A bisexual chasmogamous flower can evolve the following (three) strategies to privent self-pollination (autogamy).
(a) Dichogamy : in this mechanism, pollen release and stigma receptivity are not synchronised. In sunflower, the pollen is released before the stigma becomes receptive (protandry). In Datura, Solanum, the stigma becomes receptive much before the release of pollen (protogyny) leads to cross-pollination.
(b) Herkogamy : The male and female sex organs are placed at different position or in different directions is called Herkogamy. In these plants the pollen cannot came in contact with the stigma of the same flower . It has undergone cross pollination, e.g., Hibiscus, Gloriosa.
(c ) Self-sterlility it is a genetic mechanism which prevents the self-pollen from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil, e.g., Abotilon.
Note : Another device to prevent self-pollination is the production of unisexual flowers, but this is not beneficial as above mentioned strategies. It prevents autogamy but not geitongamy in monoecious plants such as castar and maize .
