Disease in Plants. Ward Harry Marshall

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СКАЧАТЬ 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 СКАЧАТЬ