Equisetum is the only living genus in the Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds. Equisetum is a "living fossil", as it is the only living genus of the entire class Equisetopsida, which for over one hundred million years was much more diverse and dominated the understory of late Paleozoic forests. Some Equisetopsida were large trees reaching to 30 meters tall; the genus Calamites of family Calamitaceae for example is abundant in coal deposits from the Carboniferous period. A superficially similar but entirely unrelated flowering plant genus, mare's tail (Hippuris), is occasionally misidentified and misnamed as "horsetail". It has been suggested that the pattern of spacing of nodes in horsetails, wherein those toward the apex of the shoot are increasingly close together, inspired Napier to discover logarithms.
Uses
The genus Equisetum is near-cosmopolitan, being absent only from Antarctica. They are perennial plants, either herbaceous and dying back in winter as most temperate species, or evergreen as most tropical species and the temperate species rough horsetail (E. hyemale), branched horsetail (E. ramosissimum), dwarf horsetail (E. scirpoides) and variegated horsetail (E. variegatum). They mostly grow 0.2-1.5 m tall, though the "giant horsetails" are recorded to grow as high as 2.5 m (northern giant horsetail, E. telmateia), 5 m (southern giant horsetail, E. giganteum) or 8 m (Mexican giant horsetail, E. myriochaetum), and allegedly even more. Many plants in this genus prefer wet sandy soils, though some are semi-aquatic and others are adapted to wet clay soils. The stalks arise from rhizomes that are deep underground and almost impossible to dig out. The field horsetail (E. arvense) can be a nuisance weed, readily regrowing from the rhizome after being pulled out. It is also unaffected by many herbicides designed to kill seed plants. However, as E. arvense prefers an acid soil, lime may be used to assist in eradication efforts to bring the soil pH to 7 or 8. Members of the genus have been declared noxious weeds in Australia and in the US state of Oregon. All the Equisetum are classed as "unwanted organisms" in New Zealand and are listed on the National Pest Plant Accord.
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