Roughly 3% of flowering plants are gynomonoecious, with individual plants bearing both female and bisexual flowers. Little attention has been paid to the adaptive significance of this sexual system, which is particularly prevalent in the Asteraceae. Here we investigated one hypothesized advantage of having two flower types, namely that the arrangement permits flexibility in allocation of resources to male and female reproductive functions. We examined several perennial, gynomonoecious members of the genus Solidago. These plants produce small heads consisting of several female flowers, each bearing a single small petal, surrounding several bisexual disk flowers. We carried out greenhouse experiments in which we varied one or more of three environmental variables: light, nutrients and water, and/or examined heads in different positions on the plants or produced on different dates. The effects of these variables on the proportion of female flowers were modest to none. Specifically, significant effects were found for light in 0 of 3 species, for nutrients in 2 of 4 species, for water in 0 of 1 species, for position in 1 of 3 species and for date in 1 of 1 species. Because of the small number of significant effects and their modest magnitude, we conclude that the presence of two flower types in goldenrods is probably not advantageous in allowing flexibility in allocation of resources to male and female functions. It seems likely that this sexual system has been more important either in providing for pollinator attraction of in reducing pollen-pistil interference.

Key words: capitulum, goldenrod, gynomonoecy, sex expression, sex ratio, Solidago