Fig. 154.—Common Horsetail. (× ⅙.) 1, Fertile haulms, terminating in cones (a): 2, a sterile haulm; a, tubers: 3, scale of cone with spore-boxes (mag.): 4, scale with ruptured spore-boxes (mag.): 5, 6, 7, spores (mag.).
The haulms are very plainly of two kinds; for some, which are of a pale colour, do not branch, and are developed merely to bear the pretty little cones ([Fig. 154], 1, a) which appear at their upper ends. As the purpose of the cones is to give rise to the next generation, the haulms which bear them may be called the fertile haulms. They will be described presently, when the reproduction of the plant is considered.
The sterile haulms.—The erect branching shoots ([Fig. 154], 2), bear no organs of reproduction, and are hence referred to as the sterile haulms. Their work is to provide the whole plant with carbonaceous food, and it is on this account that their branches are of the characteristic green tint. In most green plants the manufacture of carbonaceous food ([Chapter III.]) takes place mainly in the leaves. In the horsetail, however, the leaves are small and of very little importance in this respect, and the work is carried on by the sterile haulms and their branches.
The underground stem and roots.—A large part of the horsetail plant is hidden beneath the surface of the ground, and often penetrates to a great depth. In ordinary language these underground parts are called roots. They are, however, subterranean stems, as is shown quite plainly by the fact that they bear small leaf-sheaths like those on the stems above ground. In the common horsetail the underground stems often become swollen in parts by the formation of tubers ([Fig. 154]), as is the case with the potato. The true roots are very slender and thread-like, and spring from the nodes ([p. 45]) of the underground stems. They penetrate the soil in all directions, seeking for water, which they take up by the fine hairs which clothe them, like velvet-pile, a little behind their points. The stem and its haulms, like those of grasses, are stiffened by silica, which is deposited in the outer layers. This is of course obtained from the soil. It is not absolutely necessary to the life of the plant, but it is nevertheless very useful, as it enables the haulms to stand upright and spread out their branches to the light and air.
The sterile branches are thus concerned with the horsetail’s daily life. The roots provide it with water and mineral food, and the green branches supply the necessary carbonaceous matter. Stem, branches, and roots are permeated with a complete system of canals, through which the food substances find their way to the various centres of activity.
The reproduction of the horsetail.—It has been seen that some of the haulms do not bear branches, but are set aside for the production of the cones. These fertile haulms come up in March, before the green sterile haulms appear. When the cones are carefully examined they are seen to be covered by a number of shield-shaped scales. To the inner side of each scale ([Fig. 154], 3, 4) are attached from five to ten boxes, each containing a large number of little grains or spores. The boxes burst open when they are ripe, about the end of March, liberating the spores; and the fertile haulms, having performed the one duty for which they were developed, at once die down.
If the spores fall in a favourable situation they germinate, and each gives rise to a new plant. The new plant is, however, not a horsetail, but a small filmy prothallus, somewhat like the prothallus of a fern. Some of the prothalli produce male cells, while others give rise to female cells. When the minute male cells are ripe, they are set free, and are able, by means of fine, lashing threads, to swim towards the female cells of a neighbouring prothallus, through a drop of dew or rain. The two cells fuse together and give rise to a little embryo, which in due course grows up into a new plant—an ordinary horsetail with stems and branches and roots like its grandparent.
The advantage of an alternation of generations.—The life-history of the horsetail is evidently very similar to that of a fern, each exhibiting a well-marked alternation of generations. There is reason for believing that the prothallus-generation is the original form, and that the ordinary fern and horsetail were developed simply to scatter spores at intervals, and so to give new plants the advantages of fresh soil.
