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

The spasmodic temporal distribution of the rare ephemeral moss Physcomitrium sphaericum is described and an explanation of this unusual periodicity is proposed. Field observations over 44 years have established that 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. It is suggested that 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.