The normal phenological sequence in Grimmia orbicularis restricts sporophyte elongation in Mojave Desert populations to the winter months, when low temperatures and sufficient rainfall occur. During 1997, however, heavy rainfall was restricted to the summer months. Coincident with these summer rains, over 50% of the cohort sporophytes aborted, with the time of abortion traceable to the period that followed the summer rains. Abortive sporophytes were arrested at or near the end of the embryonic phase, just prior to seta elongation. It is postulated that the unusually high frequency of abortive sporophytes was caused by the absence of winter rains in conjunction with torrential summer rains. The accelerated hydration/rehydration cycles that occur during summer rains allow insufficient time for desiccation recovery processes critical for sporophyte survival.

Key words: Grimmia orbicularis, Mojave Desert, monoecy, phenology, sporophyte abortion