Horsetail
Vegetative stem of Equisetum telmateia with a whorl (at each node) of branches and dark-tipped leaves
Scientific classification
Kingdom: Plantae Division: Equisetophyta Class: Equisetopsida Order: Equisetales Family: Equisetaceae Genus: Equisetum
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 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

The horsetails comprise 15 species of plants in the genus Equisetum. This genus 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 now usually placed as the sole member of the division Equisetophyta, though some authorities place it instead in the division Tracheophyta or Archeophyta. The plants in the genus Equisetum are considered fern allies. Other classes and orders of Equisetophyta are known from the fossil record, where they were important members of the world flora during the Carboniferous period.

Vegetative stem: N = node, I = internode, B = branch in whorl, L = fused megaphylls

The name horsetail arose because it was thought that the stalk resembled a horse's tail; the name Equisetum means "horse hair". Other names, rarely used, include candock (applied to branching species only), and scouring-rush (applied to the unbranched or sparsely branched species). The name scouring-rush refers to its rush-like appearance and, because the stems accumulate abrasive silica, their being used for scouring cooking pots in the past.

The genus is near-cosmopolitan, being absent only from Australasia and Antarctica. They are winter-deciduous (temperate species) or evergreen (some tropical species), and are mostly 0.2-1.5 m tall, though E. telmateia can exceptionally reach 2.5 m, and the tropical American species E. giganteum 5m, and E. myriochaetum 8m.

These are plants without conspicuous leaves, but with hollow, jointed, ascending stems that may or may not have side-branches radiating out from the nodes, depending on species. The stem is ridged and grooved, with from (3-) 6-40 ridges. The leaves are minute, pointed-triangular, and form in a whorl at each node on the stem; there is one leaf for each ridge on the stem.

The spores are borne in a cone-like structures (strobilus, pl. strobili) at the tip of some of the stems. These reproductive stems are often unbranched, and in some species are non-photosynthetic and produced early in spring separately from photosynthetic sterile stems. Horsetails are mostly homosporous, though in E. arvense, smaller spores give rise to male prothalli. The eusporangia have an annulus that act as a moisture-sensitive spring, ejecting the spores through a weak spot of the sporangia.

Strobilus of E. telmateia, terminal on an unbranched stem

Many plants in this genus prefer sandy soils, 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, as the stalk-producing rhizome is deep underground and almost impossible to dig out. It is also unaffected by many herbicides designed to kill seed plants. The foliage is poisonous to grazing animals if eaten in large quantities.

The horsetails were a far larger and more diverse group in the distant past, before the evolution of seed plants. In the Carboniferous period, they included large trees reaching to 30 m, with the genus Calamites (family Calamitaceae) abundant in coal deposits.

The superficially similar flowering plant Mare's tail (Hippuris vulgaris), unrelated to the genus Equisetum, is occasionally mis-identified and mis-named as a horsetail.

External links

• UK National Collection (http://www.btinternet.com/~pigott/equisetum/) - includes a taxonomic list of all known species and hybrids
• The Wonderful World of Equisetum (http://members.eunet.at/m.matus/)
• Giant horsetails (http://www.fiu.edu/~chusb001/giant_equisetum.html)
• Equisetum hyemale (http://www.floridata.com/ref/e/equi_hye.cfm)

da:Padderok-familien (Equisetaceae) de:Schachtelhalmgewächse es:Equisetum fr:Prêle he:שבטבט גדול nl:Equisetum