The Genus Empetrum L.

Abstract

The family Empetraceæ is described and its position in a natural classification is discussed. According to the cons nsus of modern opinion its proper place is either next the Ericaceæ or as an additional tribe within that family. The biology of the genus Empetrum is described. Its geographical distribution is described in detail and the most important ecological features are noticed briefly. Variation within the genus is described and is shown to be chiefly in seven characters:–general habit, degree of hairiness, shape and size of leaf, arrangement of leaves, angle of leaf-insertion, sex of flowers, colour of berry. The morphological and geographical range of each of these seven characters is described in detail. Variation as seen in these characters is of three kinds: simple continuous, complex continuous, and discontinuous. It is shown that instead of an almost indefinite number of forms within the genus all the individuals fall into ten clearly differentiated groups. This is chiefly due to the combined influence of correlated variation and geographical segregation. These ten groups are described. Previous taxonomic work on the genus is briefly reviewed. The inter-relationships of the ten groups comprising the genus are fully discussed and a taxonomic treatment is suggested, which is, in the writer's opinion, expressive of the conclusions reached in the discussion. According to this classification the genus contains two species, one having a subspecies, a variety and three forms, the other having three forms. In conclusion the methods adopted in the taxonomic treatment are described and discussed. The writer gratefully acknowledges his deep indebtedness to the Directors of the Herbaria at Berlin, Upsala, Geneva, Copenhagen, Leningrad, and the Gray Herbarium for the loan of numerous specimens by which the detailed analysis of the genus was made possible. His thanks are also due to many friends who have offered valuable criticisms, advice and information, and to the Librarian of the Botanical Department of the British Museum (Nat. Hist.) for his help with the extensive literature of the genus.