Horsetail

The horsetails are a class in the Pteridophyta (ferns). They were one of the most important plant groups in the Palaeozoic era.

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

For over 100 million years they were varied, and dominated the understorey of late Paleozoic forests. They are seen in the coal measures of the Carboniferous period, and some were trees reaching up 30 metres.^[1] The group is now almost extinct, but one genus survives. They are vascular plants that reproduce by spores and not by seeds. The name horsetail came because the branched species somewhat look like a horse's tail.

Equisetum

Modern horsetails first appeared during the Jurassic period. Equisetum is the only living genus of horsetails. The name Equisetum comes from the Latin (equus = horse; and seta = bristle).^[2] The genus includes 15 species.

Horsetails are native on all continents except Australasia and Antarctica. They are perennial plants. They are either herbaceous (they die back in winter like most temperate species) or they are evergreen (some tropical species, and some temperate species). They mostly grow 0.2-1.5 m tall, though E. telmateia can reach 2.5 m, and the tropical American species E. giganteum 5 m, and E. myriochaetum 8 m.

Strobilus of Equisetum on an unbranched stem

How they usually look.

In these plants the leaves are very small, in whorls joined together to make sheaths around the stem. The stems are green and photosynthetic, also distinctive in being hollow, jointed and ridged, usually with 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 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 Springtime.

References

1. Pryer K.M. et al 2004. Phylogeny and evolution of ferns (monilophytes) with a focus on the early leptosporangiate divergences. American Journal of Botany 91: 1582-1598 (available online Archived 2007-09-26 at the Wayback Machine; pdf file)
2. 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 are coated with abrasive silica, which made them useful for cleaning ("scouring") cooking pots in the past.