The Life of the Plant

insignificant impulse to start them ; those of the latter kind on the contrary require an external supply of energy, which can be absorbed in the process. Most of the phenomena of chemical combination fall under the former of the two categories ; most of the phenomena of chemical decomposition fall under the latter. Burning, i.e. combining with oxygen, may serve as the simplest example of chemical combination. The converse phenomenon of unburning, the withdrawal of oxygen from a body, which is called reduction, may serve as the simplest example of decomposition.

We can try to demonstrate what takes place during chemical combination and decomposition, i.e. during combustion and reduction, by means of the following simple model consisting of two little balls of lead at the end of thin threads (fig. 41). The cause of any chemical combination lies in the fact that chemical substances are endowed with a tendency, a kind of gravitation, towards each other. We call that tendency chemical affinity. Particles of carbon and oxygen tend towards each other in the way these balls a and b do if I move them apart and leave them to themselves. But we know that the impact of one body against another produces heat and some- times light. Heat and light developing from this impact, i.e. from invisible collisions between particles Fig. 41.

and particles of oxygen, are indeed the heat and light that we observe in a burning flame. We thus perceive the cause of spontaneous chemical combination, and the reason why it is accompanied by the development of heat. In combining, chemical elements are only yielding to a mutual attraction, just as these balls are, but having knocked against each other they are warmed, and give off heat. The phenomena of decomposition are quite differ- ent. In order to decompose a chemical compound we must expend a certain amount of energy. In our illustration the same quantity of energy is required to drive the balls away from each other as will be mani- fested at the impact of the bodies against each other, when I withdraw my hands. This equality in the quantity of energy employed in decomposition and liberated during combination is easily illustrated by our mechanical contrivance. In order to move one ball away from the other I must raise it, must over- come the force of gravity. I estimate the quantity of energy expended in this instance by my work, and this work is measured by the product of the weight of the ball multiplied by the height to which it is raised. But just at the moment when the balls hit each other, the falling ball is endowed with energy sufficient to raise to the same height a ball of the same weight. We draw this conclusion from the fact that if it had not come into contact with the other ball it would have swung like a pendulum to the other side, and raised itself to the same height, i.e. it would have raised its own weight to the same height from which it has just fallen. In the same way, in order to break up a compound, to overcome the affinity of two chemical bodies and separate them, it is necessary to employ the same amount of energy as was liberated at the moment of their combination. If a certain amount of carbon burning in oxygen gives off, say, one thousand units of heat, then in order to set free this carbon from the carbonic acid which is formed, and overcome its affinity for the oxygen, we must of necessity employ the same one thousand units of heat. We can, indeed, decompose carbonic acid, i.e. liberate its carbon, as we

have seen, only by exposing the compound to the high temperature of burning magnesium. However, this case of the decomposition of carbonic acid by means of magnesium cannot serve as an example of a simple process of decomposition, because a com- bination of magnesium with oxygen takes place at the same time. For a long time chemists thought that the decomposition of such stable compounds as carbonic acid and water could not take place without the co-operation of a third body, possessing a still stronger affinity for oxygen, but they have comparatively recently come to the conclusion that the action of heat alone is sufficient to cause the decomposition, or the dissocia- tion of carbonic acid and water. According to modern physics heat is a kind of motion — a rapid, invisible, but palpable vibration of the particles of a body. In heating any compound body to a very high tempera- ture, we bring its particles into such a state of vibration, and loosen them to such an extent, that their mutual coherence finally breaks down, causing decomposition. To give an example : at a very high temperature water vapour does not exist any longer as such, but as a mixture of hydrogen and oxygen.

During such a process of decomposition energy is absorbed : but what happens to this energy ? It can- not disappear — that would contradict the law of the conservation of energy. It passes during the process into a latent condition of tension. Every one is familiar with instances of energy stored in a latent state among everyday mechanical phenomena : a sledge-hammer ready to fall upon a stake which is being driven into the soil ; a bow bent ready to shoot an arrow — these are simple cases of energy stored in the form of tension. This expression sounds curious, however, when applied to light or heat. Can such forms of energy as light or heat be stored ? Could I, for instance, get hold of a certain amount of light or heat, such as that evolved

during the combustion of magnesium wire, and keep it, say, until to-morrow ? I not only can, but I have already done so. When I dipped a burning wire into a vessel of carbonic acid, I employed a certain amount of this energy in the decomposition of carbonic acid with the liberation of carbon. I can burn this carbon to-morrow ; or I can bequeath it to remote posterity, and they by burning it will enjoy the light and heat that we store to-day, in using them to decompose carbonic acid.

It follows that carbon, like any other combustible material (wood in the stove, food in our body), although naturally free, always tends to combine with oxygen, and therefore represents a store of energy ; and, in every chemical process in which a non-combustible body changes into a combustible body, there is a storing up of energy. In the end we come to the conclusions : (i) that the decomposition of carbonic acid and the setting free of carbon can only take place on condition that some external source of energy is employed in the process ; and (2) that the energy so employed passes into a reserve form.

Having these facts in view let us return to our leaf. A precisely similar process takes place there. Out of non-combustible carbonic acid combustible starch, wood, etc., are formed. It is clear that this process cannot take place without the co-operation of some external source of energy. Indeed, as has already been said more than once, the decomposition of carbonic acid takes place only under the influence of light ; the activity of the leaf does not begin until a ray of light falls upon its surface. This ray of light is indeed the energy by which the decomposition of carbonic acid is brought about, and which is absorbed and stored in the process.

To make matters still clearer, let us compare the phenomena of light with those of heat. We have seen that heat Is motion, which by loosening the particles of a body causes its decomposition. But light is also motion, a regular, undulatory form of motion. The following rough comparison will help us to explain the decom- posing effect of light. Suppose two light bodies, say two wooden balls, float side by side on a smooth surface of water. We throw a stone into the water near them. Circles will radiate from the stone, and every time a new wave passes under the floating balls it will separate them, will break the connection between them, driving one of them up to its crest, and plunging the other into Its hollow. To this stone, which produces the circles in the water, we may compare the sun, with the waves of light continually running from it and diverging to infinity ; with the sole difference that these waves travel about 190,000 miles a second, and are so fine, and follow upon one another so rapidly, that on the average 50,000 of them are included within a single inch.

These waves, following upon one another at an almost inconceivable rate, come into contact in the leaf with still smaller atoms of carbon and oxygen, combined in carbonic acid, loosen them and break up their combination. The oxygen is set free while the carbon immediately enters into combinations of another kind. The first of these new compounds that we recognise under the microscope is starch. We have just seen how the heat and light of the burning magnesium can be stored. The same is true of the rays of the sun. We cannot simply seize and shut up a ray of sunlight ; but we grow plants for the purpose, and these not only extract carbon from the air by means of their leaves, but together with the carbon absorb and store, concealed in it, the sun’s rays. It is the rays of the summer sun which warm us in our wood fires ; it is again the same rays that give us light in our candles for our long winter evenings.

The form of the leaf finds its physiological signifi- cance in this function of light absorption. For this the flat form is more efficient than any other. The area of the light-absorbing leaf-surface is in some plants eighty times as large as the area of the soil they cover. It is only now that we can fully appreciate the significance of the processes that take place in the leaf. In the first place there is the assimilation of one of the most important elements that enter into the composition of the plant — carbon — and at the same time a transformation of inorganic into organic matter. As we have already said, all organic matter in plants and animals comes directly or indirectly from the leaf ; while the process of its manufacture in the leaf links up the whole organic world to the sun. The leaf serves as intermediary between any mani- festation of energy in the organic world and the sun, the universal source of energy. It is not the plant alone, but also the whole of the animal world, man included, that profits by the energy of the sun stored up by the plant. We have seen that a seed gets warm during germination. This heat is derived from respira- tion, the combustion of some part of the organic matter bequeathed to the seed by the parent plant. In the formation of this organic matter, the sun’s energy has been employed. The seed, therefore, germinating in the soil, profits by the heat of the rays of the sun absorbed by the parent plant. Similarly, when we use organic matter as food, we also take in the rays of the sun latent in it, and use them to warm our bodies or set them in motion.

This means that the leaf, the only natural laboratory where organic matter for both animal and vegetable kingdoms is prepared anew, in this assimilation of car- bon stores up the energy of the sun’s rays, thus becom- ing the source also of energy, the retailer of heat and light to the whole organic world. We have spoken in general terms till now of car- bonic acid being decomposed in the leaf, of the rays of the sun being absorbed by the leaf, and so on ; but we can express ourselves much more definitely. In speaking of the assimilation of carbon in contradistinc- tion to the assimilation of other nutrient substances, we can determine quite clearly the microscopic seat of the process. It is the green chloroplast. We can show that certain of the sun's rays are really absorbed by chlorophyll, and that it is just those rays which are absorbed which bring about the decomposition of car- bonic acid, the initial stage in the assimilation of carbon, and also the formation of starch, its final stage. The green colour, which depends upon the peculiar absorption of light by the chlorophyll in the chloroplast, is thus not an accidental property of the plant, but is closely bound up with the most essential process of its nutrition. It is not the leaf as a whole, but the chloroplast that colours it green, which serves as a connecting link between the sun and all things living upon the earth . 1

We have now studied the function of the green leaf. Plants without green organs are unable to manufacture organic matter for themselves out of carbonic acid, but are obliged to live at the expense of organic matter made by other plants. Fungi, for instance, the plants we generally call mushrooms and the microscopic moulds , 1 The necessity for maintaining in due proportion the different parts of this course prevents me from working out as fully as it deserves this most interesting chapter in the physiology of the plant. Those who wish to study more closely this side of the life of the plant will find a more detailed exposition of the subject in a chapter appended to the course entitled The Plant as a Source of Energy, which in its turn presents a popular exposition of the principal results of my special work On the Assimilation of Light by the Plant, and of my further researches in the same direction. These facts are set forth in still greater detail in a lecture entitled The Plant and the Energy of the Sun (in my Lectures and Addresses, Moscow, 1S88), and in my Croonian lecture The Cosmical Function of the Green Plant. — -Proceedings of the Royal Society, 1903.

can exist only on soil already containing organic matter : every attempt to grow them in a medium devoid of such matter proves fruitless. Plants feeding on other plants by attaching themselves to their stems or roots belong to the same category : e.g. the broom-rape , which grows on the roots of hemp ; the dodder ( Cuscuta ), which twists round the stems of hop, flax, and clover, and clings to them until it finally exhausts them alto- gether. All these plants have either ugly scales of some other colour than green in place of leaves, or else do not possess leaves at all. They are therefore incap- able of independent existence, but suck the sap of other plants. Such plants are called parasites . All of them, and especially the minute fungi, which cause various diseases in plants, give farmers much trouble and frequently rob them of entire crops.

If we consider the plant exclusively from the point of view of nutrition, we are entitled, as has already been said, to see in it simply two strongly developed surfaces adapted to the twofold medium in which the plant lives. These are the root-surface and the leaf-surface : the former, being adapted to the solid medium, the soil, is developed specially in length, because the root must come into contact with the greatest possible number of particles of the soil ; the latter, being adapted to the absorption of atmospheric particles and, especially, to the absorption of the light that falls upon it, is developed especially in one plane. Owing to such ail arrangement, under favourable conditions scarcely a particle of the soil can escape the root, nor a single ray of sunshine be lost to the plant.

The substances absorbed by the root and the leaf are totally different, but at the same time are equally necessary to the plant. Evidently the existence of each of these organs, the very existence indeed of the whole plant, requires that there should be constant intercourse between them. The organ joining the two surfaces, which bears the leaves and serves as intermediary between them and the root, is the stem. As an intermediary this stem is not an organ so essentially necessary to the plant as the root or the leaf, and it is therefore sometimes very poorly developed ; but where, on the contrary, it is well developed it plays the most prominent part in determin- ing the general aspect of the plant, and, in fact, the

aspect of the whole vegetation of a locality. Everybody knows, for instance, the meadow plantain, consisting of a bundle of leaves gathered into a rosette and lying almost flat upon the ground. In this case the stem is scarcely developed at all, and this is why the leaves are so closely brought together. Something of the kind, only on a larger scale, is illustrated by the American Agave, which grows out of doors in southern Europe, and also in our greenhouses here in the north. That plant consists simply of a bundle of very large, fleshy leaves almost seven feet in length, which once in ten years throw out a flowering stem like a huge candelabra about twenty- eight feet high. We find a stem very slightly developed also in a certain extremely curious African plant. Picture to yourselves a vast expanse of barren steppe, on some parts of which are what appear to be stumps or logs scarcely rising above the soil, and slightly hollowed like funnels with little furrows across them. On both sides of the stump, from the furrows at its edges, there stretch two broad strap-like objects from four to seven feet in length — coarse and leathery, at first greenish in colour but turning brown at the edges, and torn into narrow strips — looking, in fact, quite tattered. Here and there at the edges of the stumps there grow small branches with minute cones like those of the fir-tree. This is Welwitschia, surnamed mirabilis, wonderful, on account of all its remarkable characters. The significance of its various parts is as follows : the stump, always half-buried in the soil and merging gradually into the root, is the trunk of this tree ; it is seldom more than two feet in height, although the plant itself may live to be a hundred years old.

The two tattered shreds described above are a pair of leaves which the plant keeps during the whole of its existence ; dying at the edges they gradually grow from the base, and reach a very great age. plants to the tall graceful palms which Endlicher called Principes , i.e. the princes of the vegetable world. Their trunks grow upwards as straight vertical pillars with a crown of leaves at the top like the very columns for which it is believed they served as model. But the trunks of the palms represent only a one-sided develop- ment — development in length ; they are very tall and graceful, but they do not usually branch or increase in thickness. A totally different aspect as well as the greatest development in size is seen in the trunks of our broad-leaved trees and our fir-trees. They, throughout their existence, increase in thickness and throw out branches, and may thus reach very great dimensions. Thus, for instance, within the circle of the bark of a Californian Wellingtonia, there would even be room for dances ; a small chapel has been fitted up within the hollow of a huge chestnut tree on Mount Etna ; while travellers tell us that whole caravans find shelter under the green shade of the baobab. Although such giants do not exist in Russia even in the forests, we can still find hoary denizens of the past like the oak of Kunzewo. Its mighty trunk, of the thickness of four horse-girths, rises from the bottom of a deep ravine, while its summit towers above the lime and aspen trees that are crowded all along the edges of the ravine.

Such are the dimensions that can be attained by a stem in the fulfilment of its destiny, bearing a canopy of foliage, the large leafy surface for the absorption of the rays of the sun ; and one cannot fail to see how well it is adapted to this purpose. We have only to remember the partial obscurity which reigns in a pine wood, even on a sunny day, to realise that the needles must be distributed on the stem in the most advan- tageous way if, in spite of their insignificant breadth, they are to arrest as many rays of light as possible. Indeed, although the distribution of the leaves on a stem seems at a first glance entirely hap-hazard, a closer investiga-

tion reveals remarkable regularity in their arrangement. The first person who called attention to this fact was, I believe, the famous Leonardo da Vinci ; but it was not until the nineteenth century that the phenomenon was studied in any detail by botanists. This regularity of distribution is seen mainly in the way the leaves are distributed on the stem, so that as far as possible they neither screen nor shade one another, and at the same time leave no free spaces through which the rays of the sun may pass to no purpose. This statement can be verified by a mere glance at the rosette of leaves on the plantain. They alternate in such a way that only the ninth leaf covers the first ( i.e . the very lowest) , Cer- * tainly the more leaves are separated the one from the other the less they shade one another ; but a great development of the leaf system is only possible when the stem has reached certain dimensions. In most cases this can be attained only at great expenditure of building material, because in order to bear a great number of leaves the stem must be very steady and firm.

There are plants, however, which do produce a great number of leaves and reach a great height, while at the same time economising their building material. These are the climbing plants, the thin delicate stems of which select other plants or inanimate objects as supports. Twisting and turning round these they climb to a considerable height, and produce a large mass of foliage which they could not support unaided. Such are, for instance, the hop, the bindweed, the ivy, and many other plants growing in tropical forests and known under the general name of Hanes.

In general the stem has a twofold function : it must bear leaves and conduct the nutrient sap from the root to the leaf and from the leaf to the root. For this purpose it must evidently be equipped with something that will give it solidity, firmness, elasticity, and other mechanical properties ; but at the same time it must have some system of canals, or some other kind of passage for the conduction of saps. In order to understand how the stem proceeds to serve these purposes, we must study its structure and, first of all, the structure of the actual cells, the bricks, as we called them in our first lecture, out of which the plant is built up.

If we make a very thin transverse section of wood, we at once notice that it is perforated with very minute holes. These are the cavities of cells. We have already studied the inside of the cell, and its chemical contents. In the present instance, we are mainly interested in it as building material from the mechanical point of view ; and in this connection the main part is played by its * solid skeleton, its wall, upon which the entire form of the plant depends.

When isolated the cell is for the most part spherical in form ; when connected with other cells, as is the case in tissues, this spherical form passes into the polygonal (as is represented on fig. 42, 1) . A polygonal form shows that throughout its life-time the cell has uniformly developed in all directions, whereas if it develops mostly in two directions, i.e. along two axes, a flat tabular form will result. Such tabular cells are formed mostly at the surface of organs ; they constitute the epidermis, of the plant (fig. 42, 2). Lastly, the cell may develop almost exclusively in one direction, along one of its axes only. Then instead of a polygonal or a flat, tabular cell, a very narrow and elongated fibre appears, (as is represented on fig. 42, 5 and 6). Such long fibres form most of the wood of trees ; but even they are not the very longest, and the fibre-like cells of flax, for instance, which are used in spinning, are sometimes a thousand times longer than their own diameter, so that we could only represent them here on their true scale by a single line. Cells not only vary in general outline and external appearance, but the very wall of

the cell as such may vary widely in structure ; it is either uniform and thin (as in fig. 42, 1 and 2), or thickened everywhere in concentric layers (fig. 42, 3 and 5) ; or, lastly, these inner layers may not be uniformly deposited on the cell-wall, but only at certain places, thus forming the most curious patterns. For instance, if only small parts of the cell-wall remain unthickened, the cell as a whole instead of being uniformly transparent will appear spotted, and in transverse section will be found canals perforating the thickness of the cell-wall and corresponding to these spots (fig. 42, 1 and 3) . It is

at the same time curious that the canals of neighbouring cells generally correspond, so that these spots, pores, or pits, as they are called, are simply points where adjoining- cells are divided only by the very thin primary membrane through which the sap can easily diffuse. Sometimes even this extremely thin cell-wall disappears, and the cavities of neighbouring cells are brought into open communication with each other. On the other hand, if most of the cell-wall remains unthickened, the thicken- ings appearing on the Inner side will present the most varied forms of nets, rings, spirally-twisted bands, etc. (fig. 42, 4 and 7). Such varieties of form in thickenings and pores are generally described and enumerated with special care by micrographers. To physiologists, on the other hand, form as such, however curious it may be in itself, does not present any interest as long as its significance, the part it plays in the life of the plant, is unrecognised ; and this is precisely the case with regard to the different forms of thickening. It is only comparatively recently that attempts have been made to explain these structures from the physiological point of view. We shall briefly return to this matter a little further on.

These two considerations, viz. the general outline of the cell and the structure of its walls, do not, however, exhaust the ways in which cell structure may vary. Cells can also become fused into more complicated organs, generally known as vessels or tubes. These are generally formed by the perforation or the total dis- appearance by absorption of the transverse partitions between vertical rows of cells. For instance, if a row of cells with spiral thickenings lose the transverse partitions which divide them from each other (fig. 43, 1, on the left), a continuous tube called a spiral vessel will appear (fig. 43, 1, on right). Sometimes, however, as we have said, vertical rows of cells, in transforming themselves into tubes, do not entirely lose their transverse partitions,

but communicate with each other by means of pores, larger or smaller as the case may be. One form of such tubes is very curious : the transverse walls of the com- ponent cells are perforated with minute pores and form a sort of sieve, and the cells themselves are called sieve- tubes , One such cell fused with two others into a single tube is represented in fig. 43, II. The contents of these cells can communicate through their pores. Very minute grains of starch have been observed in these pores, and we shall soon see how important is their physio- logical significance. As well as the vessels andsieve-tubes, which are long and straight tubes, we come across others which branch and inter- weave and form a whole complicated network of com- municating canals. Such tubes generally contain white or sometimes yellow sap ; hence their name of latex tubes. Plants containing latex are known more or less to everybody ; e.g. the dandelion and the poppy which exude a white sap when slightly wounded ; the blood- wort, probably known to us all since our childhood, from the injured stems and leaf veins of which a yellow sap escapes ; lastly, the common Ficus, which is grown indoors with us, and which, like some other tropical plants, secretes an abundant latex, known under the name of india-rubber when it is dried. These different kinds of latex are contained in a complicated system of branching and interwoven tubes to be found all over the plant, but especially in its rind and leaves.

into three groups, to which different functions are mostly, if not exclusively, natural. These three groups or categories are the following : cells proper, fibres, and tubes. In the cells the processes of nutrition take place, i.e. the formation and transformation of the nutrient substances. They contain the chlorophyll ; in them the stores of albuminous matter are deposited, also starch, sugar, crystals of mineral salts, and so on. The cells are the laboratories and warehouses of the plant. Fibres serve mainly for mechanical purposes, their contents being of no value whatever. Their chief end is served by their elongated form and by their walls, which are sometimes so much thickened as to entirely fill the cavity (as is shown in fig. 42, 5). Recent investigations have shown that the material out of which these mechanical elements are formed, as well as their structure and especially their distribution in the stems, render them wonderfully well adapted to their function, which is to impart to the various parts of the plant the necessary firmness and elasticity with all possible economy of building material. Investigation has proved, for instance, the remarkable fact that the material out of which these fibres are formed is almost as strong as iron in some respects, and that the fibres are distributed accord- ing to the laws of engineering. The tubes, belonging to the third category, serve mainly as sap conductors.

Let us now see how these elements, so various in structure and function, are distributed in the plant. Cells proper form a connective or fundamental tissue, i.e. the foundation of all organs connecting all their various parts together, while fibres and tubes (or vessels) are grouped together in strands, called vascular bundles, which pass through this funda- mental tissue. We see this best of all in leaves. The middle part of the leaf between the upper and under epidermis is occupied, as we already know, by the fundamental tissue, through which run veins or nerves.