Raffi at 12:43, 8 April 2007

2007-04-08T12:43:33Z

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{{Taxobox
| color = lightgreen
| name = Horsetail
| image = Equisetum_arvense_stem.jpg
| image_width = 240px
| image_caption = Vegetative stem of ''Equisetum telmateia'' with a whorl (at each node) of branches and dark-tipped leaves
| regnum = [[Plant]]ae
| divisio = [[Pteridophyta]]
| classis = '''Equisetopsida'''
| ordo = '''Equisetales'''
| familia = '''Equisetaceae'''
| genus = '''''Equisetum'''''
| subdivision_ranks = Species
| subdivision = See text
}}

'''''Equisetum''''' is a genus of [[vascular plants]] that reproduce by spores rather than seeds. The genus includes 15 species, commonly known as '''horsetails''' and '''scouring rushes'''. ''Equisetum'' is the only one in the family '''Equisetaceae''', which in turn is the only family in the order '''Equisetales''' and the class '''Equisetopsida'''. This class is often placed as the sole member of the Division Equisetophyta (also called Arthrophyta in older works), though some recent molecular analyses place the genus within the ferns ([[pteridophytes|Pteridophyta]]), related to [[Marattiales]]. Other classes and orders of Equisetophyta are known from the [[fossil]] record, where they were important members of the world [[flora (plants)|flora]] during the [[Carboniferous]] period.

[[Image:Horsetail_vegeative_stem.JPG|left|120px|thumb|Vegetative stem: N = node, I = internode, B = branch in whorl, L = fused megaphylls]]
The name horsetail, often used for the entire group, arose because the branched species somewhat resemble a horse's tail, the name ''Equisetum'' being from the [[Latin]] ''equus'', "horse", and ''seta'', "bristle". Other names include '''candock''' (applied to branching species only), and '''scouring-rush''' (applied to the unbranched or sparsely branched species). The latter name refers to the plants' rush-like appearance; the stems were used for scouring cooking pots in the past (due to them being coated with abrasive [[silica]]).

The genus is near-[[cosmopolitan distribution|cosmopolitan]], being absent only from [[Australasia]] and [[Antarctica]]. They are [[perennial plant]]s, either [[herbaceous]], dying back in winter (most temperate species) or [[evergreen]] (some tropical species, and the temperate species ''Equisetum hyemale, E. scirpoides, E. variegatum'' and ''E. ramoissimum''). They mostly grow 0.2-1.5 m tall, though ''E. telmateia'' can exceptionally reach 2.5 m, and the tropical [[Americas|American]] species ''E. giganteum'' 5 m, and ''E. myriochaetum'' 8 m.

In these plants the [[Leaf|leaves]] are greatly reduced, in [[whorl]]s of small, segments fused into nodal sheaths. The stems are green and [[photosynthesis|photosynthetic]], also distinctive in being hollow, jointed, and ridged (with (3-) 6-40 ridges). There may or may not be whorls of branches at the nodes; when present, these branches are identical to the main stem except smaller.

The [[spore]]s are borne under sporangiophores in cone-like structures (''strobilus'', pl. ''strobili'') at the tips of some of the stems. In many species the cone-bearing stems are unbranched, and in some (e.g. ''E. arvense'') they are non-photosynthetic, produced early in spring separately from photosynthetic sterile stems. In some other species (e.g. ''E. palustre'') they are very similar to sterile stems, photosynthetic and with whorls of branches.

[[Image:Equisetum_arvense_strob.jpg|thumb|Strobilus of ''E. telmateia'', terminal on an unbranched stem]]
Horsetails are mostly [[homosporous]], though in ''E. arvense'', smaller spores give rise to male prothalli. The [[spore]]s have four [[elater]]s that act as moisture-sensitive springs, assisting spore dispersal after the [[sporangia]] have split open longitudinally.

Many plants in this genus prefer wet [[sand]]y [[soil]]s, though some are aquatic and others adapted to wet [[clay]] soils. One horsetail, ''E. arvense'', can be a nuisance [[weed]] because it readily regrows after being pulled out. The stalks arise from [[rhizome]]s that are deep underground and almost impossible to dig out. It is also unaffected by many [[herbicide]]s designe
d to kill [[seed plant]]s. The foliage of some species is [[poison]]ous to grazing animals if eaten in large quantities. ''Equisetum'' is cooked and eaten in Japan.

The horsetails were a much larger and more diverse group in the distant past before [[seed plant]]s became dominant across the Earth. Some species were large [[tree]]s reaching to 30 m tall. The genus ''[[Calamites]]'' (family [[Calamitaceae]]) is abundant in [[coal]] deposits from the Carboniferous period.

===Species===
;Subgenus ''Equisetum''
*''[[Equisetum arvense]]'' - Field or Common Horsetail
*''[[Equisetum bogotense]]'' - Andean Horsetail
*''[[Equisetum diffusum]]'' - Himalayan Horsetail
*''[[Equisetum fluviatile]]'' - Water Horsetail
*''[[Equisetum palustre]]'' - Marsh Horsetail
*''[[Equisetum pratense]]'' - Shade or Meadow Horsetail
*''[[Equisetum sylvaticum]]'' - Wood Horsetail
*''[[Equisetum telmateia]]'' - Great Horsetail
;Subgenus ''Hippochaete''
*''[[Equisetum giganteum]]'' - Giant Horsetail
*''[[Equisetum myriochaetum]]'' - Mexican Giant Horsetail
*''[[Equisetum hyemale]]'' - Rough Horsetail
*''[[Equisetum laevigatum]]'' - Smooth Horsetail
*''[[Equisetum ramosissimum]]'' - Branched Horsetail
*''[[Equisetum scirpoides]]'' - Dwarf Horsetail
*''[[Equisetum variegatum]]'' - Variegated Horsetail

===Named hybrids===
;Hybrids between species in subgenus ''Equisetum''
*''Equisetum × litorale'' Kühlew ex Rupr. = ''Equisetum fluviatile'' × ''Equisetum arvense''
*''Equisetum × dycei'' C.N.Page = ''Equisetum fluviatile'' × ''Equisetum palustre''
*''Equisetum × willmotii'' C.N.Page = ''Equisetum fluviatile'' × ''Equisetum telmateia''
*''Equisetum × rothmaleri'' C.N.Page = ''Equisetum arvense'' × ''Equisetum palustre''
*''Equisetum × robertsii'' Dines = ''Equisetum arvense'' × ''Equisetum telmateia''
*''Equisetum × mildeanum'' Rothm. = ''Equisetum pratense'' × ''Equisetum sylvaticum''
*''Equisetum × bowmanii'' C.N.Page = ''Equisetum sylvaticum'' × ''Equisetum telmateia''
*''Equisetum × font–queri'' Rothm. = ''Equisetum palustre'' × ''Equisetum telmateia''

;Hybrids between species in subgenus ''Hippochaete''
*''Equisetum × moorei'' Newman = ''Equisetum hyemale'' × ''Equisetum ramosissimum''
*''Equisetum × trachydon'' A.Braun = ''Equisetum hyemale'' × ''Equisetum variegatum''
*''Equisetum × schaffneri'' Milde = ''Equisetum giganteum'' × ''Equisetum myriochaetum''
*''Equisetum × ferrissii'' Clute = ''Equisetum hyemale'' × ''Equisetum laevigatum''
*''Equisetum × nelsonii'' (A.A.Eat.) Schaffn. = ''Equisetum laevigatum'' × ''Equisetum variegatum''

The superficially similar [[flowering plant]], [[Hippuris|Mare's tail]] (''Hippuris vulgaris''), unrelated to the genus ''Equisetum'', is occasionally misidentified and misnamed as a horsetail.

==References and external links==
*Pryer, K. M., Schuettpelz, E., Wolf, P. G., Schneider, H., Smith, A. R., and Cranfill, R. (2004). Phylogeny and evolution of ferns (monilophytes) with a focus on the early leptosporangiate divergences. ''American Journal of Botany'' 91: 1582-1598 (available [http://www.amjbot.org/cgi/reprint/91/10/1582 online]; pdf file).
*[http://www.btinternet.com/~pigott/equisetum/ UK National Collection] - includes a taxonomic list of all known species and hybrids
*[http://members.eunet.at/m.matus/ The Wonderful World of Equisetum]
*[http://www.fiu.edu/~chusb001/giant_equisetum.html Giant horsetails]

{{commonscat|Equisetaceae}}

[[Category:Equisetophyta|*]]
[[Category:Living fossils]]

Equisetum - Revision history

Raffi at 12:43, 8 April 2007