List three strategies that a bisexual chasmogamous flower can evolve to prevent self pollination (autogamy).

In chasmogamos bisexual flowers to prevent self fertilisation, following three phases are evolutioned.

$(A)$ Dichogamy : In this method pollen grain is released But for acceptance of stigma is self incompatible. In sunflower before stigma matures and accepts pollen grain, pollen grains are released. In Dhature solenum before pollen grains are released, stigma is prepared so cross pollination can be possible.

$(B)$ Herkogamy : When male and female reproductive organs are kept in different situation and in different directions it is called Herkogamy. In these plants pollen grain cannot come in contact of stigma of the same flower. So cross pollination occurs. e.g. Hibiscus and Glorissa.

$(C)$ Self sterility : This is a genetic type method. If prevents germination of pollen grain and growth of pollen tube in pistil e.g. Abotilon.

Note : In another method to prevent self pollination, the method is to produce unisexual flowers, which is not advantageous. This prevents self fertilisation but in castor and maize like plants, Geitonogamy can not be prevented.