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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