Abstract: An underlying assumption for the size-dependent sex allocation (SDS) hypothesis is that the plasticity of bisexual investments in hermaphrodites would be larger in female than in male allocation, but it has rarely been critically evaluated. Among five sexual morphs in a sexually polymorphic desert herb Geranium transversale, gynomonoecious individuals were the largest, and males were the smallest, while hermaphroditic and andromonoecious plants and females did not differ significantly in plant size. The total number of flowers increased with plant size in all five sexual morphs; in gynomonoecious and andromonoecious plants this was due to an increase in the number of perfect flowers rather than pistillate or staminate flowers. Flower size increased with plant size in hermaphroditic and gynomonoecious plants (due to an increase in the size of perfect but not of unisexual flowers). The sizes of staminate flowers in andromonoecious and male plants, and pistillate flowers in gynomonoecious plants did not increase with plant size, and pistil mass increased but stamen mass decreased with plant size. The coefficient of variation (CV) in pistil and stamen mass among 210 plants in four of the sexual morphs (excluding female plants) indicated that variation in resource allocation among individuals was higher in female than male functions. Both flower number and flower size generally increased with plant size in G. transversale, supporting the SDS hypothesis. The relatively higher plasticity (CV) in female than male allocation has not been reported before, providing a cue for understanding why the female-biased allocation associated with plant size is popular in flowering plants.

Key words: andromonoecy, floral sex allocation, Geranium, gynomonoecy, plant and flower size, reproductive and vegetative mass, sexual polymorphism.