Equisetum sp.

Family: Equisetaceae
Order: Equisetales
Class: Equisetopsida
Phylum / Division: Equisetophyta
Kingdom: Plantae

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Description

Equisetum (; horsetail) 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.

Etymology: 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 (´horse´) + (´bristle´).

Other names include candock for branching species, 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. In German, the corresponding name is (´tin-herb´) or (´nesting stalk´). In Spanish-speaking countries, these plants are known as (´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 been recognised in 1999 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.

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.

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.

Cell walls: 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.

Species: Currently, 18 species of Equisetum are accepted by Plants of the World Online. Some researchers speak of 25 species. The living members 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.

Two Equisetum plants are sold 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.

The Smith et al. classification of 2006, based on molecular phylogeny, placed Equisetum and Equisetaceae within leptosporangiate ferns. Subsequent classifications have maintained this placement.

Evolutionary history: 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 and Equisetum laterale from the Early-Middle Jurassic of Australia. Silicified remains of Equisetum thermale from the Late Jurassic of Argentina exhibit all the morphological characters of modern members of the genus. The estimated split between Equisetum bogotense and all other living Equisetum is estimated to have occurred no later than the Early Jurassic.

Subgenus Paramochaete:

– 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) transfer this here, as E. bogotense appears to be sister to all the remaining species in the genus.

Subgenus Equisetum:

– field horsetail or common horsetail; circumboreal down through temperate zones
– northern giant horsetail, syn. E. telmateia subsp. braunii (Milde) Hauke.; west coast of North America
– Himalayan horsetail; Himalayan India and China and adjacent nations above about 450 m
– water horsetail; circumboreal down through temperate zones
– marsh horsetail; circumboreal down through temperate zones
– shady horsetail, meadow horsetail, shade horsetail; circumboreal except for tundra down through cool temperate zones
– wood horsetail; circumboreal down through cool temperate zones, more restricted in east Asia
– great horsetail; Europe to Asia Minor and north Africa. The former North American subspecies Equisetum telmateia subsp. braunii (Milde) Hauke is now treated as a separate species

Subgenus Hippochaete:

– southern giant horsetail or giant horsetail; temperate to tropical South America and Central America north to southern Mexico
– rough horsetail; most of non-tropical Old World. The former North American subspecies Equisetum hyemale subsp. affine (Engelm.) A.A.Eat. is now treated as a separate species
– smooth horsetail, smooth scouringrush; western 3/4 of North America down into northwestern Mexico; also sometimes known as Equisetum kansanum
– Mexican giant horsetail; from central Mexico south to Peru
– scouringrush horsetail, syn. E. hyemale subsp. affine (Engelm.) A.A.Eat.; temperate North America
(including E. debile) – branched horsetail; Asia, Europe, Africa, southwest Pacific islands
– dwarf horsetail, dwarf scouringrush; northern (cool temperate) zones worldwide
– variegated horsetail, variegated scouringrush; northern (cool temperate) zones worldwide, except for northeasternmost Asia
– Atacama Desert giant horsetail; southern Peru, northern Chile