Horsetails (Equisetum), a genus of herbaceous plants which in itself constitutes the singular natural order Equisetaceæ. The family is distinguished from all others by the leafless, articulated, and whorled stems and branches, which in structure and character closely resemble some of the larger fossil plants now extinct. They are separated from all other plants also by their fructification, which is an ovoid or oblong terminal cone-like spike, consisting of several whorls of peltate, shield-shaped, short-stalked brown or black scales, under each of which are six or seven capsules filled with minute spores, and opening on the inner side. Under the microscope there will be seen attached to the base of each spore four thread-like filaments, somewhat club-shaped at the apex, rolled spirally round the spore when moist, but uncoiling elastically when dry.
The species of horsetail are few in number, although widely diffused in the temperate and colder regions of the northern hemisphere, becoming rare in the tropics. Nine species occur in Britain, usually in moist or marshy places, but they adapt themselves easily to a great variety of stations, and are almost ineradicable where they obtain a footing in either field or garden. Diuretic and other medicinal properties have been ascribed to them, but apparently on slight grounds. They all contain a large quantity of silica in the cuticle of their stems, which has rendered them useful in polishing metals, marbles, ivory, cabinet-work, &c. E. hyemale is the most favoured species for these purposes, and it is imported in considerable quantity from Holland under the name Dutch Rushes.