The Genus Riella in North and South America: Distribution, Culture, and Reproductive Isolation

Abstract

Plants meeting the general description of Riella americana occur in the states of Texas, New Mexico, California, Chihuahua, and Durango, as well as the republic of Argentina. Evidence suggests that the species is more widespread than earlier reported. Experimental crosses between various isolates indicate that all dioecious members of the genus in the Western Hemisphere are part of a single complex. However, in many instances cross-fertilizaation between populations leads not to viable spores but only to initial sporophyte development, followed by disintegration of the contents of the developing capsule. Judged from the basis of reproductive isolation at least three distinct groups exist within the Western Hemisphere. Two of these differ further with respect to habitat and external morphology. Members of the genus Riella have long been considered distinctly uncommon, if not rare; yet as Persson and Imam (1960) suggested, this impression probably stems in large part from the unusual habitat in which the plants are found. Few bryologists appear to spend any appreciable time searching the bottom of muddy desert pools, and, unfortunately, most phycologists are unfamiliar with the genus. I have found Riella to be fairly common throughout western Texas, New Mexico, Chihuahua, and Durango. The same probably holds true in many other arid to semiarid regions of the world as well. The present account summarizes previous information on the distribution of the genus in North and South America, briefly describes means by which new populations have been encountered, and explores the extent of interfertility between spatially isolated populations.