HORSETAIL, Equisetum, the sole genus of the natural order Equisetaceae, consists of a group of vascular cryptogam ous plants remarkable for its resemblance in general appearance to the phanerogamic genera Casuarina and Ephedra. The stem is jointed, consisting of numerous easily separable tubular sheaths toothed at the apex, and is generally furnished with whorls of similar but more slender branches. The fructification is borne at the apex of the stem in the form of a dense oval, oblong, or cylindrical spike, consisting of a number of shortly-stalked peltate scales, each of which has attached to its under surface a circle of spore cases. These open by a longitudinal slit on their inner side. The spores differ from those of ferns in their outer coat being split up into four club-shaped hygroscopic threads or elaters, which are curled when moist, but become straightened when dry. The apparent roots consist of underground stems, any portion of which broken off is capable of producing a new plant; hence the difficulty of eradicating them when once established. There are 25 known species of the horsetail, and the genus is universally distributed....
The Corn Horsetail, E. arvense, L., one of the commonest species, is a troublesome weed in clayey cornfields. The fructification appears in March and April, terminating in short unbranched stems. It is said to produce diarrhoea in such cattle as eat it. The Bog Horsetail, E. palustre, is said to possess similar properties. It grows in marshes, ditches, pools, and drains in meadows, and sometimes obstructs the flow of water with its dense matted roots. The fructification in this species is cylindrical, and in that of E. limosum,, L., which grows in similar situations, it is ovate in outline. The largest British species, E. maximum, L., grows in wet sandy declivities by railway embankments or streams, <fcc., and is remarkable for its beauty, due to the abundance of its elegant branches and the alternately green and white appearance of the stem. In this species the fructification is conical or lanceolate, and is found in the month of April, on short stout unbranched stems, which have large loose sheaths. Horses appear to be fond of this species, and in Sweden it is stored for use as winter fodder. E. hyemale, L., commonly known as the Dutch rush, is much more abundant in Holland than in Britain; it is used for polishing purposes, and also in medicine by homoeopathic practitioners. E. variegatum, Sch., grows on wet sandy ground, and serves by means of its fibrous roots to bind the sand together. The horsetails are remarkable for the large quantity of silica they contain, which often amounts to half the weight of the ash yielded by burning them, and the roots contain a quantity of starch.
