horsetail, any plant of the genus Equisetum [Lat., = horse bristle], the single surviving genus of a large group (Equisetophyta) of primitive vascular plants. Like the ferns and club mosses, relatives of the living horsetails thrived in the Carboniferous period (when they contributed to coal deposits); the group as a whole is now considered relictual. Horsetails have whorls of small scalelike leaves around a hollow, jointed stem that is green and carries on photosynthesis. They reproduce by an alternation of generations (see reproduction) similar to that of the ferns; in some horsetails, special nongreen shoots have at their tops strobili (see cone) that bear the spores. Fossil evidence indicates that many extinct horsetails were treelike and attained a far greater size than do living types, although the stems of a sprawling tropical American species ( E. giganteum ) grows to more than 30 ft (9.1 m) in length. Other species, mostly under 3 ft (91 cm), are found in all temperate and tropical regions except New Zealand and Australia; the common types of North America and Eurasia are E. arvense in drier habitats and E. hyemale, the scouring rush, in moist and wooded areas. The latter was formerly utilized for scouring purposes and it is still included in some scouring and abrasive powders; its typical coarse texture is due to the presence of silica. Other horsetails have been used for home remedies. Horsetails are classified in the division Equisetophyta, class Equisetopsida, order Equisetales, family Equisetaceae.
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