Temporal range: Callovian  to Recent
|"Candocks" of the Great Horsetail (Equisetum telmateia subsp. telmateia), showing whorls of branches and the tiny dark-tipped leaves|
Equisetum is a "living fossil" as it is the only living genus of the entire class Equisetopsida, which for over one hundred million years was much more diverse and dominated the understory of late Paleozoic forests. Some Equisetopsida were large trees reaching to 30 meters tall. The genus Calamites of the family Calamitaceae, for example, is abundant in coal deposits from the Carboniferous period.
A superficially similar but entirely unrelated flowering plant genus, mare's tail (Hippuris), is occasionally misidentified as "horsetail", and adding to confusion, the name mare's tail is sometimes applied to Equisetum.
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 derives from the Latin equus ("horse") + seta ("bristle").
Other names include candock for branching individuals, and snake grass or scouring-rush for unbranched or sparsely branched individuals. The latter name refers to the rush-like appearance 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. In German, the corresponding name is Zinnkraut ("tin-herb"). Rough horsetail E. hyemale is still boiled and then dried in Japan, to be used for the final polishing process on woodcraft to produce a smoother finish than any sandpaper.
In these plants the 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 Lycopodiophyta (clubmosses and relatives), but rather derived adaptations, evolved by reduction of megaphylls. They are, therefore, sometimes referred to as megaphylls to reflect this homology.
The leaves of horsetails are arranged in whorls fused into nodal sheaths. The stems are 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.
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. field horsetail, E. arvense) they are non-photosynthetic, produced early in spring separately from photosynthetic, sterile shoots. In some other species (e.g. marsh horsetail, E. palustre) 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 after the sporangia have split open longitudinally.
Equisetum 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. 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 two distinct lineages, which are usually treated as subgenera. Hybrids are common, but hybridization has only been recorded between members of the same subgenus.
- Subgenus Equisetum
- Equisetum arvense L. – Field Horsetail, Common Horsetail or Mare's tail
- Equisetum bogotense Kunth – Andean Horsetail
- Equisetum diffusum L. – Himalayan Horsetail
- Equisetum fluviatile L. – Water Horsetail
- Equisetum palustre L. – Marsh Horsetail
- Equisetum pratense Ehrh. – Meadow Horsetail, Shade Horsetail, Shady Horsetail
- Equisetum sylvaticum L. – Wood Horsetail
- Equisetum telmateia Ehrh. – Great Horsetail, Northern Giant Horsetail
- Subgenus Hippochaete
- Equisetum giganteum L. – Southern Giant Horsetail or Giant Horsetail
- Equisetum myriochaetum Schltdl. & Cham. – Mexican Giant Horsetail
- Equisetum hyemale L. – Rough Horsetail, Scouringrush Horsetail
- Equisetum laevigatum A.Braun – Smooth Horsetail
- Equisetum ramosissimum Desf. – Branched Horsetail
- Equisetum scirpoides Michx. – Dwarf Horsetail
- Equisetum variegatum Schleich. ex Weber & Mohr – Variegated Horsetail
- unplaced to subgenus
- Equisetum californicum (Milde) G.N.Jones
- Equisetum robustum A.Braun ex Engelm.
- Equisetum trachyodon (A.Braun) W.D.J.Koch
- †Equisetum thermale - Middle to Late Jurassic, Argentina
- Hybrids between species in subgenus Equisetum
- Equisetum × bowmanii C.N.Page (Equisetum sylvaticum × Equisetum telmateia)
- Equisetum × dycei C.N.Page (Equisetum fluviatile × Equisetum palustre)
- Equisetum × font-queri Rothm. (Equisetum palustre × Equisetum telmateia)
- Equisetum × litorale Kühlew ex Rupr. (Equisetum arvense × Equisetum fluviatile)
- Equisetum × mchaffieae C.N.Page (Equisetum fluviatile × Equisetum pratense)
- Equisetum × mildeanum Rothm. (Equisetum pratense × Equisetum sylvaticum)
- Equisetum × robertsii Dines (Equisetum arvense × Equisetum telmateia)
- Equisetum × rothmaleri C.N.Page (Equisetum arvense × Equisetum palustre)
- Equisetum × willmotii C.N.Page (Equisetum fluviatile × Equisetum telmateia)
- Hybrids between species in subgenus Hippochaete
- Equisetum × ferrissii Clute (Equisetum hyemale × Equisetum laevigatum)
- Equisetum × moorei Newman (Equisetum hyemale × Equisetum ramosissimum)
- Equisetum × nelsonii (A.A.Eaton) Schaffn. (Equisetum laevigatum × Equisetum variegatum)
- Equisetum × schaffneri Milde (Equisetum giganteum × Equisetum myriochaetum)
Distribution, ecology and uses
The genus Equisetum 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 perennial plants, either herbaceous and dying back in winter as most temperate species, or evergreen as most tropical species and the temperate species rough horsetail (E. hyemale), branched horsetail (E. ramosissimum), dwarf horsetail (E. scirpoides) and variegated horsetail (E. variegatum). They typically grow 0.2-1.5 m tall, though the "giant horsetails" are recorded to grow as high as 2.5 m (northern giant horsetail, E. telmateia), 5 m (southern giant horsetail, E. giganteum) or 8 m (Mexican giant horsetail, E. myriochaetum), and allegedly even more.
Many species in this genus prefer wet sandy soils, though some are semi-aquatic and others are adapted to wet clay soils. The stalks arise from rhizomes that are deep underground and almost impossible to dig out. The field horsetail (E. arvense) can be a nuisance weed, readily regrowing from the rhizome after being pulled out. It is also unaffected by many herbicides designed to kill seed plants. 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. Members of the genus have been declared noxious weeds in Australia and in the US state of Oregon.
If eaten over a long enough period of time, some species of horsetail can be poisonous to grazing animals, including horses. The toxicity appears to be due to thiaminase enzymes, which can cause thiamine (vitamin B1) deficiency. People have regularly consumed horsetails. The young plants are eaten cooked or raw, but considerable care must be taken. For example, the fertile stems bearing strobili of some species are cooked and eaten like asparagus (a dish called tsukushi) in Japan. The people of ancient Rome would eat meadow horsetail in a similar manner, and they also used it to make tea as well as a thickening powder. Indians of the North American Pacific Northwest eat the young shoots of this plant raw. The plants are used as a dye and give a soft green colour. An extract is often used to provide silica for supplementation. Horsetail was often used by Indians to polish wooden tools. Equisetum species are often used to analyze gold concentrations in an area due to their ability to take up the metal when it is in a solution.
The plant[clarification needed] has a long history of medicinal uses, although modern sources include cautions with regard to its use. In 2009 the European Food Safety Authority issued a report assessing health claims (invigoration, weight control, and skin, hair, and bone health) for Equisetum arvense L., concluding that none could be substantiated.
These plants have a high content of thiaminase, which may induce edema and may cause lack of limb coordination, possibly putting a person at risk of injury from falling, but also slow heart-rate or cardiac irregularity. It is mentioned in the fifth edition of the national pharmacopoeia of Argentina that response to treatment with thiamin is effective. Since horsetail contains traces of nicotine, it is not recommended for young children
There is insufficient evidence of its effectiveness as a medicine, but taken orally Equisetum giganteum is widely used in South America as a diuretic to reduce swelling caused by excess fluid retention and for urinary infections, bladder and kidney disorders. It also contains silicon which is why it is sometimes suggested as appropriate for sufferers from brittle bone disorders 
Dosage is a matter of conjecture, but dried herb capsules of 300 mg are sometimes recommended to be taken 3 times per day. More commonly it is prepared as an infusion whose content can only be guessed – one tablespoonful or half an ounce (c. 10g - 15g) of dried herbs mixed with 5 cup-fulls (c. 1 litre) of boiling water and when cool, drunk as tea or used as a compress
The best time to collect horsetails to produce medicinal tea, compress or store for later usuage, is during the midsummer. The collected shoots of the horsetails may be used fresh or dried.
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