The identity of zostera marina var. angustifolia Hornemann (Potamogetonaceae)

Since Hornemann (Fl. Dan. 9, 1816, p. 3, pl. 1501) published the name Zostera marina var. angustifolia together with a very poor drawing and the extremely short diagnosis ‘foliis subenerviis’ several interpretations of the identity of this taxon have been given. Some authors regarded it as a separate species closely related to Z. marina L., e.g. Reichenbach (1c. Fl. Germ. 7,1845, p. 3, as Z. angustifolia), and Tutin (J. Bot. 74, 1936, p. 227—230, as Z. hornemanniana). Others thought that it was a hybrid between Z. marina and Z. noltii Hornem., e.g. Ascherson (in Boissier, Fl. Orient. 5, 1882, p. 25), Prahl (Krit. Fl. Schlesw.- Holst. 2, 1890, p. 211), and Rouy (Fl. Fr. 13, 1912, p. 290, as Z. hornemanni). Recently I myself expressed the opinion that Hornemann’s variety was merely a brackish-water form of Z. noltit (Den Hartog, Sea-grasses of the world, 1970, p. 68). Thanks to the kindness of Mr. A. Hansen I was able to study two sheets of original material of Hornemann’s taxon and as a result all the above-mentioned interpretations can be ruled out. One of the two sheets is marked ‘cotypus’ and is labelled ‘Zostera marina angustifolia, e sinu Othiniensi, Hornemann’, the labelling in the characteristic handwriting of Prof. J. W. Hornemann himself. The specimens mounted on this sheet are all extremely narrow-leaved Z. marina. The specimens on the other sheet are very similar, and were collected from the same place; the labelling, however, is in the handwriting of N. Hofmann Bang, who was a close friend of Hornemann and owned the manor Hofmannsgave near the type locality.