Equisetaceae Michx. ex DC.

Terrestrial ferns. Underground stems long-creeping, freely branching, bearing wiry roots. Aerial stems green, erect, branching in whorls or unbranched, usually with a central hollow surrounded by two rings of smaller alternating cavities, jointed, longitudinally grooved, siliceous, glabrous, bearing leaves in sheathing whorls at the nodes. Leaves united laterally into a sheath, toothed at the apices, equalling the number of stem grooves. Veins undivided in each leaf. Strobili terminal on main aerial stems, or sometimes on lateral stems, or on specialised shoots lacking chlorophyll, bearing stalked peltate sporangiophores arranged in whorls. Several sporangia pendent from adaxial surface of heads of sporangiophores; each sporangium sessile, lacking an annulus and dehiscing by a longitudinal slit, containing 1000s of spores and elaters. Homosporous; spores alete, spherical, attached to four coiled elaters, granulate to smooth, chlorophyllous.

A family of one genus and 15 species (Hauke 1990). The Equisetaceae is a very distinctive family of terrestrial ferns with erect aerial stems that are usually hollow, jointed and longitudinally grooved, produce whorls of branches and leaves, bear conspicuous terminal strobili comprising whorls of peltate sporangiophores, and have a base chromosome number of 108.

Traditionally the Equisetaceae has been included within the fern allies, and is clearly related to early groups of fossil plants in the Sphenophyllales and Calamitales. However, molecular evidence now indicates that the family is part of the ferns, albeit distantly related and its exact placement remains uncertain. Pryer et al. (2004) and Smith et al. (2006) treated Equisetaceae as sister to Marattiaceae, but Rai & Graham (2010) suggested that the group is sister to the rest of the ferns. Christenhusz et al. (2011) considered the latter position to be more consistent with the fossil record but continued to include Equisetaceae within the ferns.