The Life of the Plant

In order to discover how the root fulfils its function, we must begin by studying its structure. In external appearance a root belongs to one or other of two clearly differentiated types. 1 1 either grows vertically downward as a single continuous trunk which gradually tapers to a fine thread, e.g. the beet, the carrot, flax ; or else it branches at once, almost at the surface of the soil, into a tuft of thread-like fibres, such as those of our cereals— rye, wheat, and so on. Roots of the former kind are called tap-roots ; of the latter, fibrous-roots. These two extreme types have many modifications, but they underlie all the variety of forms presented by this organ.

Every root, whether tap-root, single fibre, or lateral root, grows in the same way as the stem, by elongating and expanding at the apex. But we notice a great difference when we compare the apex of the stem with that of the root. If we remove all the leaves of a bud, thus laying bare the apex of the stem, the so-called ' growing point,’ we find that it is the youngest and also the most tender part of the stem, consisting of minute, undeveloped cells. If we examine at the same time the apex of the root, which is generally bare already since there are no leaves to remove, we sometimes see with the naked eye, but better with a lens, and better still with a microscope, that it presents an untidy and ragged appearance. It seems to be covered with a cap consisting of several rows of cells, the outer of which have already lost coherence, and only stick together by means of a kind of mucilage. This cap is nothing but the external dying layers of tissue, which cover and protect the young and tender tissue of the growing point lying under them (fig. 32). 1 Sometimes this cap can be pulled off the root-tip like a glove. The physio- logical purpose of this organ is easily understood ; it

1 Fig. 32 shows under a low power the growing point of the root with its cap. serves as a shield, under the cover of which the delicate growing point of the root pushes its way into the soil. If the growing point of the root ended with the unpro- tected youngest cells, it would evidently not be able to fulfil its function ; it is only by pushing forward the cap that it is able to force its way without serious harm through the hard, rough, and sharp particles of the soil.

At a short distance from the very tip, protected by the cap, the whole external surface of the root is covered with long thin hairs (fig. 33) d Every such hair is simply a very much elongated cell of the surface layer. Further from the tip this belt of hairs comes to an end ; there the surface of the root is protected by a hardened outer layer (fig. 33) which has lost its hairs. Still further up this layer becomes torn, and is replaced by another protective tissue like that which covers the stems of trees, and which botanists generally call cork ; like all cork it is impervious to water. Thus the root is differ- entiated into three zones : the cap at the very tip of the root, then a belt of hairs, and, lastly, the oldest part with a dried skin and a corky tissue. This oldest portion cannot absorb water and nutrient substances ; the very tip is also unable to absorb, or at any rate absorbs inadequately, a fact which can be proved experimentally. The absorbing surface of

1 Fig- 33 shows a young root (A) covered with hairs; a similar root with particles of soil sticking to the hairs (B) ; an older branched root with old parts already without hairs (C) ; and part of the transverse section of a root under the microscope, showing the structure of the hairs and their adhesion to the particles of the soil (D) . the root is in fact confined to the belt of hairs, which are in the highest degree permeable to water, far more so than the tissues composing the aerial parts of the plant.

The root is especially interesting to us as an organ of absorption, and it is important from this point of view to form some idea of its length, and also of the extent of its absorbing surface. A mere glance at the root of any plant which has been thoroughly washed and freed from particles of soil shows us how consider- able its full length must be if all its numberless branches and fibres were joined end to end. But the boldest conjecture falls far short of the reality. A German scientist undertook the following laborious task : provid- ing himself with a pair of forceps, a measuring scale, a pair of compasses, and an almost inexhaustible stock of patience, he actually measured the length of a wheat root down to its minutest ramifications. The result obtained was astounding. It turned out that the total length of the root was 520 metres. However consider- able that figure may be it does not even yet represent the whole length of the absorbing surface of the root. As a matter of fact it is the hairs which present the actual absorbing surface. Let us see how many such hairs the wheat plant possesses — no difficult matter in round numbers. Determining under the microscope how many occur on one square millimetre and then multiplying this by the number of square millimetres in the total surface of the root, we get approximately ten million. If we multiply this number by the average length of the hairs we get the enormous length of twenty kilometres (twelve and a half miles). Such is the path traversed by the wheat root together with all its hairs in the volume of soil contained in a common flower-pot. I said the path traversed by the root together with all its hairs, for in point of fact this figure does not represent their length at any one moment in the life of the plant. All the root hairs do not work simultaneously. Thus

for instance in fig. 33 C only the lower parts of the root are active ; there are no hairs higher up, and there is no need for them there ; in that region they have already done their work, and have exhausted the nutrient substances from the hard particles of the soil. If we now calculate the total surface of all the hairs, together with the fibres that bear them, on the root of a wheat plant during its life-time, we find that this surface is almost a hundred times larger than the area of land allotted in the fields to each wheat plant. If, on the other hand, we calculate the volume of these hairs which cover a length of nearly thirteen miles, we find that they could all be packed into a vessel of the size of a thimble (about 1*5 cubic centimetres).

in its hairs, presents a large surface, although its volume is insignificant, owing to the fact that this volume is stretched out to so extraordinary a length. Here Nature has had recourse to a trick similar to the one attributed by poetic legend to Dido, the foundress of Carthage. Dido obtained for the asking as much land as could be measured by a single bullock’s hide. As it turned out, that same hide was made to enclose the whole future site of Carthage, for Dido cut the hide into very narrow strips. But the strips of Dido are not to be compared with root hairs, which are considerably finer than a human hair.

It is most difficult adequately to realise the great physiological importance of this prevalent development in length. The root is thereby able at the smallest possible expenditure of building material to encompass the greatest possible number of particles of the soil, and to come into the closest possible contact with them. The data we possess are sufficient to calculate approxi- mately the distance of the particles of the soil from the surface of the root hairs of our wheat plant.

For this purpose we must turn to the statistical method, the procedure adopted by statisticians ; they, neglecting the individuality of separate persons, speak only of the average man, the average scale of living, and so on, and describe phenomena in terms of average quantities. Let us follow their example, and try to depict in the most obvious way the volume of soil occupied by an average root fibre of wheat. We know the average area in a field occupied by each plant ; we know the depth of soil occupied by the roots, and hence know the volume of soil occupied by each plant. This glass jar contains that measured quantity. Sup- pose that we wished to transfer this soil to a vessel or rather a tube one-third of a mile in length, i.e. of the length of our root, what would the diameter of such a tube be ? The answer works out to three millimetres.

If the root fibres with their hairs were introduced into a glass tube with this internal diameter, the hairs would touch the wall of the tube . 1 Therefore if all the fibres of the root were distributed quite uniformly in the soil allotted to them, every fibre w r ould occupy just a cylinder of soil, pierced through in all directions by its hairs ; and, consequently, the greatest distance of a particle of the soil from a root hair would be equal to half the distance between the neighbouring hairs of the same root ; this would be something like one-fifteenth of a millimetre. This calculation gives us therefore the greatest distance from which our average fibre must obtain its food, and it gives us an idea of the close contact into which a root comes with the hard particles of the soil. Obviously it is not every root fibre that finds itself in such favourable conditions. I repeat, this is only a statistical average giving us a clear illustration of the perfect absorbing powers of the root. This adaptation is the more perfect in that the root becomes specially developed in those parts of the soil where it finds the most nutrient substances. This fact has been proved in the following way. A plant was grown in a flower-pot filled with alternate layers of fertile and sterile soil. Roots developed very luxuriantly in the fertile soil, but only poor and meagre fibres were pro- duced in the sterile soil. This fact together with the great length of the hairs seems to indicate that roots must themselves go in search of their food, and that the liquid food furnished to them by means of water is generally inadequate. This supposition apparently finds confirmation in the fact that roots of plants grown in solutions or in soil submerged in water have very few hairs, if any at all ; and yet the plants do not seem to suffer.

It is, therefore, sufficiently evident that nutrient substances in a liquid medium penetrate the root very 1 During the lecture this fact was illustrated by means of a lamp glass and its brush. readily, so that it has no need of a particularly large surface. We have said more than once that a root probably obtains food also from the hard particles of the soil ; but how is this to be explained ? The surface layer of the root with its hairs consists of small cells, which have, of course, no apertures in their walls. The particles of the soil can come into very close contact with the root hairs, as is shown in fig. 33 D, but never pierce their walls. How can we reconcile these con- tradictory facts, that solid bodies serve as food to the root and yet do not pass through its cell-walls ? In order to explain this apparent contradiction, we have recourse to the following experiment. A glass jar is filled up to the very top with water, and then covered with a bladder. The external surface of the bladder is carefully wiped with blotting-paper, so that it appears to be quite dry. We scatter some chalk powder on the surface. Chalk is a solid body ; the bladder does not contain any apertures ; and yet we soon find that the chalk disappears from the surface of the bladder, passes through it, and appears in the solution inside the jar. We need not wait until all the chalk disappears from the surface of the bladder, for we possess very delicate reagents for detecting the presence of lime in water. This colourless liquid (ammonium oxalate) , for example, has the property of forming a white precipitate with soluble lime salts. I pour some of it into the water taken out of the jar before the experiment ; there is no precipitate. I take some of the water out of the jar some time after the chalk has been lying on the bladder ; I pour into it some of the reagent, and get an abundant white precipitate, which means that the water already contains lime ; that is, part of the chalk has passed through the bladder. This experiment, which rather surprises us at first, can be very simply explained.

However carefully we wipe the bladder with the blotting- paper, it only seems to be dry ; in reality it is always suffused with the liquid that washes its inner surface, and this liquid was not simply water, but water slightly acidified with acetic acid. Therefore the bladder is moistened with an acid, and acids, as we know, dissolve chalk. The chalk dissolves at every point of contact with the moistened bladder, and this solution passes through the bladder into the jar. All this happens imperceptibly, and so it appears as if a dry solid body passes in some incomprehensible way through a dry bladder. We gather from this experiment that if only cell - walls be moistened with an acid, solid bodies dissolving in that acid can easily pass through them.

Does not something of this kind happen in roots ? In order to prove the possibility of such a phenomenon it is necessary to show that the surface of roots gives an acid reaction. For this purpose we have only to apply to the root a piece of litmus paper, which chemists use for detecting the presence of acids. Under the action of acids the blue colour of this paper changes to red. The root-tips do actually leave a red trace on the blue paper. There are indications which point to the fact that sometimes this acid is actually acetic acid which we used in our last experiment. Moreover, the root, like every other part of the plant, is continually breathing and giving off carbonic acid. This can be proved by an experiment similar to the one demonstrating the respiration of germinating seeds. Now carbonic acid dissolves many substances which are insoluble in water. Here is, for instance, water into which phosphate of lime in the form of a very fine powder is stirred ; two very important nutrient substances, calcium and phos- phorus, have thus been mixed with the water. I pass through this water a stream of carbonic acid, and in a short time the turbidity disappears — the salt has dissolved.

give off carbonic acid ; and these acids must act as solvents on the surrounding particles of the soil, the more so because the hairs, as we have seen, come into the very closest contact with, and almost grow into, these particles (see fig. 33 D). But perhaps instead of all these indirect considerations it would be well to prove by experiment that roots produce such a solvent effect on the solid particles of the soil. Let us take for this purpose a piece of white marble carefully polished — ■ marble in its chemical composition is practically chalk — and let us bury it at the bottom of a flat flower-pot. Then let us plant something in the pot, say a bean. The roots of the bean plant will soon reach the piece of marble, and will spread over it, closely adhering to its polished surface. If in a few days we take out the piece of marble, wash it, dry it, and then examine it, holding it to the light, we shall notice on its smooth surface, which reflects the light, some dull worm-like traces. These are the imprints of the roots, which in adhering to the polished marble have eaten out their image upon it by the action of their acid surface. These impressions are certainly not very deep, but they are nevertheless perfectly clear. 1

There can be no further doubt, after what has been said, that a plant is able to obtain its food from the solid particles of the soil as well as from solutions. This fact can be made doubly sure by the following curious experiment. The root of a plant of suitable age was carefully washed and divided into two tufts, one of which was sunk in water, while the other was buried in soil, and given no water. Nevertheless, the plant has continued to develop. It has absorbed water with the one tuft of roots, while with the other it has taken from the solid particles of the soil the sub- stances necessary for its nutrition.

1 These imprints can be made clearer by rubbing them with powdered graphite. Lastly, there are such plants as lichens, for instance, which settle like froth and scum upon the inhospitable surface of rocks and stones, and even, it is said, attach themselves to the surface of polished glass, destroying these bodies in drawing out of them the mineral food they require. It is a remarkable fact that these plants are distinguished by the abundance of acids in them, especially oxalic acid.

We have now to answer the last of the three questions raised at the beginning of the lecture. Why is it that, among the various substances with which the roots come into contact in the soil, they attract especially those that are necessary to the plant ? Let us study the fact itself more closely before we answer this ques- tion. If a plant is grown in a solution of two salts, say saltpetre and common table-salt, it is soon evident that the root entirely absorbs one of these salts, namely the saltpetre, while it scarcely draws at all upon the common salt, which it does not require. Such facts were formerly disconcerting to scientists ; it looked as if roots could discriminate between the different sub- stances and choose their own food, accepting one sub- stance and refusing another. How, indeed, can such discrimination be explained ? Surely we cannot admit that a root is endowed with will-power or instinct ? The explanation is very simple, and we came across it some time ago. You remember our artificial cell and the behaviour of iron salt towards it (see chapter ii.). Similarly, both saltpetre and common salt easily diffuse, and therefore both will penetrate into the cells of the root, and hence into the rest of the plant. But the subsequent fate of the two salts inside the plant will be totally different. The saltpetre will there be decom- posed, and its nitrogen will serve to form albuminoids

and other complicated nitrogenous compounds : 1 as the result of this transformation fresh quantities of salt- petre will enter the plant, will again be transformed into the substance of the plant, and this process will continue indefinitely. It is different in the case of the common salt. According to the laws of diffusion it will pass into the plant until a solution of equal strength be formed within and without the plant ; then its further absorption will cease. If there should by any chance happen to be more of it inside the plant than outside, according to the same laws of diffusion, the superfluity will pass back again from within the plant to the solution. Now it is clear why just those sub- stances which are transformed and assimilated by the plant, being necessary to it (as is saltpetre in our experiment), are extracted from the solution, whereas those which are useless to the plant (as is the common salt in our experiment) remain untouched in the solu- tion, or, to be more accurate, almost untouched.

It is therefore quite unnecessary to presuppose any rational will, habits, tastes or instinct in order to explain the discriminating property of the root — simple laws of physics are sufficient for the purpose. We must leave the root here and pass in our next chapter to another organ, the leaf. It is obviously impossible to exhaust such a rich subject in one short chapter ; but I think that what we have now learned is sufficient to give us a general idea of the life of the root, which in such a limited space covers a course of many miles, and sucks, eats into, and breaks up the soil with its million hairs, and absorbs from it the soluble mineral substances so sparsely scattered in it — nitrogen and the elements of its ash, those eight elements without which the very existence of the plant is impossible.

1 We are entitled to make this assertion because we can grow a plant by furnishing it with saltpetre as the sole source of nitrogen. In the present lecture let us undertake the study in general outline of the life of the leaf. This task will be a little more difficult and complex than that under- taken in our last lecture, since there appears to be more confusion, among people not conversant with science, about the leaf than about any other plant organ. No other vegetable organ has suffered so much injustice. For centuries, even down to the close of the eighteenth century, people refused to see any direct utility in the leaf. From time immemorial they had admitted as indis- putable the utility of the root as an organ of nutrition, and of the flower and seed as organs of reproduction ; but the leaf continued to enjoy the superficial reputation of a showy but useless ornament : the utmost that people would consent to see in it was an organ for the excretion of noxious vapours. Yet, as we soon shall find, the leaf is as necessary as the root in the nutri- tion of the plant ; moreover, it is the leaf that obtains for the plant what is quantitatively and qualitatively its principal food. It may, indeed, be said that the leaf embodies the very essence of a plant’s life.

The erroneous ideas which prevailed for such a long time with regard to the leaf and its significance are fully accounted for by the peculiarity of the processes of nutrition which go on in this organ. These are totally different, both as regards the nature of the food absorbed and the mode of its absorption, from the nutritive processes of the animal organism, which involuntarily recur to our mind whenever we speak of nutrition. It is owing to this marked difference that these processes form the most characteristic and essential feature of vegetable life.

We already know in part what substances are absorbed as food by the leaf. They must evidently be those substances which enter into the composition of the plant, but are not absorbed by the root. We have seen that among the eleven elements enumerated in our last chapter (the twelfth, silicon, was proved to be useless to the plant), the seven elements of the ash, phosphorus, sulphur, chlorine, potassium, calcium, magnesium, and iron, together with nitrogen, enter by way of the root ; and that water also, con- sisting of hydrogen and oxygen, enters the plant by the same path. There still remains carbon — the foun- dation of all organic substance. So far we have not troubled about it in our artificial cultures of plants, though these contain a thousand times, even many thousand times, as much carbon as the seeds we took for our experiment. Quantitatively carbon forms the most important constituent of the plant (something like 45 per cent.) , yet we did not supply it to the root, but even systematically removed it from the surrounding soil. This means that a plant can live without absorb- ing carbon by its root.

These experiments are not, however, sufficient to tell us the way the plant obtains carbon in the natural conditions of life. To say that a plant can live with- out absorbing carbon through its roots is obviously different from saying that a plant is unable to absorb carbon by its roots, although this mistake is often made. It has not yet been proved that a plant is unable to obtain its carbon from the organic substance of the soil. The discussion of this point would, however, carry us too far afield ; moreover, it is of little interest, because it is easy to show that if carbon obtained in this way takes any part at all in the life of the plant, that part is in-

significant, and therefore scarcely worth our attention. If plants extracted their carbon exclusively or even chiefly from the organic substance of the soil, then soil covered with vegetation, the products of which are periodically removed in one way or another, must in course of time become poorer in humus ; on the contrary, everyday experience shows that soil becomes richer in humus when it is under cultivation, as in corn-land, pasture, or wood. In cultivating our fields we extract from them every year in the form of crops more organic substances than we introduce into them in the form of manures ; and yet soil carefully manured becomes richer in humus. It is clear that ultimately plants do not reduce the amount of organic substances present in the soil, but even increase it ; and therefore they can- not find at all events their principal source of carbon in the soil. But if not in the soil it must be in the air ; and if so, it is probably absorbed by an organ pre- eminently aerial — by the leaf. Let us see what is the source of the carbonaceous food in the air and how it is obtained by the plant.

Together with nitrogen and oxygen, atmospheric air also contains a very small quantity, about three ten- thousandth parts, of carbonic acid. This gas, although colourless and in appearance indistinguishable from air, is a compound of carbon and oxygen. No one will, I am sure, doubt the accuracy of this statement, yet we must see the proof of it, as of every other state- ment, as far as possible with our own eyes ; and it is possible in the present case. In order to detect the presence of carbon in carbonic acid, we must remove the oxygen. This can be done by causing the oxygen to combine with a body having still stronger affinity for it. Such is, for instance, the metal magnesium, the wire of which burns with a dazzling light. I light this piece of wire and sink it into a glass jar containing common air ; the wire burns down and a perfectly

white ash drops to the bottom of the jar : this is magnesia, a compound of the metal magnesium and oxygen. I repeat the experiment, sinking the wire this time into a jar of carbonic acid ; it must now procure oxygen for itself by separating it from the carbon, which will then be set free. As a matter of fact the wire does not burn so quietly this time, but crackles as if so many weak explosions were taking place, and black soot is deposited on the walls of the glass vessel. This is the free carbon.

Great stores of carbon are therefore always present in the atmosphere though in an invisible form. All water in contact with the air contains carbonic acid, and plants which grow submerged in water are thus able to obtain it. It is by experimenting with the leaves of submerged plants that the interchange of gases between a leaf and its environment is most readily demonstrated. Here are several experiments which can easily be made on any clear sunny morning.

Let us gather some leaves and place them while fresh under a glass bell, filled up to the top with water and then inverted in another glass vessel (fig. 34). We fill them in a pail of water in which it is easy to sink both bell and vessel. If we use common water, or still better water through which carbonic acid has been passed, and then set the apparatus in the light, we shall soon notice the lower surface of the leaves becoming covered with a Fig. 34. silvery layer of bubbles. A little

later a considerable amount of gas will be accumulated in the upper part of the bell, while the surplus water will be displaced into the outer vessel, as is shown in fig. 34. with water deprived of carbonic acid. We shall not see any bubbles of gas. We infer that leaves give off gas, but only in water containing carbonic acid. We are astonished at first to notice that bubbles are given off only on the lower surface of the leaf ; but this phenomenon will be explained as soon as we become acquainted with the microscopic structure of the leaf. We notice in every leaf, or rather in its lamina, two different parts, which are readily distinguishable : the nerves or veins , and in between them the ground tissue of the leaf. On closer examination we find that the upper and lower surfaces are covered with a layer of tissue, the epidermis , which is easily peeled off and if we prepare the leaf, by macerating it well in water, we can separate it into three layers, the upper epidermis, the middle part, and the lower epidermis. From this middle part, consisting of the ground tissue and the veins, we can remove the former by carefully beating the leaf with a soft brush and get a beautiful thin transparent network of ribs and veins like a cobweb. We shall consider later the significance of this part of the leaf ; meanwhile let us confine our attention to the ground tissue and the epidermis. The epidermis consists of one layer of cells, distributed in one plane ; the ground tissue is porous and spongy, with spaces of considerable size filled with air. Because of these air spaces leaves float in water ; but if we pump all the air out of them under the water, they will sink, becoming at the same time darker in colour and more transparent ; this is because water has taken the place of air in between the cells. After these preliminary explanations we shall now understand this diagram which shows in a magnified and somewhat diagrammatic form a little square of leafy tissue cut out somewhere between the veins (fig. 35). Here sections in both the transverse and the longitudinal planes are shown, as well as the lower surface of the leaf. The ground tissue consists of two