The Horsetails are a small group of flowerless plants, quite distinct from the ferns, though there are certain points in which some resemblance may be traced. We have eight British species out of twenty-five that are known to inhabit the earth. The most widely distributed of these is the Field Horsetail (E. arvense), which farmers regard as a pest. In common with the whole tribe it has a creeping underground rootstock, from which more or less erect jointed stems arise. If we break off one of these joints at its natural articulation we shall observe that the ends are solid, and that the upper extremity is crowned by a sheath ending in long pointed teeth, into which the lower end of the next joint fitted. This leaf-sheath, as it is called, is composed of a number of aborted leaves—the only vestiges of leaves the plant possesses. Just below the leaf-sheath a whorl of jointed branches is given off, each constructed in a manner similar to the upright stem. If now we cut our main joint across its middle with a sharp knife we shall find that it is tubular, a central cavity occupying about one-third of its diameter. Between this cavity and the exterior wall is a series of small tubes, somewhat egg-shaped in outline, the smaller end towards the central cavity; alternating with these and nearer the centre are a number of smaller circular tubes. This section should always be made when in doubt as to the species, for the shape and arrangement of these cavities differs in each, as do the external ridges. The accompanying cuts represent half-sections through the stems of the principal British species. In this species there are about a dozen blunt ridges on the stem, extending right to the points of the leaf-sheath. The branches are four-angled, solid, and jointed and sheathed like the main stem. The cells of the cuticle secrete silica in such quantity that the whole of the vegetable matter may be got rid of by maceration, yet the form of the stem will remain in this transparent skeleton of silica. Certain species are used for polishing metal, under the name of Dutch Rushes.

So far we have been describing what is known as the barren stem, because it ends in several unbranched joints, without any fructification. Before these barren stems appeared there arose from the rootstock a stem differing greatly in appearance, usually without branches, and lacking the green colouring matter (chlorophyll). It is pale brown in colour, of stouter build, but much shorter, for whereas the barren stem is about two feet in length, the fertile is only a few inches, or at most less than a foot. The leaf-sheath is longer, and the teeth frequently adhere two or three together. The stem terminates in a kind of cone, consisting of many whorls of flat scales, each supported by a central stalk, on the underside of which are arranged from six to nine capsules containing spores. These spores are very curious: they are globular in form, and invested with several coats, the outermost of which splits into four narrow strips, which are highly hygroscopic, and which remain attached to the spore at one point only. These elaters, as they are termed, are very sensitive to changes in the humidity of the atmosphere, as may be proved by breathing upon them, however slightly, when they will be seen (through the microscope) to be in active movement. In many ferns the spores require months to elapse before germination takes place; those of Horsetails will germinate in a few hours. Owing to its possession of chlorophyll the spore, if not placed in a situation suitable for germination, perishes in the course of a few days.

The name of the genus is from the Latin, equus, a horse, and seta, a bristle. The fertile stems appear in March and April, the barren ones at intervals later.