The influence of variation in style length on pollen flow and seed set was examined in six populations of Menyanthes trifoliata in southwestern British Columbia to evaluate Ganders' hypothesis that morphological distyly increases the fecundity of a diallelic self-incompatible plant. In five populations, Menyanthes was distylous and self-incompatible. The sixth population consisted of pins and morphological homostyles (thrums with unusually long styles). In each population morph frequency, pollen frequency, the composition of stigmatic pollen loads and seed set were estimated. Results from the six populations demonstrate that the size and composition of stigmatic pollen loads fluctuates erratically during the flowering season. Pins and thrums experienced disassortative pollination, assortative pollination and random pollination at different times during the season. Homostyles were always assortatively pollinated. Among populations there was a high correlation between morph frequency (pollen frequency) and the composition of stigmatic pollen loads. However, in anisoplethic populations of Menyanthes morphological distyly seems to compensate for the rarity of one floral form, by increasing the proportion of compatible pollen received by the opposite floral form. Comparison of the composition of stigmatic pollen loads of homostyles with those of thrums revealed that the separation of stigmas and anthers in distylous flowers of Menyanthes reduces the number of incompatible pollen grains received, and the reciprocal placement of stigmas and anthers appears to increase the number of compatible pollen grains received. Seed set in the six populations of Menyanthes was always below the potential maximum. Among populations there was a high correlation between the number of compatible pollen grains received and seed set. In at least one population, pollen availability was a major factor limiting seed set, but other factors are probably influencing seed set in Menyanthes as well. Thrums set significantly more seeds per capsule than homostyles. In populations of Menyanthes, the reciprocal placement of stigmas and anthers in distylous flowers (compared to homostylous flowers) increases the amount of compatible pollen deposited on stigmas, and this increase is associated with greater fecundity. / Science, Faculty of / Botany, Department of / Graduate