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Equisetum (/ ˌ ɛ k w ɪ ˈ s iː t əm /; horsetail) is the only living genus in Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds. [2]Equisetum is a "living fossil", the only living genus of the entire subclass Equisetidae, which for over 100 million years was much more diverse and dominated the understorey of late Paleozoic forests.
Equisetum arvense, the field horsetail or common horsetail, is an herbaceous perennial plant in the Equisetidae (horsetails) sub-class, native throughout the arctic and temperate regions of the Northern Hemisphere. It has separate sterile non-reproductive and fertile spore-bearing stems growing from a perennial underground rhizomatous stem system.
In the only extant genus Equisetum, these are small leaves (microphylls) with a singular vascular trace, fused into a sheath at each stem node. However, the leaves of Equisetum probably arose by the reduction of megaphylls , as evidenced by early fossil forms such as Sphenophyllum , in which the leaves are broad with branching veins.
The green stems grow 50–150 cm tall and 2–8 mm thick. The leaf sheaths are narrow, with 15-20 black-tipped teeth. [2] Many, but not all, stems also have whorls of short ascending and spreading branches 1–5 cm long, with the longest branches on the lower middle of the stem.
Equisetum palustre, the marsh horsetail, [2] is a perennial herbaceous pteridophyte belonging to the subclass of horsetails (Equisetidae). It is widespread in cooler regions of Eurasia and North America .
Equisetum hyemale (rough horsetail [2]) is an evergreen perennial herbaceous pteridophyte in the horsetail family Equisetaceae native to Eurasia and Greenland. It was formerly widely treated in a broader sense including a subspecies (subsp. affine ) in North America, but this is now treated as a separate species, Equisetum praealtum .
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Equisetum telmateia, the great horsetail, is a species of Equisetum (horsetail) native to Europe, western Asia and northwest Africa. [ 2 ] [ 3 ] It was formerly widely treated in a broader sense including a subspecies (subsp. braunii ) in western North America, but this is now treated as a separate species, Equisetum braunii .