Constant extreme leaf dimorphism in the moss family Calymperaceae, involving evolution of the gemmiferous leaf of some species into a rod-like essentially naked costa or a narrow tube-like or channeled shaft, occurs more or less consistently in five species of Calymperes and six species of Syrrhopodon world wide (approximately eight percent of the species of the family), but not in the closely related genus Mitthyridium. Such highly derived gemmiferous leaves—referred to here as stenophylls—are an evolutionary phenomenon that is not known for any other group of mosses. The stenophylls bear the gemmae on their tips, are rigidly erect both wet and dry, are generally longer than and mostly have smaller areas of cancellinae than the vegetative leaves. The principal functional-evolutionary benefit of this type of extreme leaf dimorphism may be the constant (plants hydrated or desiccated) presentation of the gemmae for dispersion elevated above the potentially trapping tangle of vegetative leaves of the moss turf. Also, some of the stenophylls can store water transiently in tubes or channels. The stenophylls presumably play only a minor role in photosynthesis due to their lack or near lack of green lamina; thus reduction of overall photosynthetic area per plant is a consequence of strongly expressed specialization for asexual reproduction in these dioicous mosses. Likewise, reduction in water-retaining capacity per plant is a consequence of reduction in area of the cancellinae of stenophylls. Four of the extreme leaf-dimorphic Calymperes species occur in Malesia and one in tropical America; one of the extreme leaf-dimorphic Syrrhopodon species occurs in Malesia, three in tropical America, one in eastern North America, and one in eastern Africa (including the island of Mauritius).