Plant Size Effects on Female and Male Function in Hermaphroditic Sabatia Angularis (Gentianaceae)

Abstract

Variation in the contribution of different—sized individuals to female and male function was examined for the extremes of a size—structured population of hermaphroditic Sabatia angularis (Gentianaceae). Local populations of S. angularis at Miller Dunes, Indiana consisted of 10—15% large plants (>25 flowers) and °50% small plants (<7 flowers). Components of female function studied were (1) pollen grain deposition of stigmas, (2) probability of fruit set, (3) seed production per fruit, (4) average seed mass per fruit, and (5) seed dispersal distributions. Components of male function examined were (1) pollen production on a per flower basis, (2) pollen removal rates, and (3) pollinator observations and fluorescent dye experiments to infer pollen movement. Large plants contributed disproportionately through female and male function to the production of offspring compared to small plants. In one of the two years examined, the proportion of flowers developing into fruits was significantly greater in large plants than small plants. Seed production per fruit was overall twofold greater for fruits of large plants than small plants. Plant size had no effect on mean seed mass per fruit or the distance of primary and secondary seed dispersal. Flowers of large plants produced significantly more pollen per flower and had significantly more pollen grains deposited on their stigmas than flowers of small plants. However, there was no difference in the proportion of pollen removed from flowers of small and large plants, although the absolute number of pollen grains removed was greater for large plants. Large plants appeared to have the potential for more geitonogamous pollinations than small plants.