An explanation of the intermittent occurrence ofPhyscomitrium sphaericum(Hedw.) Brid.

Abstract
The spasmodic temporal distribution of the rare ephemeral moss P. sphaericum is described and an explanation for this unusual periodicity is proposed. The presence of P. sphaericum is closely correlated with periods of summer drought. Evidence from germination experiments and the discovery of a viable buried spore bank suggest that during drying out of mud exposed by drought, spores are unearthed and dormancy is broken by the action of light. Apparently these regenerative features, together with a compressed life cycle, enable success in a habitat where conditions for growth occur infrequently and are of a short duration.