The Life of the Plant

Throughout its life the cell is continually surrounded by substances which easily diffuse into it. Thus, for instance, the carbonic acid of the air rushes constantly into every cell with which it comes into contact. But if this carbonic acid remained in the cell unchanged, only a very little of it would penetrate into the cell ; however, as we shall see later on, once in the cell it is changed : carbonic acid and water form a carbohydrate, and this transformation leaves room for the entry of fresh

quantities of carbonic acid. Therefore, the two phases of nutrition : the diffusion of nutrient substances and their transformation into the very substance of the cell, their assimilation, are closely related to each other. One process is conditioned by the other : did assimila- tion not take place there would be no more diffusion ; did diffusion not take place there would be no matter for assimilation. Moreover, since by means of such assimilation the substance is transformed into a hardly mobile or even totally immobile form, it does not diffuse away again, but accumulates in the cell.

When we examine the nutrition of the plant from such a general, physical point of view, we get a concep- tion of it quite different from the usual current ideas upon the subject. It is not the plant, nor the cell, which attracts or imbibes nutrient substances ; on the contrary it is the nutrient substance itself which rushes into the cell owing to its inherent mobility. A cell is simply a microscopic centre, where the equilibrium of the surrounding substances is constantly disturbed, a kind of whirlpool, into which these very mobile sub- stances rush in a continual stream, and within which they lose their mobility, are transformed, and become deposited. A vegetable cell is a trap which lets things pass easily one way, but does not let them out again. In this way we come to understand the fundamental feature of vegetable life : increase of mass, accumulation of matter.

As we shall soon see, these general ideas as to the nutrition of the cell will prove to be essential at almost every step in our study of the phenomena of the nutrition of the whole plant. The nutrition of the root by sub- stances in the soil, the aerial nutrition of leaves by the atmosphere, or the nutrition of one organ at the expense of another adjacent to it — to arrive at the explanation of any of these phenomena we shall have recourse to the same fundamental causes : (i) diffusion, i.e. the

property by which substances spread, rush from where they are present to where they are absent, and (2) trans- formation, i.e. the passage of substances from very mobile to less mobile or quite immobile forms. In this way the study of the fundamental phenomena underlying the nutrition of a vegetable cell brings us to the conclusion that they are really phenomena of diffusion, not essentially peculiar to living organisms, but following rather from the general properties of matter. We come to the conclusion that so funda- mental a process as nutrition is conditioned by laws common to both animate and inanimate nature.

Let us begin our survey of the living functions df a plant with the awakening of the seed after its long winter rest under the snow, or at the moment when it is cast into the earth in spring. Probably no other phenomenon in the life of a plant has attracted so much attention as this, its first manifestation. Scientists, philosophers, and poets have alike meditated upon it ; a mystic and poetic veil hangs over it ; we find in it the personification of life itself, the symbol of awakening from dreams and death. There is something indeed attractive, something that stimulates thought in this sudden awakening of activity in- an object hitherto apparently indistinguishable from the rest of inanimate nature. In fact there is something enigmatical about this hidden and arrested life which suddenly bursts forth again. Without indulging in the poetical fancies with which imagination loves to enshroud this pheno- menon, let us try to submit it to strict scientific analysis ; let us try to reduce it, complicated as it is, to its lowest terms and explain the difference between a resting seed and an active one. We may thus discover wherein the very impulse consists which provokes this activity.

Outwardly the renewed activity of a seed is manifested by its swelling and by the consequent rupture of its seed- coat, followed by the appearance first of the root and then of the plumule, i.e. of the stem with its first leaves. These organs develop and increase in size every day. It is obvious that this development must proceed at the expense of some substance, which serves as food for the growing parts. Yet, notwithstanding its rapid growth, it is just at this period of germination that the plant is practically independent of the soil. As a rule germina- tion takes place in the soil ; but here is a brush-like mass of green cress grown on felt, and there are seeds of maize and beans grown on thin gauze net and, therefore, surrounded merely by air and having their root-tips only in distilled water.

On examining germinating seeds such as beans more closely, we notice that while the root and the stem with its young leaves in- crease in size, the first pair of leaves, the cotyledons, be- come wrinkled, are gradually absorbed, and become smaller (fig. 20). This observation may serve as an indication of the fact that the develop- ment of some parts of a shoot takes place at the expense of others. Other seeds, such as grasses, present a somewhat more com- plicated structure than the seeds of beans. If we split a grain of wheat longitudinally, we find under the seed-coat two perfectly distinct parts (see fig. 21 : b r whole grain ; c, d, the separate parts). At the base of the figure, a little to one side, there is a small body which is simply a seedling, an embryonic plant, such as can easily be seen in any germinating seed (see fig. 21 5 , d , e). We notice in it a leaf-bud and the beginning of a root. The remaining larger portion of the seed Is filled with a white, uniform, mealy mass, called the endosperm

(fig. 21 c; b). The part of the embryo lying close to the endosperm is called the scutellum (fig. 21 b, d ; sc.= scutellum). It is a kind of modified leaf and represents the cotyledon of the embryo. In this case we find only one cotyledon instead of two. The nature as well as the position of the endosperm may differ in different seeds. In grasses, for instance, it is mealy ; it is such endosperm which forms the essential part of flour, the embryo being com- paratively small. The embryo lies to one side, and comes into contact with the endosperm ern - only by means of its scutellum. In the poppy, on the other hand, the embryo is surrounded by the endosperm, and em- bedded in it ; and the endosperm is not mealy, but is fatty and oily (see fig. 21 a ; Z>=endosperm, SC. em.=t mbryo). Lastly, in coffee beans the greater part of the seed consists of hard, horny endos- perm, and the very small embryo is on one side enveloped by the endo- sperm. A curious experiment will reveal the presence of the embryo. Coffee beans are known to have already lost their germinating power when they reach us ; in fact they possess that capacity only for a few days after being gathered ; but if soaked in boiling water, or, still better, in a solution of caustic alkali, we notice what appears to be the germination of a seed which is certainly dead.

In an hour, or even less, a small snow-white rootlet protrudes from a slit in the testa ; sometimes the entire little embryo is pushed out afterwards. What happens is this : the endosperm of the coffee bean, although as hard as horn, becomes soft and very elastic through the action of the boiling water or of the alkali, and in swelling compresses the embryo and squeezes it from its place. We see therefore that seeds may be of two kinds : in some we find very well developed fleshy cotyledons ; in others mealy, oily, or hard and horny endosperm. Just as cotyledons shrink and decrease in size during germination, so also the endosperm disappears little by little, being apparently absorbed. This arouses the suspicion as to whether the decrease of substance in the cotyledon or endosperm is not connected in some way with the increase in size of the shoot, i.e. whether the development of the young plant is not achieved at the expense of the food-substances stored up in the cotyledon or endosperm. But all these substances are also present in a resting seed ; why then is their displacement manifested only during germination ? The answer to this question will be apparent if we recall what we learnt in our last lecture. The nutrient substances in the endosperm or cotyledons exist in a solid and, generally speaking, insoluble form. You remember our analysis of flour, i.e. of powdered seeds. We detected there insoluble starch, insoluble gluten, and oil. All these substances are immobile and incapable of diffusion from one cell to another, a property which is quite essential to them as reserve foods, since other- wise they would not remain stored up.

Hence we have in the seed an embryo, and in a certain part of the embryo the cotyledons ; or in its immediate neighbourhood, in the endosperm, we have stores of nutrient substances in an immobile form, and on that account inaccessible to the embryo. We now inquire what conditions must be fulfilled in order that the embryo may make use of these stores, may bring into circulation this sunk capital. These conditions are well known. Water is needed — for the seed does not germinate in dry soil ; heat is needed — for a seed sown during a cold spring does not show any sign of development until the sun warms it ; lastly, air is needed — for a seed buried deeply in the soil may remain very long without germinating.

' Thus water, heat, and air are the three essential conditions which awaken the seed to life. Let us in- vestigate them in turn. First of all water. Seeds generally contain very little water (see table on p. 43). This is one of their essential peculiarities. A seed which is not dry loses its most important property — the power of concealing life, of living through winters, years, and even centuries in a dormant condition. If the seed is not dry it cannot be preserved ; we cannot get good seed in a wet autumn — grain then germinates in the sheaves or even before the corn is cut. For a seed to remain in the resting state the principal condition is thus the absence of water. As soon as the seed is brought into contact with water we notice an immediate awakening to life. The seed swells and breaks the seed-coat which protected it.

This absorption of water is generally accompanied by a considerable manifestation of energy. An English scientist named Hales studied this phenomenon as early as the beginning of the eighteenth century. He filled a small iron pot with moistened beans, and covered them with a lid upon which he placed a weight. He proved in this way that bean seeds as they swell can lift nearly two hundred pounds. Hofmeister has demonstrated that seeds swelling under similar circum- stances exercise on the walls of the vessel containing

them a pressure equal to that of several atmospheres. Anatomists make use of this property of seeds when they wish to separate the bones of the skull : the cavity of the skull is usually filled with beans, which are then moistened. The bones of the skull separate at the sutures owing to the strong and uniform pressure all over the inner surface of the skull. Such is the mechanical effect of water upon seeds ; it enables them to shed their coats which they no longer need, and to overcome the resistance of the surrounding particles of soil. But the chemical action of water is still more important : without it the solution and consequently the transfer of the stores of nutrient substances cannot take place. Water, however, is not alone sufficient for the purpose, because all these substances, as we have seen, are insoluble in water ; in order to become available for nutrition they must first of all be changed into other substances. Starch, for instance, could be dissolved in water if previously changed into the sugar, glucose. Such a transformation is possible — the preparation of starch sugar is based upon it, and it can easily be proved that such a transformation actually takes place. We have only to taste a raw barley grain and then a malted grain, i.e. a germinating barley grain, to realise that the former is tasteless whereas the latter is sweet. But perhaps the taste has deceived us ; in that case we can avail ourselves of a test I described in our last lecture. We have seen how the blue liquid — Fehling’s solution — gives with glucose a bright red precipitate. We take malt mixed with water, add to it Fehling’s solution, and get a red precipitate. We cut off a thin slice of a germinating seed, place it under the microscope, add a drop of the same Fehling’s solution, and get a red colouration in the cells. Therefore the taste as well as the more con- clusive chemical reaction prove that sugar appears in the germinating seed. But is it true that this sugar is formed from starch ? Both quantitative analysis and

microscopic investigation answer this question. The former shows that throughout the germination of the seed the quantity of starch contained in it decreases. The latter reveals a change in the starch grains : they lose their characteristic form, appear as if corroded, and sometimes break into pieces like ice that has been thawed. They do, in fact, dissolve away. Now let us try and explain the reason for such a transformation of starch into sugar. We can produce this transformation artificially by using sulphuric acid ; but seeds cannot obtain any free sulphuric acid. A special substance called diastase appears instead of it during the period of germination, producing on starch quite a similar effect. Diastase may serve as a repre- sentative of a whole group of substances generally called ferments. The word ‘ ferment * usually denotes a substance which when used in a minute quantity is able to produce the chemical transformation of other sub- stances. There are many such ferments. Bitter almonds, for instance, in themselves have no characteristic taste or scent ; these qualities are produced in them by means of a ferment, which begins to act as soon as the seed is brought into contact with water. Mustard seeds, too, would not have their pungent odour and taste unless they contained the ferment, myrosine , which with water decomposes a substance contained in them (the salt of the so-called myronic acid) and liberates the pungent mustard-oil. A very curious experiment demonstrates this phenomenon. Chemists sell mustard plasters which consist of two sheets of paper, to be laid one upon the other and then moistened with water. Neither leaf by itself constitutes the mustard plaster, but the character- istic pungent odour of mustard is produced as soon as they are brought in contact with each other. This is because one of the sheets is smeared with the ferment and the other with the substance upon which the ferment acts ; the effect of the ferment only manifests

itself when the sheets are moistened. These examples are quite sufficient to illustrate the action of vegetable ferments. A similar effect is also produced by diastase, which is easily obtained from malt liquor, i.e. liquid obtained from germinating grain. One part of this diastase dissolved in water is sufficient to turn into sugar more than a thousand parts of starch ; the warmer the liquid the more quickly does this transformation take place. Thus the nutrition of the embryo of the seed by means of the starch stored in its endosperm or cotyledons becomes quite comprehensible. It is curious that this process is exactly similar to that which takes place during the nutrition of an animal organism. The saliva, and the gastric and other juices secreted by the alimen- tary canal, contain ferments which like diastase change starch into sugar. It is quite easy to realise this : if we suck a piece of bread a little longer than usual, we notice a sweet taste. Thus both animals and plant- embryos can make use of insoluble starch by changing it into soluble sugar.

A similar change must also take place in seeds, like coffee beans or date seeds, with hard and horny endo- sperms. The character of the endosperm is in such seeds due to the very hard cellulose walls of the cell. During germination this cellulose dissolves and serves to feed the embryo. This dissolution aroused the suspicion that a special ferment was concerned, and the existence of such a ferment has since been proved. Let us pass to the second group of foods stored up in the plant, to the albuminoids. In wheat grains and flour, as we have already seen, they are present in the insoluble and therefore immobile form of gluten ; but even soluble albumens, such as the white of a hen’s egg, or the soluble albumen that occurs in vegetable endo- sperms, are immobile, because they are colloids, i.e. substances which do not pass through membranes,

In order to pass from one cell to another, and thus to serve as food to plants, albuminoids must go through a transformation similar to the transformation of starch into glucose. The study of the nutrition of an animal organism will give us once more the key to the explanation of the phenomena which take place in a germinating seed. Gastric juice contains a ferment called pep sine, which with a few drops of an acid has the property of turning insoluble albuminoids into soluble ones ; for instance, the white of a boiled egg, or the albumen of cooked meat, into soluble substances, called peptones . These are not only soluble in water, but are also capable of passing through animal and vegetable membranes. For a long time nothing of the kind was ever observed in the vegetable world, and so long the translocation of albuminoids remained unexplained; but at last almost simultaneously discoveries were made in totally different directions which demonstrated the transformation of albuminoids in vegetable organisms.

Even as early as the eighteenth century a plant called the ‘ Catch-fly ’ had been observed to seize, by means of its irritable leaves, insects which came into contact with them in their flight and then use them as food. This fact, however, had not been sufficiently appreciated : the sceptical even cast doubts upon the existence of the phenomenon, and it might eventually have been quite overlooked had not Darwin paid attention to it. Dar- win added considerably to the list of such carnivorous plants, and acquainted botanists with curious details as to their functions. We shall postpone to a future lecture the description of the mechanical side of such phenomena, and here consider them only in so far as they illustrate the fact of a plant’s capacity for using as food an insoluble albuminoid. These phenomena of digestion in plants were studied by Darwin more especially in the sun-dew, a plant fairly common in marshes. The

mucilage secreted by the hairs with which the leaves of this plant are covered and which seize upon the insects, contains a substance apparently similar to pepsine. This substance, in the presence of an acid produced by the hairs of the sun-dew when irritated, like gastric juice has the power of rendering albuminoids soluble. Insects which, in the natural course of things, fall on these leaves and fragments of meat or white of egg placed upon the leaves, as in Darwin’s experiments, alike become dissolved and assimilated by the plant. These experiments which proved that it is possible to feed a plant with insoluble albuminoids led scientists to look for ferments like pepsine in germinating seeds. Their discovery was not long delayed. Such ferments were found first in leguminous plants, and then in others such as hemp and flax, and, lastly, in malted barley. A ferment very similar to gastric juice in its effects has also been discovered in the latex of Carica papaya. The nutrition of the embryo at the expense of the stores of albuminoids is now comprehensible : the pepsine-like ferment which develops during germination acts upon the albuminoid, transforming it into a soluble, diffusible form. In addi- tion a certain quantity of albuminoid matter undergoes a still greater transformation during germination, into bodies capable of crystallisation — i.e. into crystalloids — which diffuse still more readily.

Thus the embryo of a grass seed, for instance, not only feeds upon the same starch or gluten that we use in eating bread, but also digests them in the same way as we do ; treats them with similar ferments, and changes them into glucose and peptones. We know less about nutrition at the expense of the stores of fatty substances, though we have some indications in this direction also. Oils as such are generally unable to pass through cell- walls moistened with water. They consist, however, of so-called fatty acids in combination with glycerine, a substance easily soluble in water ; and certain facts

suggest that during germination oil is decomposed into its constituents, acid and glycerine, likewise by means of a ferment. Moreover, it is well known that fatty acid when set free furthers the breaking-up of oil in water into very fine drops with the formation of so-called emulsions, such as the white oily liquids we call f milk ' — cow's milk, coconut-milk, and so on. This formation of emulsions plays a great part in the nutrition of an animal's organism ; very likely it also plays a certain part in the nutrition of the embryos of oily seeds.

The first stage in the nutrition of a young seedling has now been explained. By the action of water and ferments the immobile material stored up within the seed is brought into circulation, and becomes available to the seedling. We can easily prove that the develop- ment of the embryo takes place at the expense of the stored material. We have only to cut off the cotyledons of a leguminous plant to stop the further development of the embryo, even though its root and stem have already attained to some degree of development. The cessation of further growth in the embryo cannot be explained by the fact of its having been wounded ; on the contrary, experiments prove that it has still considerable vitality. We can cut it in pieces, in various ways, and each segment will develop if only a connection with the cotyledons containing the food-store be maintained. In fact, if we cut off the rootlet, leaving the plumule connected with the cotyledons, the stem will develop even more quickly than if it had been attached to an uninjured embryo ; and, vice verst , if we cut off the plumule, leaving the rootlet connected with the cotyledons, the rootlet will develop more strongly than if it had been attached to an uninjured embryo. In these cases one of the two organs evidently uses the food stored up for both. In seeds with endosperm (albuminous seeds) the embryo is not organically connected with the source of its food supply ; it may

be closely attached to the endosperm, or even surrounded by it, but in either case it can be separated from it without being injured ; this is why albuminous seeds are the best specimens for the study of the nutritive phenomena of the embryo. In grasses the endosperm, originally dry and farinaceous, becomes less dense on germination, till it resembles gruel or milk. Meanwhile, the outer cells of the scutellum, the part of the embryo adjacent to the endosperm (iu in fig. 21, 6 , d, e), grow out as papillae into the softened endosperm, and absorb the nutrient solution from it. The embryos of buck- wheat and of many other plants find themselves under still more favourable conditions. They simply swim in the semi-fluid mass of the endosperm, and absorb the nutrient substances with their entire surface. If in such seeds we remove the embryo from the endosperm, it will stop growing ; but its development can be maintained artificially if, after removing it from the endosperm, we enclose it within a lump of dough made of flour or starch. That the embryo obtains its food from the dough is shown not only by its successful development, but also by the signs of decomposition in the grains of starch in the immediate neighbourhood of the embryo.

We have several times spoken of the embryo absorbing the nutrient substances from the cotyledons or the endosperm, but evidently this is only a metaphorical expression, and the translocation of the nutrient sub- stances into the embryo has to be explained on the basis of the general phenomena of diffusion, which were studied in our last lecture. We have seen that during germination every nutrient substance passes into a soluble form ; and these solutions, according to the laws of diffusion, have to distribute themselves equally in all parts of the seed, including the embryo. The part played by diffusion ends, however, with this equal distribution of substances, and the attainment of equilibrium.

What is it, then, that disturbs this equilibrium, and, so to speak, transfers the centre of gravity from the endosperm to the embryo ? How can we explain this transfer of substance from the endosperm to the embryo? We can do so in the same way as in our last lecture we explained the passage of the iron salt from the outer vessel into our artificial cell, by the reconversion of the diffusing substances into insoluble compounds. The substances in solution which penetrate into the embryo are used up in the development of new organs. Glucose, a soluble carbohydrate, is thus converted into the insoluble carbohydrate cellulose, of which the walls of the new cells are built up. The soluble and diffusible albuminoids are transformed into the insoluble and non- diffusible protoplasm of these cells. This transforma- tion, as we already know, will cause the diffusion of fresh quantities of glucose into the embryo, and so on. This dissolution and precipitation of substances in a seedling, this drift of matter from endosperm to embryo, will continue as long as they are in contact with one another. Let us imagine that two persons have agreed to share their movable belongings from time to time in equal parts ; then let us suppose that one of the two persons is so imprudent as to gradually exchange his immovable property into movable, whereas, on the contrary, the other exchanges part of his movable property for immovable. In the end all the property of the former will have passed into the hands of the latter. This is precisely how an embryo acquires its food from the endosperm and the cotyledons. It absorbs the food because of its growth, and. it grows because of the food it absorbs — here cause and effect are mutually connected very intimately, as they are in any vital function.

We therefore see that underlying the nutritive phenomena of the embryo there are the same general phenomena of diffusion and transformation, by means of which the nutrition of the cell was explained in our last lecture. This parallel might, indeed, have been ex- pected, since the life of the embryo is the sum of the life of the cells which compose it. We have now proved that during the process of germination matter is only translocated from one organ to another within the seed. Notwithstanding the apparent increase in size and the growth of the young plant, we can prove by weighing the seed and the seedling that no increase in substance really takes place during this period. Simple weighing would, however, be insufficient for the purpose ; if we record the weight first of the seed and then of the seedling developed from it, we certainly do notice that the latter is heavier than the former ; but this is easily explained. We have seen that different parts of a plant contain very different quantities of water : seeds scarcely contain any water, while the plant as a whole contains a considerable quantity . 1 During germination water is absorbed first by the whole seed and eventually by the rootlet, a fact which explains the addition in weight. If, on the other hand, we had dried both the seed and the seedling at a temperature of loo°, and determined their weight in a dry state, we should have found that the plant has lost in dry weight during the process of germination, although it has increased in size. The question arises : what has become of the lost substance ? As a rule we do not notice any excretion by plants of dry or liquid matter such as takes place in animals, and even if there were any such excretion, having taken it into consideration, we should still find that the whole of the loss in weight was not by any means accounted for. We can only conclude that the seed loses some of its substance in the form of gaseous products which disappear into the air.

This supposition brings us to the consideration of the second of the three conditions of germination, defined a little while ago, namely, the importance of air. Air, as we know, consists of oxygen and nitrogen. Experi- ments point to the fact that seeds require oxygen. A seed buried deeply in the soil, or remaining under water which is never changed, does not germinate ; but it likewise does not germinate, or if already begun, the process stops short, if the seed be surrounded with air deprived of oxygen. It needs oxygen, undoubtedly. Wherein, however, does the function of the oxygen consist ?

It is easy to prove that oxygen is absorbed by the seed. Now oxygen maintains combustion ; in its absence burning bodies become extinguished. There- fore, if germinating seeds absorb oxygen and we leave them for a certain length of time in a limited volume of air, we shall deprive that air of oxygen and so rob it of the property of maintaining combustion. Ten hours ago we laid some germinating seeds at the bottom of this wide-mouthed vessel, tightly closed with a glass stopper. I open it now and introduce into it a burning taper ; it is at once extinguished. Evidently the air in this vessel does not contain oxygen any longer. The oxygen has been absorbed by the seeds.

Having observed before how close is the parallel between the nutrition of seeds and animals, we may legitimately raise the question whether also seeds do not use oxygen for the same purpose as animals ? May they not use it for respiration ? Respiration, as we all know, is in its essence combustion. We inhale oxygen ; it is carried by the blood all through the body, and oxidises or burns up part of its carbon and hydrogen, giving them off in carbonic acid and water. We can see this in the following simple experiment, which proves that the gases we inhale and exhale are different in kind, and that the gas exhaled contains carbonic acid. The