Abstract
ABSTRACT Isoetes ×jermyi, hyb nov . (Isoetaceae), is the sterile hexaploid (6x) hybrid quillwort of diploid (2x) I. echinospora x decaploid (10x) I. lacustris. It has been identified from cytologically confirmed plants at several sites in the western United Kingdom and by the morphological determination of herbarium specimens from sites in north-eastern France and Småland, Sweden. Clarification is provided for the distinctions of 6x I. ×jermyi from the similar, apparently heptaploid (7x) Pyrenean endemic I. ×brochonii and the misapplied 6x North American endemic I. ×hickeyi. The possibility that enigmatic Irish quillwort I. morei could represent this taxon is discussed and rejected. The frequency of its occurrence in the United Kingdom suggests that I. ×jermyi can be expected to occur regularly within mixed I. echinospora and I. lacustris populations across the wide area of north-western Europe where the two species are sympatric.
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