The advantage of being tall: Higher flowers receive more pollen inVerbascum thapsusL. (Scrophulariaceae)

The effective pollination hypothesis predicts that the primary fitness advantage of tall height affected by apical dominance is the ability to attract pollinators away from other plants and hence promote outcrossing. In this study, we tested whether taller plants of Verbascum thapsus are better competitors for pollinator service than shorter plants. Pollinator preference was estimated by pollen deposition on stigmas. Within each of six populations, the ten tallest and ten shortest plants were selected, and from each plant the stigma was collected from the highest open flower and from the lowest open flower on the inflorescence spike. In all six populations, pollen deposition was significantly greater on stigmas from taller plants and from flowers at the top of inflorescences. Taller plants also had more open flowers. These data indicate that pollinators are attracted to taller plants, or plants with more open flowers. Field observations did not suggest that the greater pollen deposition on higher flowers within a spike was a consequence of directional pollinator foraging. These results provide preliminary support for the effective pollination hypothesis and provide a strong basis for further studies regarding the selective advantage of apical dominance in plants.