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.

The fructification is occasionally on separate stems, which make their appearance before the barren ones, and are in general unbranched and succulent, bearing a cone on their apex. The surface of the cone is at first smooth or indistinctly reticulated, but it eventually splits into numerous octagonal, brown, shield-like discs, spirally arranged, which, separated from the stem, are found to have a stalk, and to bear on the under-side four or eight pendent sac-like bodies of a whitish hue. These are the sporangia, which open on the inside by a slit for the discharge of the spores. The inner coat of the cells of the sporangium is composed of beautifully spiral tissue. The spores arise by cell division, each being covered by a separate membrane, which ultimately forms a pair of elastic fibres attached by their middle, closely coiled round the spore, as it is formed within the cell; but, when the spore is liberated, they extend; the least moisture, however, even the breath, makes them contract.

The Equisetaceæ differ from the Ferns in having spiral instead of scalariform vessels in their structure; but they agree in having only one period of fertilization, after which they produce a crop of spores year after year, and these, when sown, give rise to the primary leaf with its antheridia and archegonia—a perpetual cycle.

The whole substance of the stems of the Equisetums is so completely penetrated with silex, that a silicious skeleton remains after the herbaceous part is destroyed, and in some species as much as thirteen per cent. of the whole plant, and fifty per cent. of its ashes are found to be pure silex. On account of the crystallization of the silex, the Horsetails form some of the most beautiful of microscopic objects. If a fragment of the cuticle is magnified and viewed by polarized light, the colours are seen to be intense and vivid, and the arrangement of the silicious particles so elaborate and symmetrical as to resemble necklaces of diamonds and coloured gems. The rows of crystals run in lines parallel to the axis of the stem, the greater number being so disposed, but the rest are grouped so as to form ovals joined together by a chain of particles forming a sort of curvilinear quadrangle, these rows of oval combinations being arranged in pairs. The effect is brilliant when they are seen in polarized light. According to Sir David Brewster every particle has a regular axis of double refraction; but Professor Bailey, of the United States of America, states that the effect under polarized light is not produced by the silicious particles, but by the organized tissues, since, after these have been destroyed, the silex shows no double refraction.

The vascular tissue of the Equisetums shows them to be of a higher class than the Ferns. The recent forms vary much in size, but even the Equisetum giganteum of Brazil, which is eighteen feet high and three-quarters of an inch in diameter, is incomparably less than the Calamites and other fossil Equiseta, which appear in the coal measures and new red sandstone. Recent species are found in Iceland and the high northern latitudes of America, in the tropics, and in most parts of the world, except Australia and New Zealand.