Abstract
The taxonomy of six species of the moss genusDitrichum subgenusDitrichum, comprising all of the known species from North America, has been reviewed. Specific characters have been reevaluated, distributional patterns were studied, and a key included which emphasizes diagnostic characters not previously stressed. Two species,D. rufescens, andD. montanum have tropical American affinities. In addition two species-pairs are recognized, each pair consisting of a diploid and tetraploid species with haploid chromosome numbers of 13 and 26 respectively.D. schimperi, a West Coast tetraploid species, appears to be more closely related toD. henryi, a diploid species which is restricted essentially to the Appalachian and Alleghanian Uplands.D. pallidum, a widespread species of the east, is a tetraploid and has a Coastal Plain counterpart inD. curritucki, a closely related diploid species. Cytological, geographical, and morphological observations are discussed.
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