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Horsetails are a group of relatively primitive vascular plants in the genus Equisetum, family Equisetaceae, subdivision Sphenophytina. The sphenophytes have an ancient evolutionary lineage occurring as far back as the Devonian period. These plants were most abundant and diverse in species about 300 million years ago, during the late Devonian and early Carboniferous periods. Fossils from that time suggest that some of these plants were as large as 8 inches (20 cm) in diameter and at least 49 feet (15 m) tall.

Today, however, this group is represented by 29 species of small, herbaceous plants all in the genus Equisetum. Horsetails are very widespread, although they do not occur naturally in the Amazon basin or in Australia and New Zealand. These plants are characterized by their conspicuously jointed stems and their reduced, scalelike leaves, which are arranged in whorls around the stem. The stems of horsetails contain deposits of silica which give the plants a coarse, grainy feel when crushed. The silica-rich horsetails are often used by campers to clean their dishes and pots, giving rise to another of their common names, the scouring rushes.Horsetails are perennial plants, and they grow from underground systems of rhizomes. Horsetails develop specialized structures known as a strobilus (plural: strobili), containing sporangiophores that develop large numbers of spores (or sporangia). In some species the strobilus develops at the top of the green or vegetative shoot. In other so-called dimorphic species of horsetails, the strobilus occurs at the top of a specialized, whitish shoot which develops before the green shoots in the early springtime.

The woodland horsetail (Equisetum sylvaticum ) occurs throughout the northern hemisphere in boreal and north-temperate forests. The common horsetail (E. arvense ) is a very widespread species occurring almost worldwide, often in disturbed habitats. This species is dimorphic, producing its whitish, fertile shoots early in the springtime and its green vegetative shoots somewhat later. The scouring rush (E. hyemale ) occurs widely in the northern hemisphere in wet places. The water horsetail (E. fluviatile ) occurs in a wide range of aquatic habitats in boreal and north-temperate regions of North America and Eurasia. The dwarf scouring rush (E. scirpoides ) is a small species of wetlands and moist shores, occurring widely in arctic and boreal habitats of the northern hemisphere.

See also Rushes.

Bill Freedman

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