The Significance of Spore Banks in Natural Populations of Athyrium pycnocarpon and A. thelypterioides

How do homosporous plants survive in variable environments? One mechanism available to plants is seed or spore dormancy (Cohen 1966, 1967). An observable consequence of seed and spore dormancy is the formation of a seed or spore bank (Harper, 1977; Grime, 1979). Work in this area goes back at least to Darwin (1859), who assayed a cupful of mud from a stream bank for viable seeds. Cohen (1966, 1967, 1968) developed models of seed and spore dormancy describing the environmental conditions favoring its evolution. He predicted that dormancy will evolve when the variance of environmental quality greatly exceeds the mean of environmental quality. Under this condition natural selection will favor individual seeds and spores that a) have delayed germination and prolonged viability, as a means of avoiding unfavorable