EQUISETUM, a genus of cryptogamous plants, the structure and affinities of which arc not yet well understood, but which many botanists regard as constituting a sub-order of ferns, whilst others prefer to make it a distinct order, equisetacew. The English name HORSE-TAIL is often given to all the species. They have a leafless, cylindrical, hollow, and jointed stem, each joint terminating in a membranous and toothed sheath, which incloses the base of the one above it. The fructification is at the summit of the stem in spikes, which somewhat resembles trobiles (cones), and are formed of scales bearing spore-cases on their lower surface. The spores are minute, oval, or round, green, and each accompanied with four elastic and hygrometrical threads. These threads are sometimes called elaters, but it is by no means certain that they arc of the same nature with the spiral filaments so called, which are mixed with the spores of many hepatiew (q.v.). Each thread terminates in a kind of club. The stems generally have lateral branches, angular, but otherwise similar iu structure to the stem, growing in whorls from the joints; sometimes the stem is simple ; or fertile stems are simple, and sterile stems are branched. The species of this genus contain a peculiar acid, called equisetic acid. Astringent and diuretic properties exist in these plants, and they were formerly used in medicine, but are not now regarded as of much value. It has been said that they are very
injurious to cattle which eat them, but this seems to require confirmation. They abound chiefly in damp soils, and sometimes so much that the plow and harrow, or the grub ber, must be employed to extirpate them: Some of them, however, grow in dry fields and gardens; whilst others are found chiefly in ditches or the banks of rivers. They exist in almost all parts of the world, and are seldom of large size, varying from a few inches to a few feet in height, but a comparatively gigantic species has recently been discovered in tropical America. The rough siliceous stems of some species are used for smoothing and polishing wood, particularly those of B. Agemale, which are imported into Britain in considerable quantities from Holland, under the name of DUTCH RUSHES. The stems of this species are unbranched, or a little branched only at the base. It is not uncommon in Britain, and is found also, rather sparingly, in North America. It has been proposed to cultivate it, as it grows well under trees, where few other plants would thrive. The stems of other species, as E. arrense, the most common of all the British species, arc used for scouring tin and pewter vessels.