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Genus of vascular plants in the family Equisetaceae

Equisetum (,/ɛ//k//w//ᵻ/'/s//iː//t//əm/; horsetail, marestail, snake grass, puzzlegrass) is the only living genus in Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds. 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. Some equisetids were large trees reaching to tall. The genus Calamites of the family Calamitaceae, for example, is abundant in coal deposits from the Carboniferous period. The pattern of spacing of nodes in horsetails, wherein those toward the apex of the shoot are increasingly close together, is said to have inspired John Napier to invent logarithms. Modern horsetails first appeared during the Jurassic period. A superficially similar but entirely unrelated flowering plant genus, mare's tail (Hippuris), is occasionally referred to as "horsetail", and adding to confusion, the name "mare's tail" is sometimes applied to Equisetum. Despite centuries of use in traditional medicine, there is no evidence that Equisetum has any medicinal properties.

The name "horsetail", often used for the entire group, arose because the branched species somewhat resemble a horse's tail. Similarly, the scientific name Equisetum is derived from the Latin equus ('horse') + seta ('bristle'). Other names include candock for branching species, and snake grass or scouring rush for unbranched or sparsely branched species. The latter name refers to the rush like appearance of the plants and to the fact that the stems are coated with abrasive silicates, making them useful for scouring (cleaning) metal items such as cooking pots or drinking mugs, particularly those made of tin. Equisetum hyemale, rough horsetail, is still boiled and then dried in Japan to be used for the final polishing process on woodcraft to produce a smooth finish. In German, the corresponding name is Zinnkraut ('tin herb'). In Spanish speaking countries, these plants are known as cola de caballo ('horsetail').

Equisetum leaves are greatly reduced and usually non photosynthetic. They contain a single, non branching vascular trace, which is the defining feature of microphylls. However, it has recently been recognised that horsetail microphylls are probably not ancestral as in lycophytes (clubmosses and relatives), but rather derived adaptations, evolved by reduction of megaphylls. The leaves of horsetails are arranged in whorls fused into nodal sheaths. The stems are usually green and photosynthetic, and are distinctive in being hollow, jointed and ridged (with sometimes 3 but usually 6–40 ridges). There may or may not be whorls of branches at the nodes. Unusually, the branches often emerge below the leaves in an internode, and grow from buds between their bases.

The spores are borne under sporangiophores in strobili, cone like structures at the tips of some of the stems. In many species the cone bearing shoots are unbranched, and in some (e. g. E. arvense, field horsetail) they are non photosynthetic, produced early in spring. In some other species (e. g. E. palustre, marsh horsetail) they are very similar to sterile shoots, photosynthetic and with whorls of branches. Horsetails are mostly homosporous, though in the field horsetail, smaller spores give rise to male prothalli. The spores have four elaters that act as moisture sensitive springs, assisting spore dispersal through crawling and hopping motions after the sporangia have split open longitudinally. They are photosynthetic and have a lifespan that is usually two weeks at most, but will germinate immediately under humid conditions and develop into a gametophyte.

The crude cell extracts of all Equisetum species tested contain mixed linkage glucan : xyloglucan endotransglucosylase (MXE) activity. This is a novel enzyme and is not known to occur in any other plants. In addition, the cell walls of all Equisetum species tested contain mixed linkage glucan (MLG), a polysaccharide which, until recently, was thought to be confined to the Poales. The evolutionary distance between Equisetum and the Poales suggests that each evolved MLG independently. The presence of MXE activity in Equisetum suggests that they have evolved MLG along with some mechanism of cell wall modification. Non Equisetum land plants tested lack detectable MXE activity. An observed negative correlation between XET activity and cell age led to the suggestion that XET is catalysing endotransglycosylation in controlled wall loosening during cell expansion. The lack of MXE in the Poales suggests that there it must play some other, currently unknown, role. Due to the correlation between MXE activity and cell age, MXE has been proposed to promote the cessation of cell expansion.

The living members of the genus Equisetum are divided into three distinct lineages, which are usually treated as subgenera. The name of the type subgenus, Equisetum, means "horse hair" in Latin, while the name of the other large subgenus, Hippochaete, means "horse hair" in Greek. Hybrids are common, but hybridization has only been recorded between members of the same subgenus. While plants of subgenus Equisetum are usually referred to as horsetails, those of subgenus Hippochaete are often called scouring rushes, especially when unbranched. Two Equisetum plants are sold commercially under the names Equisetum japonicum (barred horsetail) and Equisetum camtschatcense (Kamchatka horsetail). These are both types of E. hyemale var. hyemale, although they may also be listed as separate varieties of E. hyemale.