The Life of the Plant

Thus at any given moment we find among the mineral constituents of the soil, first, a basis, useless at present, but containing stores of food which will be available in the remote future ; next, stores of food within com- paratively easy reach of the plant ; and, lastly, a very small quantity of matter which serves the plant for immediate use. The truth of this statement can be easily proved. We have only to take the most fertile soil, to calcine it, and to treat it with an acid, in order to get an almost white residue which will prove totally sterile.

It follows that we may look upon the greater part of the soil at any given time as a substratum which merely serves to hold plants firmly, without taking any im- mediate part in their nutrition. The food of the plant must be sought in the remaining component parts of the soil, i.e. in the humus, and in those parts which dissolve in water and acids. Let us see how the twelve elements discovered in the plant are distributed in these parts. The organic humus contains four elements : carbon, hydrogen, nitrogen, and oxygen. Substances soluble in water and acids consist of salts containing all the

elements found in the ash of the plant, together with two other compounds containing nitrogen , namely saltpetre, a salt of nitric acid, and ammonia, a com- pound of nitrogen and hydrogen. Hence, the four elements of organic matter, the elements of ash and the two compounds of nitrogen, nitric acid and ammonia, are the substances to which the analysis of the soil points as the possible sources of food for the roots of the plant. Let us see which of these possible sources will prove to be the actual indispensable sources. In order to find this out we must interrogate the plant itself, by placing it under such circumstances as to oblige it to give us an answer to the question.

How otherwise, indeed, can we determine what substances are really necessary for the nutrition of the plant ? At first sight it would seem quite sufficient to analyse the plant and determine which substances enter into its composition, and to declare these the necessary substances. But doubts at once arise as to the soundness of such a conclusion. Many of the substances found in the plant might clearly be, as it were, luxuries ; their presence might even be accidental and useless, following merely as the result of their existence in the environment. We can only recognise as necessary, substances in the absence of which the very existence and development of the plant are impossible. This can be determined only by means of an exact experiment like the one which proved the sterility of the insoluble mineral substratum of the soil. The essential conditions for such an experiment are as follows : we provide one plant with all the substances discovered by analysis in the plant itself, or in the soil on which it successfully grows ; another exactly similar plant beside it we supply with these same substances, omitting one, and observe the consequences. If we do not notice any apparent difference in the development of the two plants, we may conclude that the eliminated substance is

not important for the nutrition of the plant ; but if under exactly the same conditions we get a weaker plant in the second case, we shall be justified in attribut- ing the difference to the difference in the conditions, i.e. to the absence of the eliminated substance. This chapter in the physiology of the plant affords us a series of good, simple illustrations of a strict and repeated application of the second canon of inductive reasoning. ‘ If an instance in which the phenomenon under investigation occurs, and an instance in which it does not occur, have eveiy circumstance in common save one, that one occurring only in the former, the circumstance in which alone the two instances differ is the cause, or an indispensable part of the cause, of the phenomenon.’ 1

Thus by eliminating one after another all the sub- stances discovered in the plant and in the soil, we discover which of them is absolutely indispensable as food for the plant. Let us study the principal results of these experiments. First of all our attention centres upon the organic substances, the products of decay. Everyday experi- ence proves that dark soils are more fertile than light ones. It would seem, then, that the black humus must form the main food of the plant. Yet exact experiment tells quite a different tale. We may thoroughly calcine black mould, burning away the whole of the organic matter in it, and yet it remains a soil in which a plant will normally develop. We can grow on such a white soil a plant that could not be distinguished from any grown on the best of moulds. Therefore, it is not from the humus that the plant obtains its food. It can grow without it. We have already seen that the largest part of the mineral matter in the soil, the part we have called the insoluble substratum, is useless as food ; so the circle of substances which can be looked

upon as food for the plant becomes still narrower : it is reduced to those substances which are soluble in water and acids. It would be futile, however, to attempt to grow a plant in artificial soil composed exclusively of nu- trient substances ; for instance, in plant ash. Such soil would be totally unfit for the purpose ; the nutrient substances would be in much too concentrated a form, and the plant would certainly perish. In order to serve the purposes of nutrition, these substances must be mixed and diluted with other inactive matter, such as is found in the insoluble mineral substances of the soil. But if such be the meaning of the latter we can surely substitute for it other substances, less complicated in composition. Indeed, experiment shows that artificial soils can be prepared out of sand, crushed pumice-stone, glass beads, and similar materials. By adding to such foundations the necessary nutrient substances we can obtain very fertile soils.

We are now within a step of the method for reducing artificial cultures to the simplest form imaginable. If a considerable part of the natural soil as well as the artificial soils just enumerated serve merely for purposes of uniformly distributing and, so to speak, diluting the nutrient substances, would it not be possible to use instead distilled water in which all the nutrient sub- stances necessary to the plant have been dissolved ? Experiments extending over many years, accompanied by many failures, were crowned at last with complete success. To-day, with certain precautions, we can substitute for soil an entirely transparent medium, and grow the most diverse plants in a watery solution, rearing them to the same normal size which they reach in the most fertile soils.

We take for the purpose a glass jar (fig. 25) containing three or four pounds of distilled water, and dissolve in it something like six or seven grains of a mixture of different We fix a seedling of any plant whatever to the lid of the jar in such a way that only its rootlets will be immersed in the water. Then we shall be able to ob- serve the develop- ment of the aerial part of the plant, and also that of the root, which is now entirely exposed to view.

27, 28, 31) record the results of ex- periments made in 1896 at the Nijny exhibition. I attribute a parti- cular significance to them, because it is doubtful whether such experiments in all their stages and details were ever before performed in the same way in the presence of so many thousands of spectators. With great pleasure do I recall one sceptic, a local inhabitant of Nijny, who acknowledged that he used to observe our water cultures day after day with the sinister in- tention of convicting us of quackery ; but in the end he became enthusi- astic and thoroughly con- vinced.

Our problem is now reduced to this : of all the mass of black mould surrounding the plant, it is only an insignifi- cant pinch of a mixture of certain salts that is at any given moment indispensable for its nu- trition. Now let us find out which of the chemi- cal elements entering into the composition of these salts are essential. It has been necessary for this purpose to make a series of experiments either with white sterile soil into which necessary salts had been introduced, or with culture solutions as described above.

Here are experiments showing the necessity of nitrogen for plants (fig. 26). We take two flower-pots filled with a soil calcined and washed with an acid, and consequently white and sterile. To one of them the ash of plants has been added, containing all the mineral substances that exist in a plant ; to the other the same ash has been added and also nitrogen in the form of a nitrate, namely saltpetre. Sunflower seeds exactly similar in weight were planted in both pots, two in each. They have come up, but a distinct difference is evident at the end of the experiment : the first flower- pot contains two miserable, unhealthy plants, scarcely rising above the soil; the second contains two healthy specimens with flowers and seeds, leaves and stem being as well developed as those of sunflowers grown simul- taneously in the best garden soils. 1 Yet the only difference between the two experiments consists in the fact that to the second flower-pot some saltpetre, i.e. nitrogen, was added. Similar results might have been reached had nitrogen in the form of an ammonium salt, instead of saltpetre, been used. The inference is that plants need nitrogen.

Here is another experiment. We take several jars containing nutrient solutions (fig. 25) ; some of the jars contain all the necessary salts, others the same salts minus that of potassium. We place exactly similar buck-wheat seeds in every jar. After a certain time we notice that the former jars contain healthy plants which flower and produce ripe seeds, while in the rest, instead of developing well, the plants are weak or have perished. We may repeat these experiments many times -over, and always get the same results. The inference is that the plant needs potassium, that it cannot exist without potassium.

You see on this table the results of similar experi- ments also made with buck- wheat seeds (fig. 27). The first, third, and fifth row received a complete nutrient solution, the second received no nitrogen, 1 Fig. 26 represents on the left-hand side plants grown with saltpetre (for the sake of comparison a leaf of a garden specimen is given), and on the right plants grown without saltpetre. This is the classical experi- ment of Boussingault. the fourth no potassium nor phosphoric acid. The results speak for themselves.

striking of all in its results. Among the salts found to be indispensable for the nutrition of the plant, there stands that of iron ; it enters into the composition of the ash of the plant in very minute quantities. This salt cannot be used as a solution in water cultures, because it forms with another substance equally indis- pensable for the nutrition of the plant, phosphoric acid, a precipitate insoluble in water. We shake up this white insoluble precipitate in the liquid, so as to bring it into contact with the surface of the roots. We take several jars : some with entirely clear solutions, which means that they do not contain any iron salt ; others with a certain degree of turbidity, owing to the presence of the iron salt. Suppose we grow a plant such as maize in each of these solutions. At the end of two or three weeks we already notice a sharp difference between them. While the full nutrient solution produces a normal plant that will flower and produce ripe seeds, the other will produce a plant with only a few narrow and unhealthy leaves that will soon die altogether (fig. 28). These leaves, moreover, will show a remark- able peculiarity : the first two or three of them will be of the usual green colour, but the rest will be white. It is clear that the absence of iron has stopped the development of the plant, and has resulted in a peculiar disease, a ' pallid sickness,’ called chlorosis. The following simple experiment will attest the accuracy of this inference ; we have only to add some of this iron salt to the solution hitherto without it to see the sickly condition coming to an end, the plant becoming green and growing ; moreover, we have only to moisten one part of a totally white and sick leaf with an iron salt to see a green spot appearing soon after on that very place. 1 Our attention has already been drawn more than once to the similarity between the vital functions of vegetable and animal organisms ; the action of the iron salts presents a striking illustration of this point. Such unfortunate cases as the following may have been

1 In the middle of Fig. 28 we see a vigorous specimen of maize still in flower reaching the top of the green house; on either side are two specimens of a smaller variety (Cinquantino) already bearing full-grown cobs ; in between are two specimens grown without any iron salt. moned. He begins at once by examining the gums, and then prescribes pills or medicine. The patient takes the medicine and after a time regains his healthy appearance. The remedy contains iron. The same

happening quite lately : somebody of your acquaintance feels ill, the illness being accompanied, among other symptoms, by an unusual pallor. A doctor is sum- iron that brings back healthy colour to a faded cheek brings back the natural green colour to a white leaf. Results similar to those just described in connection with nitrogen, potassium and iron, can be obtained in the same way in respect of phosphorus, sulphur, chlorine, lime, and magnesium. All these substances have proved to be indispensable for the nutrition of the plant, which sooner or later perishes without them.

But among the constituents of the ash of the plant we find silicon. Silicon, together with oxygen, forms silica, which in a pure form occurs in nature as rock crystal, and a little less pure as quartz, white sand, etc. The same silica forms the main constituent of glass. This silica is also found in many plants, in their cell-walls, making them brittle like glass ; if we burn such a cell we are left with a glassy skeleton, which under the microscope preserves the outward form of the living cell in its minutest details. By very unpleasant experience eveiy one has had opportunities of learning the existence of such glassy cells. The stinging hair of the nettle is simply a long-pointed cell, the walls of which, especially at the top, are as brittle as glass, because they are full of silica ; this is why they pierce the skin so easily, break in the wound, and inject their poisonous sap. Large quantities of silica are contained in the straw of cereals and in the stem of the horse-tails. The latter are so hard that carpenters use them for polishing wood.

Silica, then, is found very generally among plants, and we might suppose it to be indispensable to the plant. An idea has grown up to the effect that it not only adds hardness to the external tissue of cereals, but even gives solidity and firmness to the whole body of the straw ; it has been supposed that by increasing the supply of silica in our cultivated cereals, these would be less readily laid by wind and heavy rain, which do so much damage to corn. But direct experiment has put an end to all these

suppositions, which before seemed so probable. Cultures in artificial soils as well as in solutions deprived of silica have proved that quite normal specimens of cereals develop even in the entire absence of silica ; a plant must therefore be able to exist without silica. Further, experiments on a larger scale have been made actually in the open fields, the soil being manured with silicates ; but these experiments gave a negative result. Plants in a silicated soil were laid worse than those in untreated soil. It might have been suggested that the manure had not reached the plant, but analysis proved that the plants had really become richer in silica. This in- comprehensible result was to a certain extent explained when, after the general analysis of the plant, par- ticular analyses were made of its several parts. It appeared then that it was the leaves and not the stem or the straw which became richer in silica ; and therefore the increase of silica might work rather to the detriment of the plant, making it top-heavy and more liable to fall, instead of contributing to its steadiness. Eventually it transpired that a plant can exist without silica, and that its presence has nothing to do with the firmness of the straw, as was formerly believed. We shall see in one of our subsequent lectures that the laying of crops is to be explained by other causes, and therefore can be averted by other means.

So, then, in striking silica out of the list of the elements present in the ash given in the second chapter, and by substituting for it the indispensable nitrogen, we get eight elements that exhaust the list of substances which must necessarily be supplied to the root in order to nourish the plant. Four of them — nitrogen, phosphorus, sulphur, and chlorine— form acids. These acids by combining in pairs with the four metals, potassium, calcium, magnesium, and iron, produce four salts. These four salts satisfy all the requirements of the root ; they furnish all the nutrient solutions used for the

experiments we have made. The most sterile soils, when watered with such a solution, become fertile in the sense of becoming quite fit for the purpose of feeding the plant. Such are the results, so brilliant in their simplicity, to which the study of the physiology of the root has brought us. Let us, however, remember that this simplicity is the result of many years’ stubborn labour of scores of scientific investigators. A question naturally arises here : would it be right: to conclude that all the other substances which form the main bulk of the soil are quite useless to the plant ? Evidently not. Some of the substances, while they do not serve as food at any given moment, may be of use as food at some future time ; others, without taking any direct part in nutrition, may indirectly contribute towards it. For instance, besides saltpetre and ammonia, the soil contains considerably larger quantities of nitrogen in the form of organic matter. Yet this nitrogen is of no immediate use for the purposes of nutrition ; soil, which contains nitrogen only in that form, is almost sterile ; but this nitrogen may gradually change into ammonia and nitric acid, and then serve as food. This is an example of a substance useless at any given moment, yet serving as a future supply of food. Substances in the soil may be useful to the plant in still other ways. They are of value to it by reason of their capacity for retaining moisture, and for absorbing heat, and they help to hold the nutrient substances and distribute them uniformly.

With regard to this last property the capacity of the soil for absorption is remarkable. If we fill a funnel with soil and water it with some nutrient solution, and then collect the water after it has passed through the soil, we find that it contains very little of the nutrient substances. Ammonia, phosphoric acid, and potassium are especially absorbed ; all these, as we have seen, being elements necessary to the plant. This remarkable property of the soil is of great importance in the economy of nature. Substances necessary to the plant, i.e. those that exist in the soil in very limited quantities, are thereby prevented from being washed away by the rain, and are kept in the soil, which only gradually gives them up to the water circulating between its hard particles.

Nitric acid (as saltpetre) forms an exception to this rule, as it is rather easily washed from the soil ; yet, as we have seen, it supplies the plant with nitrogen, the most important of nutrient elements. The investi- gations of scientific agriculturists are drawing the attention of farmers more and more urgently to the necessity of utilising this substance as fully as possible by means of cultivated plants. It is in this very rela- tion of the plant to the nitric acid in the soil that an explanation has been sought for the part played by leguminous plants in the rotation of crops. Until quite lately their r6le seemed very mysterious. Leguminous plants contain more nitrogen than cereals, and yet nitrogenous manures affect them less than cereals. Moreover, when leguminous plants are cultivated in alternation with cereals in an unmanured soil, cereal crops are gathered as heavy as any succeeding the bare fallow. This seemed to show that leguminous plants, instead of exhausting the soil, even enrich it, an opinion which would have been strictly justified could it only have been proved that leguminous plants absorb nitrogen from the air instead of from the soil. Yet this was for a long time contradicted by exact experi- ments.

The only other possible explanation of the relation of the leguminous plants to nitrogen lay in the fact that, developing a network of roots which sank very deeply into the soil, and growing in the soil for a longer time, leguminous plants absorbed more completely the stores of nitric acid, which under other circumstances were washed away by the rain and therefore lost to the farmer. Such a utilisation of the nitric acid in an explanation about the end of the eighties. This discovery is one of the most brilliant contributions of recent years to the theory of the nutrition of the

plant, and we may therefore dwell on it at some length. As has already been said, saltpetre manures, while they exercise such an essential influence upon cereals, sometimes remain entirely without effect upon legu- minous plants. One such experiment 1 is represented in fig. 29 (lower half) . Two pots of oats (marked by the letters KP.) received all the necessary mineral man- ures with the exception of saltpetre ; two other pots (marked by the letters KPS.) received the same mineral manures with the addition of saltpetre : the result speaks for itself. A similar experiment was made with peas (at the top of the figure), and the result was negative ; the presence of saltpetre was not manifested in any way whatever. This means that peas can procure nitrogen for themselves, even if it is not present in the soil.

Evidently beans can ob- tain nitrogen from the air ; but under what circum- stances ? In raising this question investigators recalled the fact that certain small swellings were noticed even by the ancients on the roots of leguminous plants. These nodules (fig. 30) appear in consequence of the roots becom- ing infected by certain bacteria, apparently widely distributed in the soil. This can be proved very simply. A leguminous plant is grown in an aqueous solution in

such a way that some of the roots are in one vessel, while the rest are in another. The solution in one of the vessels is previously boiled, whereas a small quantity of water, in which soil containing bacteria has been standing, is added to the other. Nodules do not on the roots immersed in the boiled and sterilised the assimilation of nitrogen from the air depends upon the presence of such bacteria in the soil can be proved in the following way (fig. 31). A row of glass vessels containing peas are filled with soil deprived of nitrogen, but infected with soil-washings containing bacteria ; while another row are filled with soil sterilised by means of heat, and watered with the same soil-washings previously boiled, and therefore also sterilised. The result is marvellous : it is only plants

(fig. 31, odd numbers) grown in the soil that contained the bacteria which have formed nodules, and have developed normally; the others (fig. 31, even num- bers) have perished. This proves that the property which differentiates peas and likewise ■ all leguminous plants from cereals, namely the property of assimilat- ing free nitrogen from the air, is bound up with the capacity of their roots for becoming infected by certain soil-bacteria. How and where this assimilation of nitrogen takes place has not yet, however, been fully explained.

In view of the fact mentioned above that the soil- water is such an extremely weak solution of nutrient salts, it may well be asked whether it is really sufficient for the nutrition of plants ? We can answer this question by reference to the following calculations. We know the quantity of rain that falls on a certain area of land ; we also know the quantity of nutrient substances that this rain water can extract from the soil ; on the other hand we know the quantity of ash contained in a whole crop, gathered from the same area of land. These data are sufficient to provide an answer to our question whether this liquid food is sufficient. It is only the most fertile soils for which the answer is in the affirmative, in all other cases the answer is negative. In general the liquid food alone is not sufficient for the plant. Evidently it must also use substances insoluble in the soil-water. But in that case the root must, so to speak, seek out its own food, searching round all the neighbouring particles of the soil in order to find among the mass of sterile matter minute particles of the nutrient substances so sparsely scattered in it. This brings us directly to the consideration of the second question we have raised. Having ascertained wherein the food of the root consists, let us now try to find out how it gets it.