULL OF FROG : DORSAL VIEW ....

58. DIAGRAMMATIC SECTION THROUGH HEAD OF FROG

59. DIAGRAM OF VERTEBRATE LIMBS ....

60. FROG : SKELETON OF LIMBS AND GIRDLES

61. FROG : DORSAL VIEW OF HEART ....

62. FROG : LATERAL VIEW OF THE GREAT VESSELS

63. ISOLATED BONE CORPUSCLE .....

64. THE NEURONE THEORY OF THE SENSORY AND MOTOR REFLEX

NERVOUS MECHANISM DIAGRAMMATICALLY REPRESENTED . 304

65. TYPES OF NERVE CELLS ...... 306

66. CHROMOSOMES DIVIDING ...... 315

67. SEGMENTATION OF THE OVUM ..... 332

68. THE BLASTOSPHERE ...... 333

69. SECTION THROUGH AN INVAGINATE GASTRULA . . . 334

70. SECTION THROUGH A LATER GASTRULA .... 334

71. LONGITUDINAL VERTICAL SECTION THROUGH A YOUNG EMBRYO . 335

72. TRANSVERSE SECTION THROUGH ANTERIOR REGION OF A YOUNG

73. LONGITUDINAL HORIZONTAL SECTION THROUGH EMBRYO, SHOWING

ENTEROCCELES AND MESENCHYME .... 338

74- A TRANSVERSE SECTION OF THE BLASTODERM OF THE CHICK, 340

I

LIST OF PLATES

FACING PAGE

I. VENTRAL VIEW OF A PARAMCECIUM. PARAMCECIUM DIVIDING BY TRANSVERSE FISSION. DIFFERENT STAGES IN THE CONJUGATION OF PARAMCECIUM . . . -7°

II. I. TROPHOZOITE OF MONOCYSTIS. 2. TWO GAMETOCYTES ASSOCIATED. 3. GAMETOCYTES SURROUNDED BY A COMMON CYST. 4 AND 5. STAGES IN THE FORMATION OF GAMETES.

6. FUSION OF THE GAMETES TO FORM SPOROBLASTS.

7- SPOROBLAST SURROUNDED BY A CYST AND LENGTHENING.

8. SPOROBLAST HAS DIVIDED INTO EIGHT SPOROZOITES.

9- A SPERM MORULA BEING PENETRATED BY A SPOROZOITE . 82

II. GENERAL REPRESENTATION OF A TRANSVERSE SECTION OF DICOTYLEDONOUS STEM. A SEGMENT OF THE UPPER FIGURE INCLUDING ONE VASCULAR BUNDLE . . Il6

IV.

V.

VI.

II. II

X.

X.

a

BLOOD CORPUSCLES EPITHELIAL TISSUES .

CONNECTIVE TISSUES

. 276 280 . 288

I. CARDIAC MUSCLE FIBRE. 2. AN ENLARGED PIECE OF FIG. I IN THE REGION OF THE JUNCTION STRIPE (INTERCALATED DISC). 3. A TRANSVERSE SECTION OF MAMMALIAN BONE. 4. AN ENLARGED PIECE OF FIG. 3 PASSING THROUGH TWO BONE CORPUSCLES

DIFFERENT FORMS OF MUSCLE FIBRE

. 298

I. MULTIPOLAR NERVE CELL FROM NERVE CHAIN OF ANNELID WORM. 2. UNIPOLAR NERVE CELL FROM NERVE CHAIN OF A LEECH. 3 AND 4. MULTIPOLAR NERVE CELLS FROM SPINAL CORD OF OX. 5. MEDULLATED NERVE FIBRE .

DIFFERENT STAGES IN MITOSES .

maturation of the germ or sex cells

XV

• 314

Chapter I

LIVING bodies possess many familiar properties which give them an unmistakable character. We may be unable to distinguish in them growth or movement, the phenomena which we associate most readily with life, but we know that we can produce, by easy mechanical or 1 chemical methods, an arrest or alteration of the processes of life. This arrest or alteration, that we call death, we look 1 confidently to be followed by gross changes in the bodily substance. The form and consistency alter : the pleasant neuhality of the odour is replaced by offensive exhalations, and the substance resolves itself into water, ashes, and gases.’ If we treat the living substance, be it part of a growing root, a green leaf, or part of the fresh tissue of an animal, by strong heat, by placing it in a test-tube held over a flame, first a quantity of water is driven off, and then the substance chars, and there issues a thick smoke in which ammonia and sulphur can be detected. We are thus certain o the presence of hydrogen and oxygen, nitrogen and sulphur, and the charred remains are chiefly carbon. These

elements we shall find compose the chief constituents in living material.

If an exceedingly thin slice or shred of such living material placed in water or in diluted glycerine, and examined under the microscope, it may be seen to be composed of closely packed, distinct pieces, called cells. If the substance chosen is a thin film picked with a needle from the surface of a leaf, or isolated pieces scraped with a blunt edge from the inside of one’s own cheek, these cells in the former case exhibit definite cell-walls and in the latter a definite outline. In both there will be seen lying in the protoplasmic substance of the cell a central thickening or nucleus. The cells are therefore manifestly separate elements, and constitute what we may regard as the structural units of the organism. In some tissues, and especially in those of animals, the outlines of cells are not readily seen. What is more important to notice is that these cells, when taken alive from fresh tissues, and preferably from young growing tissues, are composed of a semi-transparent, colourless material, looking like thin gum into which small transparent granules have been stirred. The substance which has this appearance is protoplasm, and is the living part of the cells of all animals and plants. Animals and plants are alive and growing ; their protoplasm is alive and growing ; and we know protoplasm only as a living substance. Chemical analysis kills it, and dead material is not living protoplasm. We know that what we call protoplasm is a mixture, not a single chemical substance. It is alive, and therefore constantly building up food-materials into itself ; constantly breaking down part of itself in the process of doing the work of living ; constantly, again, forming substances like cell-walls,, like enamel, or wax, or horn, which are derived from protoplasm, but are not protoplasm. Thus protoplasm, as we look at it under the microscope, and as we must carry it in our minds, is a flux of chemical materials, some of them food in various stages of the process of building up into living, substance, some of them broken-down, waste products from the living material which has been used up,— and some oft them substances manufactured by the living material. To see it one chooses young growing cells, for in older cells the living material frequently is obscured by the various substances it has made.

Protoplasm, then, is not a definite chemical compound, but a jelly-like substance one can see with the microscope when one knows where and how to look for it. However, it can be shown that protoplasm consists chiefly of the chemical compounds known as Proteins. The general composition of these bodies is —

Oxygen from 20-9 to 23’5 per cent.

Carbon „ SI’S „ 55’S „

Besides Proteins, protoplasm always contains a large bulk of water, small quantities of carbo-hydrates and fats, and traces of iron and of phosphates and sulphates of potassium, calcium, and magnesium. Hence it is probable that, if in the material called protoplasm a special chemical compound be present, the molecules of this chemical compound are much more complex than the molecules of Proteins.

Protoplasm is dissolved by weak acids or alkalies and by solutions of various salts, though in this process of solution it is destroyed, and cannot, like a simple chemical substance in solution, be recovered unaltered by precipitation or evaporation. Strong alcohol coagulates it, but this again changes it as water is withdrawn in the process. Heating it to about 4o° Cent, coagulates it just as white of egg the best-known protein, is coagulated by boiling. It must be noticed that all these methods kill animals or plants just as they alter the living material of these animals and plants. Different proteins are soluble in different solutions and coagulate at different temperatures.

But without the aid of chemistry enough may be observed about protoplasm to give it a very definite place in the world miafCtS^First °* all> Pr°toplasm has the power of movewhiri f hlS4,Cai! be Seen Particularly well in the delicate cells ! J Tr ^ f!rs 0f many plants- The staminal filaments ' with n { u Virginian sPiderwort) are set thickly th dellcate blue hairs. A newly-opened flower should be chosen, and one or two of the blue hairs must be removed with great care and placed on a slide in water under a glass coverslip. The hairs are seen under the microscope to be composed of elongated barrel-shaped cells disposed in single rows. On careful focusing, the inner wall of each cell is seen to be lined with a layer of protoplasm, thin in

Fig. i. — a. Cell from staminal filament of Tradescantia. The protoplasmic threads are light, and in them are contained the nucleus and chlorophyll granules. The spaces between the threads are filled by coloured cell sap.

b. A White or Amoeboid Corpuscle from the blood of a frog, showing changes of shape undergone during five minutes.

c. Group of Yeast-cells exhibiting active budding.

some regions, heaped up in others (Fig. I, a). In or near the middle of the cell is seen the nucleus, a rounded, more opaque, solid-looking mass. The nucleus is embedded in another irregular mass of protoplasm, and from this to the layer round the cell- wall there pass delicate strands irregularly arranged and often branching and running into each other. In this network of protoplasm may be seen granules of different shapes and sizes ; some are opaque, some readily catch the light if one moves the mirror of the microscope backwards and forwards. When the eye has become accustomed to the semi-transparent protoplasm, it may be seen that constant streaming movements take place, especially in the fine strands running to and from the nucleus. In some of these, the protoplasm and granules are moving towards the nucleus, in others, away from it ; while occasionally in the same strand there are two currents passing in opposite directions. In this restless activity the shape and direction of the strands are always changing, some disappear, new ones arise, the cross connections break down and are reformed in new places, and the whole appearance of the network changes from time to time. Living protoplasm moves, and this movement is the manifestation of a molecular unrest, producing a condition which we may term irritability.

Another kind of movement can be seen in white bloodcorpuscles. If a drop of fresh blood, taken from a newt or frog, be placed on a perfectly clean slide and covered by a thin cover-slip, the microscope reveals the presence in it of a number of large oval cells with regular outline and reddish tinge (PI. IV 2). But among these maybe seen a less number of smaller, irregularly shaped, transparent masses of protoplasm. As the preparation would soon dry by evaporation if unprotected, a film of oil should be drawn with a brush round the edge of the cover-slip. Then, if we watch one of these white or colourless corpuscles, we see that the shape slowly changes (Fig. 1, b). At first, most probably, it appears covered with delicate prickles, which are really extensions of the protoplasm of the cell. Sometimes some of these processes lengthen, become thicker, and even may end on themselves. A group of processes may get pushed out on one side, while those on the other become withdrawn.

oreovei the shape of the whole cell is constantly altering, and the cell itself slowly creeps or moves through the liquid in which it is floating, by the extension of processes on one side and their retraction on the other. Careful focusing shows that, as in the hair-cell of Tradescantia , the protoplasm appears to consist of a clearer substance with granules embedded in it, and both clear substance and the granules share in the streaming movements. In this case, however, the movements, instead of being confined within a motionless cell wall, cause movement of the whole cell from one point to another. This second kind of movement is called amoeboid, and the processes which are pushed out and withdrawn are named pseudopodia. With other forms of protoplasmic movement the student will become familiar, but these two forms may be taken as sufficient to show the fact of movement.

The movements of the protoplasm of Tradescantia , or of the white corpuscles, sometimes cannot be seen when the preparation has just been made, whereas after a few minutes they are very visible. The shock of removal to the glass may have arrested their motion, and slight pressure with the point of a needle on the surface of the cover-slip may again produce quiescence, while activity is resumed if the stimulus has been slight enough to do no damage. Gentle warmth stimulates the movements until about 450 Cent, has been reached, when all movement ceases. Electric shocks, and the application of many chemical substances, accelerate the movements. All these show the second striking fact about protoplasm — that it is irritable, that is, able to respond to stimuli. Besides the stimuli mentioned, a very little observation of the protoplasm in living animals and plants shows that light, food, variations in the fluids, purity or impurity of the fluids in which the organisms are living, serve as stimuli, increasing or diminishing protoplasmic activity. This power of response of protoplasm, however, depends not only upon the presence of external stimuli, but also upon the inherent molecular structure of the protoplasm itself. Without that structure the external agents would be powerless to call forth responses.

Next, protoplasm absorbs food. Sometimes, as with animals, the food consists of the bodies of other animals and of plants : sometimes, as in most green plants, the food is purely inorganic. The details of the nutrition of protoplasm will have to be considered later on. Protoplasm constantly is exchanging gases with the surrounding air 01 water ; but the details of the respiration or breathing of protoplasm must also be treated at length later. As a result of feeding, protoplasm grows. Sometimes, as in the body of an adult, growth is only sufficient to keep pace with the waste of tissue which goes on in all living animals and plants. But in the greatest number of cases growth means actual increase in size. The separate cells, however, do not grow indefinitely large. In growing tissues or in unicellular organisms, after a certain size, which is different in different kinds of cells, a cell on the point of overgrowth gives off a bud which becomes another cell, or which directly divides into two daughter cells. If we examine with the microscope some of the white frothy substance from a brewer’s vat called yeast, we see that most of the separate round cells which have been floating in the sweet nutritious juices in the vat are actively budding (Fig. i, c). This is the simplest kind of reproduction, and in reality is a form of growth.

Finally, protoplasm excretes, or turns out waste products. In the processes of life, substances generally coming from broken down protoplasm are extruded by the protoplasm. Many of these substances are soluble in water, and are turned out in a watery fluid. The actual occurrence of this can be seen beautifully in many single-celled animals. If one takes some pond or ditch water and examines it under a microscope, many small animals and plants may be seen. But if it be poured overnight into a flat pan in which two or three clean glass slides have been placed, a number of single-celled organisms will settle down on the slides during the night. If these slides be lifted out carefully, wiped on the under surface and covered with a cover-slip, one can hardly miss finding a minute single-celled animal, from the spherical wall of which there radiate out a number of slender knobbed pseudopodia. The protoplasm is clear

U.

and granular; there is a small nucleus ; but most obvious is a round spot that looks empty, and that has a diameter at

Fig. 2. — a. A Single-celled Animal (belonging to the Acimiaria or suctorial Protozoa ) with delicate spherical cell-wall, and long knobbed pseudopodia ; within the protoplasm lies a small nucleus and a large contractile vacuole.

b. The same ; the contractile vacuole has disappeared, the contents being extruded : the shrivelling of the cell-wall shows the loss of bulk undergone.

least half the diameter of the whole animal (Fig. 2, a). As one looks at it, it suddenly disappears ; the round disc of the cell becomes shrivelled, and in the water a little whirlpool is seen, as if an oily liquid had been squeezed out (Fig. 2, b). Slowly the spot reappears, gets larger and larger, and bursts again, and in a few minutes it may be seen to fill and empty several times. This spot is called a contractile or pulsating vacuole, and a contractile vacuole is the most visible form of protoplasmic excretion. In most cells the process of excretion goes on slowly throughout the cell, and no special vacuole is seen. But it may be taken for granted that excretion, whether visible or not, goes on in all protoplasm. One must not confound it with the extrusion of solid indigestible

Fig. 3. Protoplasm of Amtvba actinophora. Fig. 4. — Protoplasm of nucleus with part of ganglion cell of an ox. (From Btitschli.)

surrounding protoplasm from particles a process which can be seen in some living cells

especially animal cells.

Protoplasm, then, is the living, semi-transparent, jelly-like su stance composing the bulk of the living cells of animals an plants. It is made up chiefly of water and of the chemiea substances known as proteins, and it possesses the pro-

G,^k°frM0YJement’ Irritabili‘y. Feeding, Respiration, Growth, Reproduction, and Excretion.

_ n?G1 magnification it may be seen that many of the granules appear like the crossing points of a fine mesh-work,

I and that protoplasm apparently consists of a firmer meshwork, the strands of which are refractive, and a more fluid substance which fills the interstices of the mesh-work (Figs. 3 and 4). Professor Biitschli has shown recently that an artificially produced very fine froth or foam exhibits under the microscope precisely the same appearance (Fig. 5). In his artificial foams the minute bubbles consisted of tiny drops of a kind of soap in solution, each surrounded by a thin film of oil. When a drop of this foam is brought in contact with

Fig. 5. — Foam of Olive Oil and Carbonate of Potassium, very viscous, much magnified and pressed into fibrils by the cover-slip. The upper figure is a higher magnification of part of the lower figure. (From Btitschli.)

water, streaming movements, closely resembling the movements in protoplasm, are produced, and the drop of foam moves slowly through the water. It is therefore quite possible that the network of protoplasm really is a fine froth, the strands being a fluid surrounding droplets of another fluid — the apparently more liquid part of protoplasm.

There are several theories as to the physical constitution of protoplasm, but it would serve no useful purpose to further consider them here,

Chapter II The Building Up Of Protoplasm

PROTOPLASM and substances formed by it are the material of the tissues of all animals and plants ; and all animals and plants, in consequence of this, have the powers of MoYement, Irritability, Assimilation, Respiration, Growth and Reproduction, and Excretion. The individual cells of the animal or plant bodies may be built up into complicated tissues and organs which serve special purposes, and in the elaborate systems of higher animals and plants these tissues and organs may assist, or regulate, or interfere with each other’s work. But, in every case, the actual work done is done by individual cells, and the result that we see is the combined work of the large number of individual cells present in the organs. For instance, the hands may take food to the mouth, the teeth chew it, the muscles of the tongue and mouth and gullet force it down ; but it is ultimately the individual cells lining the intestines that absorb and really assimilate the meal. Similarly all the powers of animals or plants can be traced down to individual cells, down to protoplasm itself.

Practically, then, the living world is protoplasm. In the processes of life, protoplasm is constantly being used up. A starved organism loses in weight, and even while food is abundant, protoplasm is constantly undergoing change and being broken down. When an animal or plant dies and decays, protoplasm is destroyed. How, then, is this substance built up from the inorganic materials in the world ?

If we consider the food supply of the land, it is clear that flesh-eating animals practically only turn the protoplasm of

ii their prey into their own protoplasm, and that their life is dependent on the life of other animals. Animals with an omnivorous diet live partly on animal protoplasm, partly on the tissues of plants. Other animals live entirely on plants. If, seeking an answer to our problem, we turn to plants, we can dismiss many which, like moulds and fungi, live on living or decaying organic matter, and we are left with the green vegetation of the earth. The food supply of the sea is less easy to understand. The vast majority of marine animals are predatory. Most seabirds are fish-eaters ; most fish live on other fish, or on small swimming animals like the larvae of Crustacea which abound in the waters. Others live on the shellfish of the bottom, or on worms and anemones and coral polyps. Most of these lower forms of life are themselves carnivorous, and at first sight it seems as if there was no answer to our problem. The amount of floating seaweed or the seaweed round the coasts is not nearly enough to replace the vegetation of the land. Some debris comes down from the land ; the sewage in the Thames, for instance, ultimately supplies food for the rich fishing stations off Yarmouth or on the Dogger Bank ; the sewage of the Clyde fattens the herrings of Loch Fyne. But the greater part of the open sea is not supplied with organic remains from the land. If, however, we examine a bucket of surface water, there are to be found in it innumerable microscopic plants, and these, like the green vegetation of the earth, form the first stage in the building up of protoplasm ; and, like most plants with which we are familiar, very many of these minute plants have a green colour. The starting-point of the food supply of land and sea is green vegetation. The formation of protoplasm from inorganic materials depends on the substance to which this green colouring is due, and which is called Chlorophyll (green of leaf). If some green leaves are soaked in alcohol, the green colouring matter is dissolved and forms a clear solution, bright green in colour. This solution is fluorescent, for it is green when the light shines through it, but the colour appears red under a strong reflected light.

When examined under the spectroscope its spectrum is seen

THE BUILDING UP OF PROTOPLASM 13

to give strong absorption bands in the red and oiange end, but only a faint and diffuse absorption in the blue and violet. There probably is a connexion between this large absorption of thermal energy and the striking functions of chloiophyll. Every living cell containing chlorophyll in the presence of sunlight performs chemical work. It absorbs carbonic acid from the air, partly dissociates the carbon and oxygen, returning the liberated oxygen to the air. The carbon is not retained as pure carbon, but, probably in the act of being separated from the carbonic acid, is associated with hydrogen and oxygen in the form of a simple carbohydrate that is, a compound of carbon, hydrogen, and oxygen, in which the hydrogen and oxygen are in the same proportion as they are in water.

This leads to another striking fact. Carbon combines readily with oxygen, and in the process sets free energy in the form of heat. What then takes place in plants, by the agency of chlorophyll, is a turning of radiant energy of sunlight into potential energy ; the radiant energy is stored up in the form of a chemical compound of such a kind that, by union with free oxygen, it will liberate the energy again.

From one point of view plants and animals, or the protoplasm of which they consist, may be regarded as centres of force, as things capable of doing work ; and here, as the secret of their food-supply, as the first stage in the building up of protoplasm, is to be found a supply of energy, a means by which the radiant energy of sunlight is stored up in a form which can be used. The plants which possess chlorophyll store up the energy : the animals which feed upon plants use this store for their own lives, but retain enough in their own bodies to serve for the carnivorous animals which eat them. Carbon or Carbohydrates, when supplied with oxygen, form carbonic acid, and give out energy as heat. Although the chemical details of the processes of life — what is called the metabolism of protoplasm — are very complicated, it may be said generally that protoplasm takes in oxygen, performs the work of life, and gives out carbonic acid ; and that it is enabled to do this by the capacity chloro-

*4 phyll has for absorbing the energy of sunlight and storing it up in the form of carbon compounds with less oxygen than the proportion in carbonic acid (C02).

Speaking generally, dead protoplasm, at the moment of its death, has approximately the same composition as living protoplasm. But yet dead and living protoplasm are very different things. Dead protoplasm was once living protoplasm, and but a moment before was protoplasm in the full play of the energies of life. Yet it is incapable of doing what living protoplasm can. Left to itself it gradually undergoes dissociative changes, it becomes less and less in bulk and ultimately disappears, all except a mere trace of mineral dust. But it is not destroyed. For in the course of its disintegration it evolved heat, carbonic dioxide gas, water, and some ammoniacal or other nitrogenous body in one form or another. There has been a transformation but not destruction. There is no such thing as destruction in the literal sense. The living world, like the universe, is a seat of constant change, not of creation or destruction. Nothing is new, nothing is lost, all is change and recombination of old materials and existent forces. Out of inorganic matter protoplasm is built up, into inorganic matter it becomes dissociated as the normal function of its living activities, and to inorganic matter it returns as the transient destiny which follows its death.