AbstractThe germination and establishment abilities of different types of propagules under natural conditions is an area in bryophyte reproductive biology that is largely unknown. In some species gametophyte fragments, rather than spores, may be primarily responsible for establishment of new colonies in nature, and the presence of other mosses may have an effect on establishment processes. These ideas were tested experimentally using four species ofTortulagrown in a glasshouse under semi-natural conditions. The relative germination and growth abilities of spores and gametophytic fragments were tested under each combination of the following treatments: (1) continuous hydration (using a mist system), or periodic drying; (2) substrate of bare sand, a clump ofDicranum scoparium, or (for spores only) the clump ofTortulafrom which sporophytes were taken. Fragments produced new shoots much more quickly than spores under either hydration condition but spores germinated at a higher rate. Germination rates of both spores and fragments were greatly reduced (usually to zero) in clumps ofTortulaorDicranumunder either hydration condition. Some differences between species were noted. In a further experiment filter-sterilized water extracts of each species were applied to spores on agar. Again, strong effects on spore germination and protonemal growth were noted. Patterns of inhibition differed between species and reflected those found in the previous experiment, indicating that the inhibition effect may be due to a water soluble chemical rather than the physical properties of the moss clumps. These studies present the first experimental evidence for the chemical inhibition of spores by mature plants. When taken together with previous reports of chemical interactions among protonemal systems, considerable ecological significance of these results is suggested.