Equisetum (class Sphenopsida, family Equisetaceae) The sole surviving genus of its class and family, comprising the horsetails, which typically are plants of damp places. They are vascular cryptogams with hollow stems, tiny, scale-like leaves, branches in whorls, and apical, spore-producing cones. The spores germinate into prothalli bearing the sex organs. The sperms are multiciliate. There are 29 species, almost cosmopolitan in distribution except for Australasia. In Carboniferous times the Sphenopsida produced tree-like forms. One such is Calamites, a common coal-measure fossil, which grew to heights of 30 m and, apart from its size and woody stems, closely resembled Equisetum.

Equisetum See SPHENOPSIDA.