| This thesis examines how environmental variables influence the evolution and maintenance of unisexuals in angiosperms. The first objective was to assess how resource conditions affect male floral strategy and evolution in Sagittaria latifolia. Males have prolonged flowering periods compared to hermaphrodites. Experimental results suggest that males have a linear gain curve under a spectrum of resource conditions. The second objective was to determine the stability of a female phenotype in trioecious populations of Mercurialis annua. While there were associations with density in natural populations, in "experimental plots the frequency of females in the progeny decreased regardless of density and presence/absence of males. The stable co-existence of all three sex phenotypes is unlikely. These data provide some of the first experimental evidence to test conditions which promote the evolution of males in a dioecious plant and stability of a new sex phenotype in the presence of two other sexes.;Keywords: dioecy, monoecy, hermaphroditism, trioecy, androdioecy, Sagittaria latifolia, Mercurialis annua, floral display, evolution of males, gain curves, mating arrays, sex-plasticity hypothesis, sex allocation, female invasion, sex ratio. |
