Equisetum

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Equisetum

 

(horsetail), a genus of perennial herbs of the family Equisetaceae. Many species are evergreens. From the strongly branched rhizome depart longitudinally grooved aboveground shoots, which, like the rhizome, are segmented into regularly alternating nodes and hollow internodes. The nodes bear whorls of poorly developed leaves; the leaves are fused below in the sheath that embraces the base of the internode. Also departing from the nodes are branches that are arranged in whorls and that penetrate the fused bases of the leaves. The epidermis of the stem is impregnated with silica, which gives the stem rigidity. Photosynthesis is effected by the green stems and branches, since the leaves are poorly developed. Horsetails reproduce by spores, which develop in sporangia on corymbiform sporophylls. The spores are equipped with hygroscopic filaments, or elaters, that promote distribution.

There are about 30 species, distributed throughout the world except in Australia and New Zealand. Most species are slender and low, measuring less than 1 m in height. Exceptions include the climbing tropical South American E. giganteum, which reaches 12 m in length and about 3 cm in diameter, and the Mexican E. schaffneri, whose stems reach about 2 m in height and 10 cm in diameter.

The USSR has 13 or 14 species, growing in swamps, forests, meadows, fields, and bodies of water. A number of species, including E. variegatum, E. scirpoides, and E. sylvaticum, are eaten by deer, certain domestic animals, and game animals (for example, hares, partridges, and black grouse). E. hiemale, which contains a great deal of silica, is used by the local population for polishing. E. arvense, a pernicious weed whose rhizome penetrates deep (1 m or more) into the soil, is sometimes used medicinally—as a diuretic—in the form of an infusion or liquid extract. E. palustre is poisonous to agricultural animals.

M. E. KIRPICHNIKOV