Outbreeding species with annual gametophytes reproduce successfully only if multiple spores arrive at a safe site at nearly the same time and in nearly the same place (e.g., Cousens et al., 1988). Perennial, non-gemmiferous gametophytes may free species from the temporal constraint, but much less so from the spatial constraint, i.e., they are still dependent upon the deposition of several spores in nearly the same place. By providing a mechanism for much greater local dispersion of gametophytes, gemmae may alleviate both temporal and spatial constraints.
When the gametophyte form and the breeding system of a sufficient number of species have been characterized, comparisons such as these may yield even greater insight into the influence exerted by the gametophyte generation on species dispersal. Published descriptive data suggest the existence of some variation in the form of gametophytes even within genera and families. Initially, some species appear to assume the annual gametophyte strategy, then, if not successful in sporophyte production, convert to the strategy of perennial game-tophytes. Examination of such variation for correlations with habitat types may be instructive in understanding the evolution and plasticity of growth form.
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