Equisetum

Equisetum, the only existing genus of the order Equisetaoeae and the class Equisetinae of the Pteridophyta (q.v.), comprising some twenty-five species, found in most parts of the world, except Australasia. They are isosporous, like most ferns; but the spores have three coats, of which the outermost is spirally split into four hygroscopic elaters (q.v.), which uncoil in dry air, and serve to link the spores together when dispersed. On germination they produce dioecious prothallia, those least nourished being smaller and bearing antheridia. The antherozoid is comparatively large, twice or thrice coiled, and dilated posteriorly. The female prothallus, like a curly endive leaf half an inch long, bears archegonia like those of ferns. The sporophyte consists of a creeping rhizome sending up erect branches that are generally annual. These are articulated and strongly fluted, each ridge ending at the node above in one leaf-sheath of a ring, which lie in the grooves of the next internode. The interriodes are hollow, the nodes solid; there is a fibro-vascular bundle in each ridge with a central (carinal) air-cavity, and sometimes other (vallecular) air-passages in the cortical, tissue one below each furrow. In the furrows of the stems are numerous stomata. In some species the "fructification" terminates a short special unbranched branch which, having no chlorophyll, is brown; in others it terminates a green branch which bears whorls of branches at each node. It consists of a cone made up of whorls of sporophylls each of which is a pettate sorus bearing from six to nine sporangia round its margin. All the cell-walls of the plant are strongly impregnated with silica, amounting to more than half the ash of the plant, so that large quantities of E. hyemale are imported as "Dutch rushes," and used in bundles for polishing purposes. None of the existing species approach the dimensions of the fossil Equisetinee of Palaeozoic rocks, such as Calamites, Annularia, and Asternphyllites. These all differ apparently from our Equisetaceee in producing spores of two kinds (megaspores eind microspores), being, that is, "heterosporous."