Unisexuality of flowers prevents

The transfer of pollen from anther to stigma is called pollination. Autogamy also called self-pollination involves the transfer of pollen grains of one flower to the stigma of same flower. Self-pollination occurs only in bisexual or hermaphrodite flowers. Geitonogamy involves the transfer of pollen grains from a male flower to stigma of female flower of same plant. Thus, geitonogamy operates only in monoecious plants, i.e., the plants having male and female flowers on different places. Allogamy or xenogamy also known as cross-pollination involves the transfer of pollen grains of male flower to the stigma of genetically different female flower. It takes place via various agencies like wind, water, insects, etc.