A Natural Hybrid between Weissia and Astomum and Notes on the Nomenclature of the North American Species of Astomum

Abstract

Hybrid sporophytes, resulting from natural hy- bridization between Weissia controversa and Astomum ludovi- cianum, are intermediate in characteristics between those of the parents. Some of the hybrid sporophytes failed to produce spores while in others the spores were frequently deformed and were largely or completely sterile. The three North Ameri- can species of Astomum are transferred to the genus Weissia. Both Weissia controversa and Astomum ludovicianum occur in abundance on the heavy clays of southern Louisiana. They fre- quently grow together and both produce sporophytes at about the same time, towards the end of the rainy season, in early spring. Thus conditions for their hybridization are potentially favorable in this area. Indeed we both have occasionally seen individual sporo- phytes, growing with those of Weissia and Astomum, which seemed to be intermediate in appearance between those of the two species. In March of this year, one of us (W.D.R.), while collecting in a low, wooded area in Lafayette Parish, found a lump of clay bearing a population of fruiting plants that seemed to include, among other species, Weissia controversa, Astomum ludovicianum, and a large number of individuals bearing sporophytes intermediate between those of the Weissia and Astomum. The lump of clay was, in fact, an almost perfect experimental set-up. It has been turned up by the passage of a vehicle through the wet woods; on one side of the lump was an almost pure stand of Weissia, and on the opposite side there was a dense growth of Astomum. Between the Weissia and the Asto- mum grew the plants with intermediate sporophytes. In addition there were also present a number of plants of Astomum muehlenbergianum,