Equisetum is a "living fossil", the only living genus of the entire subclassEquisetidae, 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]
The name "horsetail", often used for the entire group, arose because the branched species somewhat resemble a horse's tail. Similarly, the scientific nameEquisetum is derived from the Latinequus ('horse') + seta ('bristle').[6]
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.[7] In German, the corresponding name is Zinnkraut ('tin-herb'). In Spanish-speaking countries, these plants are known as cola de caballo ('horsetail').
Equisetumleaves 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.[citation needed] 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 of Equisetum telmateia subsp. braunii, terminal on an unbranched stemMicroscopic view of Equisetum hyemale (rough horsetail) (2-1-0-1-2 is one millimetre with 1⁄20th graduation). The small white protuberances are accumulated silicates on cells.
Spores
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.[9]: 12–15
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.[10]
Cell walls
The crude cell extracts of all Equisetum species tested contain mixed-linkage glucan : xyloglucan endotransglucosylase (MXE) activity.[11] 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.[12][13] 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.[14] 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]
Taxonomy
Species
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.[15] While plants of subgenus Equisetum are usually referred to as horsetails, those of subgenus Hippochaete are often called scouring rushes, especially when unbranched.[citation needed]
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.[16][citation needed]
The oldest remains of modern horsetails of the genus Equisetum first appear in the Early Jurassic, represented by Equisetum dimorphum from the Early Jurassic of Patagonia[17] and Equisetum laterale from the Early-Middle Jurassic of Australia.[18][19]Silicified remains of Equisetum thermale from the Late Jurassic of Argentina exhibit all the morphological characters of modern members of the genus.[20] The estimated split between Equisetum bogotense and all other living Equisetum is estimated to have occurred no later than the Early Jurassic.[19]
Subgenus Paramochaete
Equisetum bogotenseKunth – Andean horsetail; upland South America up to Costa Rica; includes E. rinihuense, sometimes treated as a separate species. Previously included in subg. Equisetum, but Christenhusz et al. (2019)[21] transfer this here, as E. bogotense appears to be sister to the remaining species in the genus.
Subgenus Equisetum
Equisetum arvenseL. – field horsetail, common horsetail or mare's tail; circumboreal down through temperate zones
Equisetum diffusumD.Don – Himalayan horsetail; Himalayan India and China and adjacent nations above about 1500 feet (450 m)
Equisetum fluviatileL. – water horsetail; circumboreal down through temperate zones
Equisetum palustreL. – marsh horsetail; circumboreal down through temperate zones
Equisetum pratenseEhrh. – meadow horsetail, shade horsetail, shady 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, northern giant horsetail; Europe to Asia Minor and north Africa, also west coast of North America. The North American subspecies Equisetum telmateia braunii(Milde) Hauke. may be treated as a separate species Equisetum brauniiMilde[21]
Equisetum calderi – Yukon, Northwest Territories, Nunavut and possibly North Slope of Alaska.
Equisetum giganteumL. – southern giant horsetail or giant horsetail; temperate to tropical South America and Central America north to southern Mexico
Equisetum hyemaleL. – rough horsetail, rough scouring rush; most of non-tropical northern hemisphere. The North American subspecies Equisetum hyemale affine(Engelm.) A.A.Eat. may be treated as a separate species Equisetum prealtumRaf.[21]
Equisetum laevigatumA.Braun – smooth horsetail, smooth scouring rush; western 3/4 of North America down into northwestern Mexico; also sometimes known as Equisetum kansanum
The genus Equisetum as a whole, while concentrated in the non-tropical northern hemisphere, is near-cosmopolitan, being absent only from Antarctica, though they are not known to be native to Australia, New Zealand nor the islands of the Pacific. They are most common in northern North America (Canada and the northernmost United States), where the genus is represented by nine species (E. arvense, E. fluviatile, E. hyemale, E. laevigatum, E. palustre, E. pratense, E. scirpoides, E. sylvaticum', and E. variegatum). Only four (E. bogotense, E. giganteum, E. myriochaetum, and E. ramosissimum) of the fifteen species are known to be native south of the Equator. They are perennial plants, herbaceous and dying back in winter as 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 "giant horsetails" are recorded to grow as high as 2.5 m (8 ft) (E. telmateia, northern giant horsetail), 5 m (16 ft) (E. giganteum, southern giant horsetail) or 8 m (26 ft) (E. myriochaetum, Mexican giant horsetail), and allegedly even more.[24]
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.[25][citation needed] Since the stems have a waxy coat, the plant is resistant to contact weedkillers like glyphosate.[26] 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.[27] Members of the genus have been declared noxious weeds in Australia and in the US state of Oregon.[28][29]
People have regularly consumed horsetails. For example, the fertile stems bearing strobili of some species are cooked and eaten like asparagus[31] (a dish called tsukushi (土筆) in Japan[32][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.[33][34][35] The young plants are eaten cooked or raw, but considerable care must be taken.[36]
If eaten over a long enough period of time, some species of horsetail can be poisonous to grazing animals, including horses.[37] The toxicity appears to be due to thiaminase, which can cause thiamin (vitamin B1) deficiency.[36][38][39][40]
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.[41] Analysis of the scratch marks on hadrosaur teeth is consistent with grazing on hard plants like horsetails.[42]
Folk medicine and safety concerns
Extracts and other preparations of E. arvense have served as herbal remedies, with records dating over centuries.[36][38][43] 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.[44] As of 2018[update], there is insufficient scientific evidence for its effectiveness as a medicine to treat any human condition.[36][43][44]
^"Equisetum L."Plants of the World Online. Board of Trustees of the Royal Botanic Gardens, Kew. 2017. Retrieved 23 August 2020.
^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.
^Daniel F. Austin (2004). Florida Ethnobotany (illustrated ed.). CRC Press. p. 283. ISBN9780203491881.
^Husby, C (2013). "Biology and functional ecology of Equisetum with emphasis on the giant horsetails". Botanical Review. 79 (2): 147–177. doi:10.1007/s12229-012-9113-4. S2CID15414705.
^Rutishauser, R (November 1999). "Polymerous leaf whorls in vascular plants: Developmental morphology and fuzziness of organ identities". International Journal of Plant Sciences. 160 (S6): S81–S103. doi:10.1086/314221. PMID10572024. S2CID4658142.
^Stace, C. A. (2019). New Flora of the British Isles (Fourth ed.). Middlewood Green, Suffolk, U.K.: C & M Floristics. ISBN978-1-5272-2630-2.
^Gould, R. E. 1968. Morphology of Equisetum laterale Phillips, 1829, and E. 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.
^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. Retrieved 14 November 2013.
^Henderson JA, Evans EV, McIntosh RA (June 1952). "The antithiamine action of Equisetum". Journal of the American Veterinary Medical Association. 120 (903): 375–8. PMID14927511.
^Fabre, B; Geay, B.; Beaufils, P. (1993). "Thiaminase activity in Equisetum 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, doi:10.1073/pnas.0812631106, PMC2708679
