[Pg 194] 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 [Pg 195] 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.