Horsetail, perennial plant of genus Equisetum, the only living representative of the very ancient and primitive class Sphenopsida, tree-sized members of which were prominent in the land vegetation of the Carboniferous era (353-300 million years ago).

Structure

The stems are usually hollow, have cylindrical sheaths of reduced leaves at the nodes, and arise from creeping rhizomes (underground stems). When present, branches are often in whorls at the nodes. Stem internodes are commonly ridged longitudinally, the ridges bearing silica-containing tubercles or bands. They are herbaceous or shrubby and rarely exceed 1 m in height.

Reproduction

Horsetails show a form of alternation of generations (a sexual phase alternating with an asexual one), in which each generation is an independent plant. Spores are produced in spore cases borne on stalks which form a fruiting, terminal cone on the fertile stem. The spores germinate, forming plants (prothallia) on which are borne antheridia and archegonia (structures respectively producing sperm and eggs). The prothallium is the sexual generation. The fertilization of the egg and its subsequent development produces the familiar horsetail plant, the asexual generation.

Distribution and Habitat

Fifteen species of Equisetum (mostly of worldwide distribution) and many sterile hybrids (some widespread) now exist. Ten species occur in Canada: smooth scouring rush (E. laevigatum) mainly in the central regions; giant horsetail (E. telmateia) on the West Coast and in the Okanagan Valley; common horsetail (E. arvense) and variegated horsetail (E. variegatum) transcontinentally and far N of the treeline. Six other species are transcontinental, but do not occur in the High Arctic. Most species occupy moist environments. Common horsetail and scouring rush (E. hyemale) are sometimes considered weeds.

Human Use

Settlers used silica-encrusted stems of both these species for cleaning and sanding. The former is still used to hone woodwind reeds. Common horsetail has been reported poisonous to livestock in Canada.