The EquisetumSpecies (Horsetails) of the British Isles


L. Watson and M. J. Dallwitz

Equisetum × trachyodon A. Br.

“Mackay’s Horsetail”.

E. mackaii Newman.

The rhizomes not tuberous. The shoots all green and alike vegetatively, the sterile and cone-bearing shoots emerging at the same time.

The main stems 30–100 cm high; 2–4 mm in diameter; erect to decumbent; greyish green; very rough; sparingly branched, the branches solitary and similar to the main stem, or simple; persisting through the winter. The main stem internodes not swollen; 8–12 grooved; grooves moderately deep. The main stem internodes with a central hollow; central hollow much less than half the diameter of the internode (about 1/3); endodermis comprising two layers, one outside and the other inside the ring of vascular bundles. The main stem sheaths at first green with a black band at the top, but becoming entirely black; 2–4 mm long; appressed. The teeth four ribbed; subulate, usually black; persistent. The primary branches when present, simple. Stomata sunken.

The cones only about 0.5 cm long (often deformed and partly abortive); apiculate. Spores abortive.

Distribution. Iceland, Norway, Upper Rhineland; south-west Greenland; northern temperate North America. Largely confined in the British Isles to Scotland (Aberdeen and Kincardine by the Dee, and Perth), and Ireland (widespread there, but absent from the centre and most of the south-east); on shady banks of lakes and streams.

Classification. Subgenus Hippochaete; = E. hyemale × E. variegatum.

Illustrations. • E. trachyodon: Sowerby and Johnson (1859). • E. x trachyodon: as E. trachyodon, Eng. Bot. 1896 (1886). • 10 British Equisetum species and hybrids: Sowerby and Johnson, 1863.

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.’.