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|Vegetative stem of Equisetum telmateia with a whorl (at each node) of branches and dark-tipped leaves|
The horsetails are a class in the Pteridophyta (ferns). They were one of the most important plant groups in the Palaeozoic era. They are seen in the coal measures of the Carboniferous period, and some were trees reaching up 30 metres. The group is now almost extinct, but one genus survives. They are vascular plants that reproduce by spores and not by seeds. The name horsetail arose because the branched species somewhat resemble a horse's tail.
Equisetum is the only living genus of horsetails. The name Equisetum comes from the Latin (equus = horse; and seta = bristle). The genus includes 15 species. Horsetails are native on all continents except Australasia and Antarctica. They are perennial plants, either herbaceous, they back in winter (most temperate species) or they are evergreen (some tropical species, and the temperate species Equisetum hyemale, E. scirpoides, E. variegatum and E. ramosissimum). They mostly grow 0.2-1.5 m tall, though E. telmateia can exceptionally reach 2.5 m, and the tropical American species E. giganteum 5 m, and E. myriochaetum 8 m.
In these plants the leaves are very small, in whorls joined together to make sheaths around the stem. The stems are green and photosynthetic, also distinctive in being hollow, jointed and ridged, usually with 6-40 ). There may or may not be whorls of branches at the nodes; when present, these branches are identical to the main stem except smaller.
A superficially similar flowering plant (Hippuris, "mare's tail"), unrelated to the genus Equisetum, is occasionally misidentified as a horsetail.
- Other names include candock (applied to branching species only), and scouring-rush (applied to the unbranched or sparsely branched species). The latter name refers to the plants' rush-like appearance; the stems are coated with abrasive silica, which made them useful for cleaning ("scouring") cooking pots in the past.
- Pryer, K. M., Schuettpelz, E., Wolf, P. G., Schneider, H., Smith, A. R., and Cranfill, R. (2004). Phylogeny and evolution of ferns (monilophytes) with a focus on the early leptosporangiate divergences. American Journal of Botany 91: 1582-1598 (available online; pdf file).