Gametophytic population density influences sex expression in the fernCeratopterisBrongn. Extensive variation occurs among gametophyte families, both in the response of sex expression to density and in the covariate (density) adjusted frequency of hermaphroditic gametophytes. These differences are probably a function of genetic variability in the antheridiogen mediation of sex expression. In general, the frequency of hermaphroditic gametophytes decreases and becomes asymptotic and more stable with increased density. Higher gametophytic densities probably promote maleness through the more rapid attainment of effective concentrations of antheridiogen. Environmental factors that modify sex expression, e.g., gametophytic density and the response of any given gametophytic family to density, may ultimately influence the mating system.
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