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Gender in flowering plants is governed by a complex interplay of genetic and environmental factors. The perennial aquatic herb Sagittaria latifolia displays remarkable variation within and among populations in gender expression, from monoecious populations composed of plants with differing numbers of female and male flowers to completely dioecious populations with separate female and male plants. Here we examine the role of environmental factors in modifying gender expression in these two sexual systems by investigating the relation between ramet size and flower number in three sex phenotypes (hermaphrodite, female, and male). In particular, we assess the extent to which hermaphroditic plants have the option of altering the number of female and male flowers to accommodate their specific environmental circumstances. We sampled 12 populations from diverse wetland habitats in southern Ontario, Canada, estimated their phenotypic gender, and examined the relations between ramet size and a range of vegetative and floral traits. On average, plants from dioecious and monoecious populations did not differ in leaf length, a correlate of ramet size. However, in monoecious populations with significant numbers of both male and hermaphroditic ramets, hermaphrodites were larger and produced more flowers than males. This contrasting pattern was also observed when plants were grown under glasshouse conditions. In monoecious populations, variation in ramet size did not affect the production of male flowers, whereas female flower production varied positively with plant size. These relations enabled statistical prediction of the dependence of gender on plant size and the frequency distribution of gender within monoecious populations. These relations also imply that the male phenotype can be determined environmentally, whereas the female phenotype cannot. The size dependence of floral sex ratios in monoecious populations provides novel insights into the likely evolutionary pathway by which dioecy has evolved from monoecy in Sagittaria.