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Horsetail
Vegetative stem of Equisetum telmateia with a whorl (at each node) of branches and dark-tipped leaves
Scientific classification
Kingdom: Plantae Division: Pteridophyta Class: Equisetopsida Order: Equisetales Family: Equisetaceae Genus: Equisetum
Species
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. These compose the entire class Equisetopsida, the sole member of the division Equisetophyta (Arthrophyta in older works), though some recent molecular analyses place the genus within the ferns (Pteridophyta), related to Marattiales. Other classes and orders of Equisetopsida 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, 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, being absent only from Australasia and Antarctica. They are perennial plants, 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. ramosissimum). They mostly grow 0.2-1.5 m tall, though E. telmateia can exceptionally reach 2.5 m, and the tropical American species E. giganteum 5 m, and E. myriochaetum 8 m.

In these plants the leaves are greatly reduced, in whorls of small, segments fused into nodal sheaths. The stems are green and 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 spores 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.

Strobilus of E. telmateia, terminal on an unbranched stem

Microscopic view of Equisetum hyemale'. 2-1-0-1-2 is one millimeter with 1/20th graduation. White and small protuberances are accumulated silicic acid on cells. Boiled and then dried E. hyemale is still used in Japan for the final polishing process on woodcraft to produce a smoother finish than with sandpaper.

Horsetails are mostly homosporous, though in E. arvense, 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.

Many plants in this genus prefer wet 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. The stalks arise from rhizomes that are deep underground and almost impossible to dig out. It is also unaffected by many herbicides designed to kill seed plants. The foliage of some species is poisonous 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 plants became dominant across the Earth. Some species were large trees reaching to 30 m tall. The genus Calamites (family Calamitaceae) is abundant in coal deposits from the Carboniferous period.

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

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, Mare's tail (Hippuris vulgaris), unrelated to the genus Equisetum, is occasionally misidentified and misnamed as a horsetail.

• 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 online; pdf file).
• UK National Collection - includes a taxonomic list of all known species and hybrids
• The Wonderful World of Equisetum
• Giant horsetails
• HDRA Organic Weed Management: Field horsetail - includes Occurrence, Biology, Persistence and Spread, Management and Discussion
• Equisetum hyemale L., scouringrush horsetail

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Equisetaceae