Patterns and consequences of stigma height variation in a natural population of a distylous plant, Primula sieboldii

1. In order to reveal the direction and magnitude of phenotypic selection o n the stigma height of heterostylous morphs in a natural population of Prim ula sieboldii E. Morren (Primulaceae), morphological traits including stigm a and anther heights, flowering phenology, local density of potential matin g partners, size of ramet, number of pollen grains deposited from the oppos ite morph and seed set were measured in two flowering seasons. 2. Both components of female reproductive success, pollen receipt and seed set, were significantly larger in the long-styled than in the short-styled morph in both years. 3. Selection analyses based on bivariate correlation and path-analysis mode ls revealed positive relationships between stigma height and pollination or seed set in the short-styled morph only, although the effect was statistic ally significant only in one year. In contrast, seed set in the long-styled morph was correlated with ramet size. 4. Morph difference in reproductive success and the natural selection revea led for higher stigmas in the short-styled morph should be ascribed to the narrow corolla tube of the species that strictly restrict the path of the p roboscis of the pollinators.