The EquisetumSpecies (Horsetails) of the British Isles
E. umbrosum Meyer ex Willd.
The rhizomes black, dull, hairy only on the sheaths; not tuberous. The shoots distinguishable as fertile and sterile: both types produced at the same time, but those bearing cones remaining non-green and unbranched until after spore dispersal, and only later becoming green and branching so as to resemble the sterile stems vegetatively.
The main stems 20–60 cm high (sterile), or 20–25 cm high (fertile); 1–2 mm in diameter (measurement available only for sterile stems); erect; green; very rough (in sterile stems); bearing whorls of slender branches at the nodes (the sterile stems, sometimes also in the mature fertile stems), or simple (fertile stems, commonly); dying down in autumn. The main stem internodes 8–20 grooved. The main stem internodes with a central hollow (in sterile stems); central hollow about half the diameter of the internode to more than half the diameter of the internode (about half or rather more, according to Clapham et al.); endodermis comprising a single layer outside the ring of vascular bundles. The main stem sheaths 3–8 mm long in sterile stems; 8–20 toothed (on sterile stems), or 10–20 toothed (on fertile stems). The teeth of sterile stem sheaths brown with a blackish rib, acute, subulate in sterile; those on sterile stems pale with dark ribs; persistent. The primary branching symmetrical. The primary branches numerous; spreading (usually), or spreading to drooping; simple (and numerous, in both fertile and steriles stems). The primary branch internodes 3(–4) grooved. The primary branch sheaths 3(–4) toothed (the teeth deltoid, acute). The first branch internodes of sterile stems at least as long as the subtending sheaths, at least on the upper parts of the stem (though sometimes shorter on the lower parts of the stem). The primary branch internodes solid. Stomata not sunken.
The cones 1.5–4 cm long; blunt. Spores released in April.
Comments. The identity of Sowerby & Johnsons illustration of E. umbrosum needs checking, in view of the erecto-patent branching and blunt-topped shape of the sterile stem depicted.
Distribution. Europe, from Iceland and arctic Russia to eastern France, Tirol, Rumania and the Caucasus; northern and central Asia, and North America from Nova Scotia and Alaska to New Jersey and Colorado. In the British Isles, in scattered (mainly eastern) localities from Yorkshire and Westmorland to Orkney, and in Ireland in Fernanagh, Donegal and Antrim. Grassy stream banks, etc., ascending to about 1000 m.
Classification. Subgenus Equisetum; Section Subvernalia.
Illustrations. • E. pratense (Sowerby and Johnson). • 10 British Equisetum species and hybrids (Sowerby and Johnson).
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: 21st June 2009. http://delta-intkey.com’.