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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 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.

The genus is near-cosmopolitan, being absent only from Australasia. They are winter- deciduous (temperate species) or evergreen (some tropical species), and are mostly 0.2-1.5 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.

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 eu sporangia 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 herbicideA herbicide is a pesticide used to kill unwanted plants. Selective herbicides kill certain targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often based on plant hormones.s 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 treeThis article is about the biological organisms known as trees. For other meanings of the word see tree (disambiguation). oak tree in Denmark A tree can be defined as a large perennial woody plant. Though there is no set definition of size, it is generallys reaching to 30 m, with the Genus Calamites (Family Calamitaceae) abundant in coalCoal is a fossil fuel extracted from the ground by mining. It is a readily combustible black or brownish-black sedimentary rock. It is composed primarily of carbon and hydrocarbons, along with assorted other elements, including sulfur. Often associated wi deposits.