Disease in Plants

But the above supposed case is realised, as Pfeffer showed in 1886, when he found by a series of beautiful experiments that certain aniline dyes can accumulate in living root-hairs, and other living cells, whereas others cannot pass the living protoplasm. After accumulating for some time, the dye may either remain stored there, or may eventually diffuse out.

Pfeffer made another discovery, of equal importance, namely, that under the influence of dilute organic acids, such as citric acid, the permeability of the living protoplasm may be altered, so that it allows substances to pass which could not otherwise have traversed it. De Vries had also shown that the condition of the protoplasm affects its power of retaining the colouring matter in the sap of the Beet: so long as the protoplasm is alive, the crimson sap is retained, even when the cell is plasmolysed, but immediately it begins to die the colour escapes through it. A similar case exists when the chlorophyll-corpuscles retain their colour in living cells known to be charged with acids: so long as the protoplasm is alive and normally active the green bodies are protected.

These, and numerous other experiments of the same kind, prove that the healthy root-hair is a living instrument for taking up dilute solutions out of the soil, and holding them in the sap-cavity for a time. If killed, by frost for instance, it loses these powers.

The researches of the last ten years have also shown that a time comes when the turgid cell, if an isolated one, and if sufficient supplies of water are present, is so tightly distended that the surplus water begins to diffuse out again under the pressure proper to the hydrostatic conditions set up.

Now we arrive at a very critical point.

When the water, or dilute solution of various substances, begins to exude under pressure from the living root-hair, what is to prevent its escape into the soil? And if it thus diffuses out, where is the object of absorption?

The questions are obviously pertinent, and they may seem the more so in that the cells adjoining the root-hair on its inner side are also turgid, and possess similar properties to those of the root-hairs. To establish a condition of things which shall bring about the inward flow of the absorbed water, one of the three following cases is conceivable. (1) The cells, as we pass radially into the root, have different properties on the wall of the two sides; or (2) they are more and more greedy of water owing to some process of extraction of their water by tissues in the centre of the root; or (3) these successive series of cells possess osmotically more powerful contents at periods coincident with the escape of the water from the now osmotically weaker root-hairs.

A little reflection will show that where we have a group of such cells as the above, all capable of absorbing water and dilute solutions and of becoming turgid, movements of the absorbed water must go on until all the cells are in equilibrium, as regards their osmotic pressures.

Now the living rootlet is just such a system, the various cells of which are in different conditions of osmotic pressure at any given time: some of these cells are old, and their protoplasm is allowing sap to filter out under pressure: others are in the height of their vigour, and their protoplasm extremely impervious to the highly osmotic sap-constituents which it itself is forming actively: others are too young to have attained their full turgescence: while others again are in stages intermediate between the above.

There is another point of importance, however, to explain some peculiarities in the absorption of these dilute solutions of salts, etc., by the root-hairs from the soil, and by cells lying deeper in the plant from these root-hairs.

It is easy to understand that if a root-hair absorbs a given substance—say calcium sulphate, for illustration—and hands it over to other cells unchanged, a time must be supposed to arrive when, the sap of all the cells being equally charged with calcium sulphate, no more could be absorbed: the rate of absorption of this particular substance, and the quantity absorbed, up to the hypothetical point of equilibrium chosen, would then depend simply on the ease with which its molecules traversed the living protoplasmic membrane, and the degree of their solubility in the sap.

But now suppose the following new factor to come in. Suppose that calcium sulphate undergoes decomposition in some one of the internal cells of the system of absorbing cells, or that it is even merely crystallised out in such a cell, or in any other way removed from solution (e.g. by deposition in cell-walls). This alters the state of affairs considerably. The separation of the molecules from the sap-solution is itself a cause for the flow of more of the solution to the cell concerned, and such causes of diffusion are very common in the plant.

The importance of this principle consists in that it lies at the base of the whole question of selective absorption, application of manures, and the rotation of crops; and those who are acquainted with the excellent analytical results of De Saussure, Boussingault, Wolff, Trinchinetti, Gödechen, etc., and the water-culture experiments of Sachs, Nobbe, and others, will understand what an illuminating effect on these points was produced by the above generalisation, which we owe especially to Pfeffer's splendid researches into the nature of osmotic phenomena.

It will now be clear, I hope, why we regard the living root-hairs as instruments—as pieces of living machinery—for the active absorption of water, with substances dissolved in it, from the soil; and it will also be evident, I think, that no one can form a proper conception of this matter of absorption, so important in all agricultural questions, unless he pays attention to these biological phenomena. It was hopeless to expect to understand these matters merely in the light of chemical analyses of plants and soils, and one expression of this hopelessness was the belief in the power of roots to select only the substances useful to it. We now know that the expression "selective power of roots" has a totally different meaning from that implied in the minds of the last generation of agriculturalists, and it would be easy to devise experiments, with solutions of different strength, where the plant should be made to take up relatively large quantities of harmless, but useless minerals, etc., and to starve in the midst of plenty of the elements proper to its structure, simply because the former are offered in a form in which they easily traverse the protoplasm of the root-hairs, while the latter are presented in a form unsuitable for absorption. That all these matters are of importance in regard to manuring and choice of soils, etc., needs no emphasising.

These remarks, of course, do not detract from the value of good comparative chemical analyses, when viewed in the light of physiological knowledge, as I need hardly say; but they do, and emphatically so, attack the position that such analyses alone can explain the problems of agriculture.

On the other hand, we must not rest satisfied with the suggestions so far put forward to account for the processes referred to, since it is impossible to overlook the fact that in their present form they merely afford proximate explanations, and are too crudely mechanical for finality.

Notes on Chapter VI

In addition to the works referred to in the last chapter, the student should consult Pfeffer's Physiology, pp. 86-149, and pp. 410-441. With reference to water cultures, Sachs' Lectures, XVII., may also be consulted. The standard work on ash constituents of plants is Wolff, Aschen-analysen, 1871 and 1880, an indispensable book of reference in this connection, though there are others, quoted in Pfeffer, where further literature may also be found.

CHAPTER VII.

THE BIOLOGY OF SOIL

Soil not a dead matrix—Organic materials—The living organisms of the soil—Their activities—Their numbers and importance. Abandonment of the notion that chemical analysis can explain the problem.

It is customary to regard the soil, between the particles of which the root-hairs of plants are distributed, as if it were merely a dead matrix of smaller or larger pieces of rock, such as sand, gravel, stones, etc., and organic remains, such as bits of wood, leaves, bones, etc., with water and air in their interstices. As matter of fact, however, soil is a much more complex body than was suspected until comparatively recent times.

It is, of course, beyond the scope of this book to go into the different varieties of soils, their structure or arrangement, and the chemical nature of their constituent rocks and the débris mingled with the latter. For the same reason I must pass over the curious properties of soils in relation to the solutions they yield to water in contact, the manner in which they retain some of these solutions and allow others to pass easily, and the remarkable double decompositions which go on in them. Moreover, I must assume as known the chief physical properties of ordinary soils with respect to the phenomena of capillarity, absorption of heat, action of frost, and so forth.

But all ideas as to the nature of soil based merely on the study of its chemistry and physics are misleading, and it is in just the establishment of this truth that modern discoveries in Agricultural and Forest Botany have played so important a part.

From the facts that organic débris is found chiefly at the surface of the earth, and that the smallest particles are held in suspension by the water near the surface, it is comprehensible why such organic remains abound in the upper parts of the soil, where the rootlets with their absorbing root-hairs are also found, because they must have oxygen. The rule is, therefore, that an ordinary soil consists of upper strata, rich in organic materials and in oxygen, and a subsoil, poorer in these substances.

Among these organic materials are countless myriads of living beings, especially fungi and bacteria, which require oxygen and organic materials for their subsistence, and it depends on the open or close, moderately moist or damp, warm or cold nature of the soil, and on some obviously connected factors, how far down these aërobic organisms can thrive. As we go deeper down they become fewer and fewer, and gradually disappear, and (neglecting certain anaërobic bacteria of putrefaction) they are rarely found in marked abundance more than a few inches below the surface soil.

These aërobic fungi and bacteria are the great agents of continued fertility of a soil, and it is they which, living and multiplying in the moist and well-aerated warm interstices of a rich open soil, carry out the useful destruction of organic matter, breaking it up into mineral and gaseous bodies, which are then dissolved in the water bathing the root-hairs or escape into the atmosphere. In this work of destruction they are aided by the oxygen of the air and the solar heat: their own fermentative action is also accompanied by a marked rise of temperature, and the carbon-dioxide and other products of their activity all go to complicate the chemical changes going on in the soil around the roots.

Duclaux has calculated that Aspergillus niger, a common mould fungus, can break down organic substances, such as carbohydrates, at such a rate that a metre cube of the fungus would decompose more than 3000 kilogr. of starch in a year, and this may serve as an example giving some idea of the possibilities in soil.

Analyses of waters containing large quantities of organic matter, as they enter such open soils as those referred to, compared with the drainage water after passing through the upper strata, show that the carbonaceous and nitrogenous materials are broken down to more or less completely oxidised simpler compounds, and that the following chief changes result. The ammonia and some other nitrogenous bodies remain behind in the soil, as also do the phosphoric acid and much of the potash; whereas large quantities of nitric and nitrous acids, together with much sulphuric acid, chlorides, and calcium salts pass away in the drainage. These facts are obviously highly important in agriculture.

Experiments on sewage farms have shown also that the upper soil retains most of the bacteria of the sewage. Koch found at Osmont, near Berlin, that whereas the different sewage waters contained numbers so enormous that each cubic centimeter probably held 38,000,000 germs, the different drainage waters held only 87,000 per c.cm.; and the whole process of water-filtration through sandy soils depends on these well-known facts.

Recent experiments in connection with soil-filtration, however, bring out the further facts that the oxidations which organic matters undergo in the soil—and without which they are useless to the higher plants—are enormously enfeebled if the upper layers of soil are sterilised, so as to deprive them of the myriads of aërobic bacteria, fungi and yeasts which they normally contain, and there can no longer be any doubt as to the importance of the biology of the soil in connection with the preparation of materials suitable for absorption in solution by the root-hairs of agricultural and other plants.

The researches of the last ten years have brought to light a long list of forms, comprising yeasts, such as Hansen's Saccharomyces apiculatus, fungi and bacteria which live and grow in the soil, finding their water and food supplies in the interstices, and under conditions which we now know to be very diverse. They are usually more numerous, in species and individuals, in cultivated farm and garden soils than in woods, prairies, and untilled lands; but the geological nature of the strata, the closeness and otherwise of the soil, its damp or dry character and its average temperature (which depends on many things besides latitude or altitude) and other factors co-operate to rule their distribution and numbers. The fact that cultivated land is so well supplied with manures, air, etc., is of great importance in relation to their relative abundance there, and it is extremely probable that the use of artificial manures lessens their numbers considerably as compared with land on which stable and other animal manures are employed.

A list of the soil-bacteria which have been isolated and more or less carefully cultivated and examined would comprise about fifty species; but it is certain that, as at present classified and named, many more species are to be discovered in any ordinary soil.

The fungi are apparently even more numerous than the bacteria, and we may rest satisfied for the present with the general statement that the life-actions of the myriads of individuals of these organisms in the soil completely alter the question of soil-water as understood by the last generation of agriculturalists.

But there is another aspect of this question of soil-organisms which has grown in importance of late to such an extent that we are more than ever justified in regarding the biology of soil as far more vital to the interests of the plant than its physical or chemical properties. With many of the fungi in the soil the roots of plants have to compete—just as plant competes with plant—for water, salts, and other food-materials. The toadstools which are so conspicuous in fields and forests spring from mycelia which ramify in the ground, and are busily breaking down the remains of other organisms, and just such fungi are known to store up relatively large quantities of salts of potassium and phosphorus—the very salts which are so valuable to crops and occur so sparingly in most soils, but which the extensively spread fungus mycelia can gradually accumulate. Some of these fungi, moreover, are more active in their antagonism, and actually attack and pierce the roots as destructive parasites, but I pass these by for the present, as they form the subject for further consideration when we come to the diseases of plants.

It is obvious that the competition of fungi with root-hairs for mineral salts, oxygen, etc., may be at times acute, and it is extremely probable that cases of so-called sterility of soil, where a particular soil is found unsuitable for a crop, may sometimes be due to this over-competition.

The researches of recent years, however, and especially those of Frank, Winogradsky, Hellriegel, and Stahl, have brought to light a series of relationships between certain of these soil-organisms and the higher plants which place the matter of soil-biology in quite new lights.

On the one hand it has been discovered that groups of bacteria are the active agents in bringing about the destruction of organic nitrogenous matter with the formation of ammonia, in oxidising this ammonia to nitrous and to nitric acids, which combine with bases in the soil to form the corresponding salts; while, on the other hand, other forms can decompose the nitrates and reduce them to nitrites, or set free ammonia or even nitrogen from them. Moreover, there are certain species which can fix the free nitrogen of the atmosphere, and start the cycle of up-building of this inert element into the complex higher compounds we term organic. It is impossible to over-estimate the importance of these processes of nitrification and denitrification going on in the soil about the root-hairs of the higher plants.

But, in addition to this circulation of nitrogen in the soil, it turns out that the life-actions of bacteria, and not mere chemical decompositions, are largely responsible for the circulation of carbon, of iron, of sulphur and other elements formed from the decomposition—also by bacterial and fungal agency—of animal and vegetable remains in the soil.

Even more startling are the biological relations in the soil between the absorbing roots of the higher plants and some of these bacteria and fungi, for it has now been established beyond all doubt that certain fungi enter the living roots and there flourish not as mere destructive parasites, but as messmates not only tolerated by the plant, but even indispensable to its welfare. It is probable that nearly half the plants of our fields, moors, and forests entertain such fungi in their root-tissues. The curious, and long-known nodules on the roots of leguminous plants—peas, beans, clover, etc.—are filled with bacteria which enable these plants to avail themselves of the free nitrogen of the air, and so enrich the soil with nitrogenous substances.

The roots of most forest trees, orchids, and plants of the moorlands, meadows and marshes are similarly occupied by fungi, which in some way convey salts—probably especially phosphates and potassium compounds—to the plant in return for the small tax of organic carbon-compounds it exacts from the latter. In some cases at any rate, as Bernard has lately shown, the very existence of the plant depends on its seedling roots obtaining this advantageous attachment and co-operation (symbiosis) of the fungus immediately on germination.

These remarks must suffice to illustrate this part of my subject, and to emphasise the statement that the question whether a given plant can be grown in a given soil, is by no means one of simply the physical and chemical constitution of the latter. The plant will have to run the gauntlet of a long series of vicissitudes brought about by the presence or absence, relative proportions and vigour, and specific nature of the organisms in the soil at its roots, and it is easy to see that many cases of disease may be due to the absence of advantageous bacteria or fungi, or to circumstances which disfavour their life, as well as to the predominance of competing organisms.

It will now be evident that the old points of view must be abandoned, and with them, especially, the widely prevalent notion that chemical analyses of the plant and soil can explain the real problems of agriculture.

It was of course an enormous advance in the science when, thanks to the splendid labours of the chemists, at the end of the last century and the beginning of this, we obtained that preliminary knowledge of the constitution of the air, and of the composition of the water, acids and salts, etc., which plants require for their food-materials and life-processes. Much was gained by De Saussure's establishment of the fact of oxygen respiration, though we now understand by the term something very different from, and much more complex than, what he understood by it, as, also, much had been gained by the previously acquired knowledge of the gas-exchanges in carbon-assimilation: nor must we forget the services of those who proved, by laborious analyses, continued for long periods, what chemical compounds are found in the tissues of plants, and in the soils at their roots and the atmosphere which surrounded them. We must also remember many other contributions which have been furnished, and are still being furnished by the chemist; and I for one hope that his labours will continue to go hand in hand with those of the physiologist.

But, when all due honour is paid to the scientific chemist, it must still be allowed that his problems are different from the real problems of agriculture. To take one set of instances alone. The chemist can analyse a given soil or a given manure, and can even go a long way towards making them, but his analyses do not tell us what conditions are necessary in order that their ingredients may be presented to the roots so as to be absorbed and become built up into the plant. Chemistry told us that carbon was fixed from the air, but physiological experiments determined how this meant the synthesis of certain definite carbohydrates—this, too, in the face of the powerful authority of the chemist Liebig, who supposed that the vegetable acids were the results of the assimilation of carbon. Wolff, De Saussure, and other chemists have done yeoman service in showing that different plants, growing in the same soil, contain different proportions of mineral substances; but it was by means of water-cultures, and other physiological researches, such as those of Pfeffer on osmotic phenomena and of Schwarz and Molisch on root-hairs, that the puzzling question of selective absorption, by means of the living root-hairs, came into the arena of our knowledge.

In every case—and, as already said, I am not undervaluing the work done—the chemist has left us only on the threshold of the real problem. He has stood outside the factory in which the real work we want to know about is being carried on, and has told us of so many tons of this material being carried in at the gates, and of so many tons of that coming out; he has even burnt down the factory, and all its contents and machinery, and has then told us how many tons of the various materials were there at the time; but this is not what we want, valuable as the information is, and still more will be. What we want, and what we expect to obtain, is more information regarding what is done with the materials in the factory: what machinery they are put into, and how they are put in: what stages they go through, and how the stages follow one another: what wear and tear has to be endured, and how we can step in and stop the working of the machine for our own benefit at the best possible time.

Конец ознакомительного фрагмента.

Текст предоставлен ООО «ЛитРес».

Прочитайте эту книгу целиком, купив полную легальную версию на ЛитРес.

Безопасно оплатить книгу можно банковской картой Visa, MasterCard, Maestro, со счета мобильного телефона, с платежного терминала, в салоне МТС или Связной, через PayPal, WebMoney, Яндекс.Деньги, QIWI Кошелек, бонусными картами или другим удобным Вам способом.