Reducing geitonogamy (pollen transfer amongflowers within the same plant) has been suggested as amajor selective force for plants with multipleflowers. The occurrence of geitonogamy is generally different amongflowers within inflorescences; however, no researchers have examined whether plants enlarge their display sizewithout increasing the possibility of geitonogamy by presenting moreflowers at positions where they are less likelyto be geitonogamously pollinated. We observed that bumblebee pollinators foraged upward within the tower-shaped inflorescences of protandrousMegacodon stylophorus(C. B. Clarke) Harry Sm. BecauseM. stylophorusdidnot strictly bloom bottom-up, there were substantial frequencies of geitonogamous pollination resulting fromupward and horizontal pollen transfer. Although there was a strong correlation between total numbers offlowersplants produced and numbers offlowers presented on single days, proportions offlowers possiblygeitonogamously pollinated were weakly correlated with total numbers offlowers. This might have been becauseplants with more resources enlarged their display size by producing moreflowers on lowerfloors whereflowers hada low probability of being geitonogamously pollinated. This study shows that the tower-shaped inflorescences ofM.stylophorusenlarge their size without more cost of geitonogamous mating, suggesting that geitonogamy acts as animportant selective agent in the evolution of inflorescence architectures.
