Disease in Plants. Ward Harry Marshall

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      Enough has perhaps been said to establish the general truth that the plant is a complex machine for storing energy and material from outside, and we have seen that modern research has at least gone a long way towards determining how the living machine works.

      It is hardly necessary to point out that important practical consequences may result from these phenomena of the accumulation of surplus starch or other carbohydrates in the leaves during the day, and of their disappearance during the night into the lower parts of the plant. For instance, foliage cut for fodder in the morning is far poorer in starch than if cut in the evening, and it would be very instructive to have experiments made on a large scale to test the result of feeding caterpillars or rabbits, for instance, with mulberry, vine, or other leaves in the two conditions.

      Again, we now see what complications may arise if a parasitic organism gains access to the stores of carbohydrates in process of accumulation, or attacks and injures the machinery which is building up such materials, etc.

      Notes to Chapter IV

      The student who desires to pursue this subject further should read Sachs' Lectures, XX. and XXV., and Pfeffer's Physiology, pp. 442-566, but he will hardly arrive at the best that has been done without consulting Pfeffer's "Studien zur Energetik der Pflanzen" in the Abhandl. der Math.-Phys. Classe der Kgl. Sachss. Gesellsch. der Wiss. (Leipzig, 1892), p. 151; and Kassowitz, Allgemeine Biologie (Vienna, 1899), Bk. I., pp. 1-127.

      CHAPTER V.

      ROOTS AND ROOT-HAIRS

      Older views as to root-hairs—Root-hairs and their development—Surface—Variations—Conditions for maximum formation—Minute structure—Adhesion to particles of soil—Functions.

      On the roots of most plants are to be found delicate, silky-looking, tubular prolongations of some of the superficial cells, known as root-hairs. Malpighi (1687) seems to have been the first to observe them, and he took them for capillary tubes. Grew (1682) seems to have been responsible for the view that the roots act like sponges in taking up water.

      Simon (1768) was probably the originator of the idea that these root-hairs were excretory tubules, a view that became very popular at the beginning of this century.

      Meyer (1838) was perhaps the first to give a comparative account of them, and he supposed them to be delicate prolongations of the root-surface to facilitate the absorption of water.

      The real importance of these organs, however, has only become apparent since Sachs, in 1859, recognised their relations to the particles of soil between which they extend and to which they cling.

      In 1883 Schwarz made a very thorough study of their biological character, and in 1887 Molisch gave us new facts as to their physiology. Our knowledge of them has been rendered very much more intimate by the researches of Pfeffer and De Vries on osmotic and plasmolytic phenomena, and they serve as an excellent study of some of the best results of modern physiology.

      In the normal case, such as is exemplified by a seedling wheat or bean, the root-hairs arise some distance behind the growing tip of the root, an obvious adaptation which prevents their being rubbed off by the soil, as they would be if developed on parts still actively lengthening. As those behind die off, new ones replace them in front, and so we find a wave of succession of functionally active root-hairs some little distance behind the tip of the root: the same order of events holds for each new rootlet as it emerges from the parent root, and so successive borings in the soil, made by the diverging root-tips, are thoroughly explored by these root-hairs.

      Measurements have shown that in various plants the surface of root on 1 mm. of length is increased by the root-hairs in proportions given in the following table:

      —which sufficiently establishes the general proposition that the area of the root-surface is enormously increased by these hairs.

      But this does not give us any definite idea of the length of the cylinders of soil explored by these surfaces, until we find that plants such as an ordinary sunflower, hemp, or vegetable-marrow may have roots penetrating into a cubic meter of soil, in all directions, and so closely that probably no volume so large as a cubic centimeter is left unexplored. Clark found by actual measurement that the roots of a large gourd, if put end to end, extended over 25 kilometers, and Nobbe gives 520 meters for the roots of a wheat. Vetches may go nine feet deep, and oats more than three feet. The Sal, a tree of the forests of India, has roots which penetrate to a depth of 50 to 60 feet.

      Some rough notion of the lengths, superficies and penetrating capacities of the roots of a large tree may be gathered from the above, but it is doubtful whether we can form any adequate ideas as to the millions of root-hairs which must be developed along the course of these subterranean boring organs.

      One of the most striking results of modern enquiry into these matters, is the discovery that the number and superficial area of these root-hairs, on one and the same plant, may vary to a large extent according to the structure, as it were, of the soil, and the degree of moisture it is capable of retaining; or, with the same soil, according to the amount of water which it receives and holds. Correlations have also been observed between the development in length and surface of the rootlets themselves.

      The following illustrations will suffice to show this:

      Six young wheat-plants in soil kept constantly wet, developed roots the total length of which measured 365 mm. each, on the average, and almost devoid of root-hairs.

      Six similar plants in soil only moderately moist, averaged 668 mm., and were well furnished (though not densely covered) with root-hairs.

      Six similar plants in soil which would be termed dry, averaged 371 mm., but were densely covered with rich crops of root-hairs.

      Further researches have shown that the conditions which rule the development of the root-system and root-hairs in the soil are very complex, and not always easy to trace. The most general statements we can make are the following:

      There is an optimum degree of moisture in the soil which promotes the maximum development of root-hairs. If the soil is too wet they are not developed.

      These facts are of importance as correlated with the ease or difficulty experienced by the roots in obtaining water, and plants such as our ordinary agricultural plants show this very distinctly.

      Although, as shown in the experiments with wheat, the short roots in dry soil were more densely covered with root-hairs than the much longer roots in moderately moist soil, subsequent closer investigation shows that the total quantity and area of root-hairs is less in the former case than in the latter.

      The greatest number of root-hairs are developed on roots which are growing at their best: too much moisture may prevent the formation of root-hairs: too little may induce dense growths of root-hairs locally, but the total number is reduced.

      Another set of events which exerts influence on the development of root-hairs is the composition of the dilute solution—water containing dissolved salts—which surrounds them in the soil.

      Thus, Schwarz found that when similar oat and wheat plants were grown with their roots in solutions of various salts, the results differed as follows:

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