Flower dimorphism and the maintenance of andromonoecy in Sagittaria guyanensis ssp. lappula (Alismataceae).

• Staminate flowers of andromonoecious species are thought to be produced to increase reproductive success through enhancing male function or diverting resources from unneeded pistils to fruits. This does not explain why andromonoecy occurs within genera with monoecy, since staminate flowers of monoecious plants can also serve these functions. • Here the male allocation of staminate and perfect flowers was measured in the annual herb Sagittaria guyanensis ssp. lappula , an andromonoecious species in a predominantly monoecious genus. Staminate flowers contained anthers that were larger and more numerous than those of perfect flowers, and their petals were also larger. This contrasts with most investigations where staminate flowers typically have equal or lower investment in male structures than perfect flowers. • Seed set was not significantly different between bagged and open-pollinated flowers. Pollinator observation and a pollen-dyeing experiment indicated visits to the flowers rarely occurred. • The presence of functional stamens in perfect flowers could be selected to allow reproductive assurance in case of inadequate pollination in andromonoecious species, rather than as a mechanism for optimal resource allocation.