Equisetum

Equisetum (/ˌɛkwɪˈsiːtəm/; horsetail) is the only living genus in Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds.^[2]

Quick Facts Scientific classification, Type species ...

Equisetum Temporal range: Early Jurassic–present PreꞒ Ꞓ O S D C P T J K Pg N
Vegetative stems of Equisetum telmateia (great horsetail), showing whorls of branches and the tiny dark-tipped leaves
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Division:	Polypodiophyta
Class:	Polypodiopsida
Subclass:	Equisetidae
Order:	Equisetales
Family:	Equisetaceae
Genus:	Equisetum L.
Type species
Equisetum arvense L.
Species
See text
Synonyms[1]
Allostelites Börner Hippochaete Milde

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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 30 m (98 ft) tall.^[3] 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.^[4] 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.^[5]

This section needs additional citations for verification. (August 2018)

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').^[6]

Other names include candock for branching species, marestail, puzzlegrass, 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.^[7] 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.^[8]

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.^[9] Unusually, the branches often emerge below the leaves in an internode, and grow from buds between their bases.

The genus Equisetum as a whole, while concentrated in the non-tropical northern hemisphere, is near-cosmopolitan, being absent naturally only from Antarctica, Australia, New Zealand, and the islands of the Pacific Ocean. They are most common in northern Europe, with ten species (E. arvense, E. fluviatile, E. hyemale, E. palustre, E. pratense, E. ramosissimum, E. scirpoides, E. sylvaticum, E. telmateia, and E. variegatum); Great Britain has nine of these species, missing only E. scirpoides of the European list.^[26][9] Northern North America (Canada and the northernmost United States), also has nine species (E. arvense, E. fluviatile, E. laevigatum, E. palustre, E. praealtum, E. pratense, E. scirpoides, E. sylvaticum, and E. variegatum). Only five (E. bogotense, E. giganteum, E. myriochaetum, E. ramosissimum, and E. xylochaetum) of the eighteen species are known to be native south of the Equator.

They are perennial plants, herbaceous and dying back in winter in most temperate species, or evergreen as most tropical species and the temperate species E. hyemale (rough horsetail), E. ramosissimum (branched horsetail), E. scirpoides (dwarf horsetail) and E. variegatum (variegated horsetail). They typically grow 20 cm–1.5 m (8 in–5 ft) tall, though the subtropical "giant horsetails" are recorded to grow as high as 5 m (16 ft) (E. giganteum, southern giant horsetail) or 8 m (26 ft) (E. myriochaetum, Mexican giant horsetail), and allegedly even more.^[27]

One species, Equisetum fluviatile, is an emergent aquatic, rooted in water with shoots growing into the air. The stalks arise from rhizomes that are deep underground and difficult to dig out. Field horsetail (E. arvense) can be a nuisance weed, readily regrowing from the rhizome after being pulled out. It is unaffected by many herbicides designed to kill seed plants.^{[28][citation needed]} Since the stems have a waxy coat, the plant is resistant to contact weedkillers like glyphosate.^[29] 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.^[30] Members of the genus have been declared noxious weeds in Australia and in the US state of Oregon.^[31][32]

All the Equisetum are classed as "unwanted organisms" in New Zealand and are listed on the National Pest Plant Accord.^[33]

People have regularly consumed horsetails. The fertile stems bearing strobili of some species can be cooked and eaten like asparagus^[34] (a dish called tsukushi (土筆) in Japan^{[35][failed verification]}). Indigenous nations across Cascadia consume and use horsetails in a variety of ways, with the Squamish calling them sx̱ém'x̱em and the Lushootseed using gʷəɫik, or horsetail roots, for cedar root baskets.^[36][37][38] The young plants are eaten cooked or raw, but considerable care must be taken.^[39]

If eaten over a long enough period of time, some species of horsetail can be poisonous to grazing animals, including horses.^[40] The toxicity appears to be due to thiaminase, which can cause thiamin (vitamin B1) deficiency.^{[39][41][42][43]}

Equisetum species may have been a common food for herbivorous dinosaurs. With studies showing that horsetails are nutritionally of high quality, it is assumed that horsetails were an important component of herbivorous dinosaur diets.^[44] Analysis of the scratch marks on hadrosaur teeth is consistent with grazing on hard plants like horsetails.^[45]

Extracts and other preparations of E. arvense have served as herbal remedies, with records dating over centuries.^[39][41][46] In 2009, the European Food Safety Authority concluded there was no evidence for the supposed health effects of E. arvense, such as for invigoration, weight control, skincare, hair health or bone health.^[47] As of 2018^[update], there is insufficient scientific evidence for its effectiveness as a medicine to treat any human condition.^[39][46][47]

E. arvense contains thiaminase, which metabolizes the B vitamin, thiamine, potentially causing thiamine deficiency and associated liver damage, if taken chronically.^[39][41] Horsetail might produce a diuretic effect.^[39][41] Further, its safety for oral consumption has not been sufficiently evaluated and it may be toxic, especially to children and pregnant women.^[39]

• List of plants poisonous to equines