Equisetum is a "living fossil", the only living genus of the entiresubclassEquisetidae, which for over 100 million years was much more diverse and dominated theunderstorey of latePaleozoic forests. Some equisetids were largetrees reaching to 30 m (98 ft) tall.[3] The genusCalamites of the familyCalamitaceae, for example, is abundant incoal deposits from theCarboniferous 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 inspiredJohn Napier to inventlogarithms.[4] Modern horsetails first appeared during theJurassic period.
A superficially similar but entirely unrelatedflowering plant genus, mare's tail (Hippuris), is occasionally referred to as "horsetail", and adding to confusion, the name "mare's tail" is sometimes applied toEquisetum.[5]
The name "horsetail", often used for the entire group, arose because the branched species somewhat resemble ahorse's tail. Similarly, thescientific nameEquisetum is derived from theLatinequus ('horse') +seta ('bristle').[6]
Other names includecandock for branching species,puzzlegrass, andsnake grass orscouring-rush for unbranched or sparsely branched species. The latter name refers to therush-like appearance of the plants and to the fact that the stems are coated with abrasivesilicates, making them useful for scouring (cleaning) metal items such as cooking pots or drinking mugs, particularly those made oftin.[7]Equisetum hyemale, rough horsetail, is still boiled and then dried inJapan to be used for the final polishing process onwoodcraft to produce a smooth finish.[8] InGerman, the corresponding name isZinnkraut ('tin-herb') orSchachtelhalm ('nesting stalk'). InSpanish-speaking countries, these plants are known ascola de caballo ('horsetail').
Equisetumleaves are greatly reduced and usually non-photosynthetic. They contain a single, non-branchingvascular trace, which is the defining feature ofmicrophylls. However, it has recently been recognised that horsetail microphylls are probably not ancestral as inlycophytes (clubmosses and relatives), but rather derivedadaptations, evolved by reduction ofmegaphylls.[9]
The leaves of horsetails are arranged inwhorls fused intonodal 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.[10] Unusually, the branches often emerge below the leaves in an internode, and grow from buds between their bases.
Vegetative stem: B = branch in whorl I = internode L = leaves N = nodeStrobilus ofEquisetum braunii, terminal on an unbranched stemMicroscopic view ofEquisetum hyemale (rough horsetail) (2-1-0-1-2 is onemillimetre with1⁄20thgraduation). The small white protuberances are accumulatedsilicates oncells.
Thespores are borne undersporangiophores instrobili, 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 tosterile shoots, photosynthetic and with whorls of branches.[11]: 12–15
Horsetails are mostlyhomosporous, though in the field horsetail, smaller spores give rise to maleprothalli. The spores have fourelaters that act as moisture-sensitive springs, assisting spore dispersal through crawling and hopping motions after thesporangia have split open longitudinally.[12] They are photosynthetic and have a lifespan that is usually two weeks at most, but will germinate immediately under humid conditions and develop into agametophyte.[13]
The crude cell extracts of allEquisetum species tested containmixed-linkage glucan : xyloglucan endotransglucosylase (MXE) activity.[14] This is a novel enzyme and is not known to occur in any other plants. In addition, the cell walls of allEquisetum species tested containmixed-linkage glucan (MLG), apolysaccharide which, until recently, was thought to be confined to thePoales.[15][16] The evolutionary distance betweenEquisetum and the Poales suggests that each evolved MLG independently. The presence of MXE activity inEquisetum 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 betweenXET activity and cell age led to the suggestion that XET is catalysing endotransglycosylation in controlled wall-loosening during cell expansion.[17] 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.[citation needed]
Currently, 18 species ofEquisetum are accepted byPlants of the World Online.[1] The living members are divided into three distinct lineages, which are usually treated assubgenera. The name of the type subgenus,Equisetum, means "horse hair" inLatin, while the name of the other large subgenus,Hippochaete, means "horse hair" inGreek.Hybrids are common, but hybridization has only been recorded between members of the same subgenus.[18]
TwoEquisetum plants are sold under the namesEquisetum japonicum (barred horsetail) andEquisetum camtschatcense (Kamchatka horsetail). These are both types ofE. hyemale var.hyemale, although they may also be listed as separate varieties ofE. hyemale.[19][citation needed]
The oldest remains of modern horsetails of the genusEquisetum first appear in the Early Jurassic, represented byEquisetum dimorphum from the Early Jurassic of Patagonia[24] andEquisetum laterale from the Early-Middle Jurassic of Australia.[25][26]Silicified remains ofEquisetum thermale from the Late Jurassic of Argentina exhibit all the morphological characters of modern members of the genus.[27] The estimated split betweenEquisetum bogotense and all other livingEquisetum is estimated to have occurred no later than the Early Jurassic.[26]
Equisetum bogotenseKunth – Andean horsetail; uplandSouth America up toCosta Rica; includesE. rinihuense, sometimes treated as a separate species. Previously included in subg.Equisetum, but Christenhuszet al. (2019)[28] transfer this here, asE. bogotense appears to be sister to all the remaining species in the genus.
Equisetum arvenseL. – field horsetail or common horsetail; circumboreal down through temperate zones
Equisetum brauniiMilde – northern giant horsetail, syn.E. telmateia subsp.braunii (Milde) Hauke.; west coast of North America
Equisetum diffusumD.Don – Himalayan horsetail; Himalayan India and China and adjacent nations above about 450 metres (1,480 ft)
Equisetum fluviatileL. – water horsetail; circumboreal down through temperate zones
Equisetum palustreL. – marsh horsetail; circumboreal down through temperate zones
Equisetum pratenseEhrh. – shady horsetail, meadow horsetail, shade horsetail; circumboreal except for tundra down through cool temperate zones
Equisetum sylvaticumL. – wood horsetail; circumboreal down through cool temperate zones, more restricted in east Asia
Equisetum telmateiaEhrh. – great horsetail; Europe to Asia Minor and north Africa. The former North American subspeciesEquisetum telmateia subsp.braunii (Milde) Hauke is now treated as a separate speciesEquisetum brauniiMilde[28][1]
Equisetum giganteumL. – southern giant horsetail or giant horsetail; temperate to tropical South America and Central America north to southern Mexico
Equisetum hyemaleL. – rough horsetail; most of non-tropical Old World. The former North American subspeciesEquisetum hyemale subsp.affine(Engelm.) A.A.Eat. is now treated as a separate speciesEquisetum praealtumRaf.[28][1]
Equisetum laevigatumA.Braun – smooth horsetail, smooth scouringrush; western 3/4 of North America down into northwestern Mexico; also sometimes known asEquisetum kansanum
The genusEquisetum as a whole, while concentrated in the non-tropical northern hemisphere, is near-cosmopolitan, being absent naturally only fromAntarctica,Australia,New Zealand, and the islands of thePacific 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, andE. variegatum);Great Britain has nine of these species, missing onlyE. scirpoides of the European list.[31][10] 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, andE. variegatum). Only five (E. bogotense,E. giganteum,E. myriochaetum,E. ramosissimum, andE. xylochaetum) of the eighteen species are known to be native south of the Equator.
They areperennial plants,herbaceous and dying back in winter in most temperate species, orevergreen as most tropical species and the temperate speciesE. hyemale (rough horsetail),E. ramosissimum (branched horsetail),E. scirpoides (dwarf horsetail) andE. 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.[32]
One species,Equisetum fluviatile, is an emergentaquatic, rooted in water with shoots growing into the air. The stalks arise fromrhizomes that are deep underground and difficult to dig out. Field horsetail (E. arvense) can be a nuisanceweed, readily regrowing from the rhizome after being pulled out. It is unaffected by manyherbicides designed to killseed plants.[33][citation needed] Since the stems have a waxy coat, the plant is resistant to contact weedkillers like glyphosate.[34] However, asE. arvense prefers an acid soil,lime may be used to assist in eradication efforts to bring thesoil pH to 7 or 8.[35] Members of the genus have been declared noxious weeds inAustralia and in the US state ofOregon.[36][37]
People have regularly consumed horsetails. The fertile stems bearing strobili of some species can be cooked and eaten like asparagus[39] (a dish calledtsukushi (土筆) inJapan[40][failed verification]). Indigenous nations acrossCascadia consume and use horsetails in a variety of ways, with theSquamish calling themsx̱ém'x̱em and theLushootseed usinggʷəɫik, or horsetail roots, for cedar root baskets.[41][42][43] The young plants are eaten cooked or raw, but considerable care must be taken.[44]
If eaten over a long enough period of time, some species of horsetail can bepoisonous to grazing animals, includinghorses.[45] The toxicity appears to be due tothiaminase, which can cause thiamin (vitamin B1) deficiency.[44][46][47][48]
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.[49] Analysis of the scratch marks on hadrosaur teeth is consistent with grazing on hard plants like horsetails.[50]
Extracts and other preparations ofE. arvense have served asherbal remedies, with records dating over centuries.[44][46][51] In 2009, theEuropean Food Safety Authority concluded there was no evidence for the supposedhealth effects ofE. arvense, such as for invigoration, weight control, skincare, hair health or bone health.[52] As of 2018[update], there is insufficient scientific evidence for its effectiveness as a medicine to treat any human condition.[44][51][52]
^abcd"Equisetum L."Plants of the World Online. Board of Trustees of the Royal Botanic Gardens, Kew. 2024. Retrieved15 September 2024.
^Dunmire, John R.; Williamson, Joseph F. (1995). "EQUISETUM hyemale". In Brenzel, Kathleen N. (ed.).Western Garden Book. Menlo Park, CA: Sunset. pp. 274, 606.ISBN0376038500.
^Gould, R. E. 1968. Morphology ofEquisetum laterale Phillips, 1829, andE. bryanii sp. nov. from the Mesozoic of south‐eastern Queensland. Australian Journal of Botany 16: 153–176.
^abcdChristenhusz, Maarten J M; Bangiolo, Lois; Chase, Mark W; Fay, Michael F; Husby, Chad; Witkus, Marika; Viruel, Juan (April 2019). "Phylogenetics, classification and typification of extant horsetails (Equisetum, Equisetaceae)".Botanical Journal of the Linnean Society.189 (4):311–352.doi:10.1093/botlinnean/boz002.
^Fitter, Richard; Fitter, Alastair; Farrer, Anne (1984).Collins Guide to the Grasses, Sedges, Rushes and Ferns of Britain and Northern Europe. London: Collins. pp. 188–191.ISBN0-00-219136-9.
^Gunther, Erna (1973).Ethnobotany of western Washington: the knowledge and use of indigenous plants by Native Americans (Revised ed.). Seattle, WA: University of Washington Press.ISBN9780295952581.
^abcd"Horsetail". MedlinePlus, US National Library of Medicine, National Institutes of Health. 8 December 2017. Retrieved14 November 2013.
^Henderson JA, Evans EV, McIntosh RA (June 1952). "The antithiamine action ofEquisetum".Journal of the American Veterinary Medical Association.120 (903):375–8.PMID14927511.
^Fabre, B; Geay, B.; Beaufils, P. (1993). "Thiaminase activity inEquisetum arvense and its extracts".Plant Med Phytother.26:190–7.
^Williams, Vincent S.; Barrett, Paul M. & Purnell, Mark A. (2009), "Quantitative analysis of dental microwear in hadrosaurid dinosaurs, and the implications for hypotheses of jaw mechanics and feeding",Proceedings of the National Academy of Sciences,106 (27):11194–11199,Bibcode:2009PNAS..10611194W,doi:10.1073/pnas.0812631106,PMC2708679,PMID19564603
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