Equisetum

Equisetum (/ˌɛkwɪˈsiːtəm/; horsetail, snake grass, puzzlegrass) is the only living genus in Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds. Equisetum is a 'living fossil', the only living genus of the entire class Equisetopsida, which for over 100 million years was much more diverse and dominated the understory of late Paleozoic forests. Some Equisetopsida were large trees reaching to 30 meters tall. The genus Calamites of the family Calamitaceae, for example, is abundant in coal deposits from the Carboniferous period. The pattern of spacing of nodes in horsetails, wherein those toward the apex of the shoot are increasingly close together, inspired John Napier to invent logarithms. A superficially similar but entirely unrelated flowering plant genus, mare's tail (Hippuris), is occasionally referred to as 'horsetail', and adding to confusion, the name mare's tail is sometimes applied to Equisetum. Despite centuries of use in traditional medicine, there is no evidence that Equisetum has any medicinal properties. The name 'horsetail', often used for the entire group, arose because the branched species somewhat resemble a horse's tail. Similarly, the scientific name Equisetum is derived from the Latin equus ('horse') + seta ('bristle'). Other names include candock for branching individuals, and snake grass or scouring-rush for unbranched or sparsely branched individuals. The latter name refers to the rush-like appearance of the plants and to the fact that the stems are coated with abrasive silicates, making them useful for scouring (cleaning) metal items such as cooking pots or drinking mugs, particularly those made of tin. In German, the corresponding name is Zinnkraut ('tin-herb'). Rough horsetail E. hyemale is still boiled and then dried in Japan to be used for the final polishing process on woodcraft to produce a smoother finish than any sandpaper. In Spanish-speaking countries, these plants are known as cola de caballo, meaning 'horsetail'. In these plants the leaves are greatly reduced and usually non-photosynthetic. They contain a single, non-branching vascular trace, which is the defining feature of microphylls. However, it has recently been recognised that horsetail microphylls are probably not ancestral as in Lycopodiophyta (clubmosses and relatives), but rather derived adaptations, evolved by reduction of megaphylls. They are, therefore, sometimes referred to as megaphylls to reflect this homology. The leaves of horsetails are arranged in whorls fused into nodal sheaths. The stems are usually green and photosynthetic, and are distinctive in being hollow, jointed and ridged (with sometimes 3 but usually 6–40 ridges). There may or may not be whorls of branches at the nodes.