POLLINATION AND FRUIT-SET IN ASCLEPIAS: A REAPPRAISAL

Experimental pollinations of Asclepias tuberosa L., the first for this species and second large-scale effort for the genus, reveal trends toward local population differentiation. The species possesses a low level of self-compatibility, the first documented report for the genus. Analysis of flowers visited by natural pollinators in populations across the eastern half of the United States demonstrated a linear relationship between pollinia removal and pollinia insertion. Clarifications in older literature regarding the specificity of the pollination mechanism, effective levels of pollination in natural populations, and the performance of experimental manipulations are made, and it is concluded that fruit-set in Asclepias is regulated by the interaction of both mechanical aspects of pollination and physiological aspects of fertilization and fruit development. A model incorporating these restraints is seen to predict fairly well the observed fruit to flower ratio of about 1:100 in natural populations.