Hybrids within Equisetum subgenus Hippochaete in England and Wales

Abstract

Hybrid horsetails in Equisetum subgenus Hippochaete are superficially similar. Distinguishing the different taxa depends on the micromorphology of the stem ridges. In the case of the hybrids E. ×trachyodon (E. hyemale×E. variegatum) and E. ×meridionale (E. ramosissimum×E. variegatum) both inherit silica tubercles that run vertically along the stem ridges but only E. ×meridionale has silica cross-bands on the stem ridges; these are difficult to discern in the field, even with a ×20 hand-lens and good light. The past misidentification of some populations of E. ×meridionale as E. ×trachyodon in the British Isles is therefore understandable. Recorded populations of E. ×trachyodon in England are re-assessed, with some found to be E. ×meridionale. Associated species and habitats are compared. Using flow cytometry, all known British populations of E. ×meridionale were determined to be diploid. An identification key which incorporates recent knowledge of the morphology of the species and hybrids in subgenus Hippochaete in Europe is presented.

Keywords:

• Equisetum ×meridionale
• E. ×trachyodon
• E. ramosissimum
• Llyn Penrhyn
• Red Rocks

The authors would like to thank C.N. Page and H. McHaffie for their assistance with historical information, records and herbarium material; B. Tenbergen (Museum für Naturkunde, Herbarium MSTR, Münster, Germany) and A.M. Paul (Natural History Museum, London) for facilitating herbarium loans from E and BM; and H.W. Bennert (Ennepetal, Germany), V.M. Dörken (Konstanz, Germany), and S. Körner (Bochum, Germany) for their work on imaging. We should also like to thank for information and assistance P. Acock, H.J. Ash and members of the Cheshire Zoo Rare Plants Group, S. Bird, M.J. Crawley, K. Godfrey, E.F. Greenwood, Q. Groom, G. Halliday, R. James, G.M. Kay, L. Kelly, A.R.G. Mundell, A. Newton, S.L. Pilkington, A.J. Richards, C.A. Stace and J. Woodman.