THE Equiseta are leafless, herbaceous plants, annually renewed from a creeping rhizome, and growing in marshy land, in pools and ditches, on the banks of rivulets, and in rivers, from Lapland and Siberia to within the tropics. There is but one genus, and few species. The largest of the ten or eleven species, which are indigenous in Great Britain, is not more than five or six feet high, but they are of greater size in warm climates. The Horsetails begin their lives precisely like the Ferns; for, when a spore begins to germinate, it forms a marchantioid leaf or prothallus lying flat on the ground, upon which are produced antheridia, full of cells, in each of which there is a spermatozoid with numerous cilia. Archegonia are also formed on the prothallus. These, after fertilization, give rise to the perfect plant, which throws out a rhizome, whence new shoots are produced. ‘The structure of the rhizome is very different from that of ferns. In an early stage it consists of a central column of cellular tissue, sending off about eight radiating plates, which connect it with an external cylinder of the same tissue, and opposite to each of which there is, in the central column, a vascular bundle consisting of annular vessels passing into spiral. At a later period, tissue grows from the walls into the cavities, in such wise that they are more or less perfectly obliterated.’[75] From the rhizome, which often extends to a great length, the stems of the Equisetums rise in the form of rough, rigid, hollow cylinders, striated longitudinally, and articulated at intervals by separable joints. Each articulation is invested at its base by a toothed membranaceous sheath, from beneath which, in the greater number of the species, whorls of branches spring, jointed like the stem, and similar to it even to the number of teeth in the sheaths and striæ on the surface, but unbranched. [Illustration: Fig. 70. Equisetum giganteum:—_a_, fragment of stem with branches; _b_, cone or spike of fructification; _c_, one of the scales of the cone; _d_, spore with its elastic filaments.]