Horsetail: Meaning (information, definition, explanation, facts)

This article is about the fern ally Genus Equisetum. The flowering plant Mare's tail (Hippuris vulgaris), unrelated to the Genus Equisetum, is also known as "horsetail".

The horsetails and scouring-rushes 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 sometimes placed as the sole member of a 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 fossils, but all are extinct so far as is known.

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

Usually, the name horsetail is applied to the branching species, while the name scouring-rush is applied to the unbranched or sparsely branched species. The name horsetail arose because it was thought that the stalk resembled a horse's tail; the name Equisetum means "horse hair". The name scouring-rush refers both to its rush-like appearance and to the fact that the stems accumulate silica and were used for scouring dishes in the past.

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.

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 30m, with the Genus Calamites (Family Calamitaceae) abundant in coal deposits.