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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:

Oats in a 15 per cent. solution of calcium chloride developed no root-hairs, though they formed in a 5 per cent. solution, and were very numerous in a 0.5 per cent. solution, or in water alone. In a 10 per cent. nutritive solution the plants developed no root-hairs, though they were abundant in a 1 per cent. solution.

Wheat plants with their roots in a 15 per cent. solution of potassium nitrate bore no root-hairs, but they were numerous in a 2 per cent. solution of the same salt.

These are extreme cases, for, although the roots were not killed, they were strongly inhibited in their growth by the more concentrated solutions. However, experiments of this kind at least bring vividly before us what variations are possible, and suggest that similar events on a smaller scale may occur in a soil which yields large quantities of soluble substances, e.g. when freshly manured. Obviously these facts have a practical significance as regards kind of soil, drainage, season (e.g. drought or wet), etc.

But there are other factors which rule the development of root-hairs, and some experiments by Lesage show that the correlations between the development of root-hairs and roots are probably much more complex than had been suspected; for he finds that if the lateral rootlets of a Bean, in a water culture, are suppressed, the main rootlet develops numerous and very long hairs to compensate the loss in surface, a matter of obvious importance in the discussion of cases where roots have been injured in the soil.

Before proceeding further it is necessary to look a little more closely into the structure of a single hair.

It is a tubular prolongation of a single cell of the external covering of the young root, usually about 1 to 3 mm. in length, and 0.01 to 0.10 mm. in diameter. In special cases the root-hairs of some water plants may reach 5 to 18 mm. in length, but of course I am referring to the ordinary land plants of agriculture and forestry. This tubular prolongation is closed and rounded off at the distal free end, and opens at the proximal end into the cell of which it is a protrusion.

The whole structure is bounded by an extremely delicate and elastic wall of cellulose, which Frank says is of special composition, almost too thin to measure in many cases, but often somewhere near 0.005 to 0.001 mm. in thickness. This thin membrane is remarkably permeable by water, or dilute solutions, as is shown by the rapidity with which a root-hair collapses if exposed to evaporation, or with which dense solutions abstract water from it, or with which solutions may be seen to penetrate it under the microscope.

Overlying the thin cell-wall proper, on the outside, is a thin gelatinous layer, a product of alteration of the outermost lamellæ of the former.

Closely lining the proper cell-wall on the inside, is an extremely thin layer of living protoplasm, and somewhere in this protoplasm is a distinct cell-nucleus.

The interior of the tube is filled with cell-sap, and it is the osmotic pressure of this cell-sap which keeps the whole living instrument tense and rigid, and the thin protoplasmic film close pressed against the cellulose cell-wall.

Nothing whatever can pass into the cell-sap, or out from it, without traversing both the lining of living protoplasm and the cell-wall.

If we gently pull a living root, of wheat, pea, mustard, etc., from a normal soil, we find particles of soil so closely adherent to the root-hairs that they cannot all be washed off without tearing the hairs: the root-hairs establish relations of contact with these particles, so close that they are cemented to the solid surfaces by means of the gelatinous layer already referred to. This peculiarity has the following consequences. In the first place, the enormous holdfast, ensured by the millions of points of adherence, enables the plant to withstand even powerful lever actions from above, and provides fixed points against which the root-tips can work as they drive deeper into the soil. In the second place, the intimate contact of the root-hairs and particles of soil, ensures that the films of water held by surface-action on the soil-particles and root-hairs shall be in continuity with the water saturating the cell-walls of the latter, and therefore with the protoplasm and cell-sap in their interior. The importance of this at periods when the soil is "dry" will be obvious, when we reflect that no soil is ever naturally so dry that surface-films of water are absent from the particles.

The fact that the root-hair contains living protoplasm, enables us to understand to a certain extent the results of the following experiments.

If we have a leafy and healthy plant, with roots, bearing numerous root-hairs, properly established in suitably moist soil in the pot, the roots cease to absorb water if the temperature of the soil falls below a certain minimum, though they recommence to do so if the temperature is raised again: this has nothing to do with the temperature of the upper parts of the plant, or of the air, and the latter may be so high that the plant rapidly droops from loss of water at the leaves, which is not being compensated owing to the inactivity of the roots.

Similarly we may have the air so cold, at a time when the soil is warm enough to keep the root-hairs actively at work, that the plant becomes surcharged with water, which escapes from the leaves like drops of dew. The temperatures necessary to cause these disturbances in the action of the living root-hairs vary for different plants, and even for different varieties of the same species.

Similar arrestation of the functions of the roots may be brought about by removing the oxygen from the soil around the root-hairs, and replacing it by carbon-dioxide, or the vapour of chloroform. If not kept too long in such a condition, the plant recovers rapidly on admitting atmospheric oxygen, which is always present in a normal well-drained soil both as gas in the capillary interspaces, and dissolved in the water on the surfaces of the particles. If the access of oxygen is delayed, however, as often happens in rainy seasons and in wet soils, the root-hairs are killed, and rot sets in. A good instance of this has lately been given by Heinricher in the case of potatoes.

Notes on Chapter V

For the further pursuit of this subject the reader should consult Sachs' Lectures, II. and XV.; Sorauer, A Popular Treatise on the Physiology of Plants, 1895, chapters II. and IV., and Pfeffer's Physiology, pp. 149-163. The principal paper on root-hairs referred to in the text is Schwarz, "Die Wurzelhaare der Pflanzen," in Unters. aus dem bot. Inst. zu Würzburg, I. Heft 2, 1883, p. 140, where a very exhaustive account of these organs will be found.