XVI International Botanical Congess

Self-incompatibility in distylous Turnera was studied using fluorescence microscopy of pollinated styles. The sites" of incompatibility in shorts occurred just below the stigma, but longs showed penetration further into the style. Shorts lacked callose plugs upon selfing, in contrast to the longs. Callose plug development occurs 60-120 minutes after pollination in all crosses but the short selfs. We tested a hypothesis for the evolution of homostylous species. The results of this investigation support the recombinant hypothesis. For crosses where the homostylous species are pollen recipients, pollen tubes from shorts, but not longs, reach the base of the style. In crosses where the homostylous species are the pollen donors, pollen tubes reach the base of the style in the longs, but not in shorts.