The EquisetumSpecies (Horsetails) of the British Isles


L. Watson and M. J. Dallwitz

Extinct Sphenopsida

Giant Horsetails, Articulates, Sphenopsids, Calamites.

Comments. The Sphenopsida (= Equisetopsida, Arthrophyta), of which the 16 species of Equisetum are the the sole surviving representatives, were represented by diverse herbaceous and woody forms, readily recognised by their characteristic jointed stems, associated with whorled leaves, branches and sporangia-bearing organs and correlated with unique anatomy (see the examples illustrated). Equisetum is referred to the Order Equisetales, which in Carboniferous forests were abundantly represented by the arborescent articulates commonly known as ‘calamites’. The general appearance of the more impressive of these can readily be envisaged by imagining vegetative shoots of Equisetum arvense, substantially secondarily thickened and enlarged to about 16 m or more in height. By the early Mesozoic, these had apparently become extinct, with the rather scanty fossil record of Sphenopsida consisting by then mainly of casts and impressions of stems resembling those of modern Equisetum. Given the existence of late Paleozoic fossil fragments of plants seemingly closely similar to Equisetum itself, it is evident that the modern Horsetails exemplify persistence of a very ancient type.

Illustrations. • Equisetalean fossils: Calamites, Asterophyllites, Annularia.

To view the illustrations with detailed captions, go to the interactive key. This also offers full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2004 onwards. The Equisetum species (horsetails) of the British Isles. Version: 7th March 2015.’.