Male plants of Cyrtomnium hymenophylloides are described in detail for the first time. They difer from female plants only in gametangial features, particularly in having a splashcup perigonium similar to that of male plants in most dioicous species of Mniaceae. The isomorphic paraphyses of male and female plants are unique within the Mniaceae. Male plants producing
perigonia were found in nine herbarium specimens from parts of northeastern Alaska and arctic Canada that may have served as refugia during the Wisconsinan (Pleistocene) ice-sheet maximum. Female plants are much more common, occurring in arctic Alaska and Canada, Greenland, and southward in the Canadian Rocky Mountains, and in parts of eastern Canada to the Catskill Mountains of New York. No sporophytes have been found in North America and Greenland, and outside this area only one sporophyte is known. Stems of this moss are shown to fragment into leafy pieces of variable size, and the subsequent dispersal of these, and the initiation of new growth from axillary and apical buds, appear to be the principal means of reproduction. We assume that wind is the major dispersal agent in the Arctic, but water and gravity may play a role in local dispersal in forested regions of the northeastern United States where the species displays a relictual distribution.