Q.
Go through the following points
(I). Dicliny (unisexuality of flower)
(II). Dichogamy (protoandry or protogyny)
(III). Self sterility / self incompatibility
(IV). Heterostyly
(V). Herkogamy (Barrier between anther and stigma of a bisexual homogamous flower)
The above contrivances prevent -
Sexual Reproduction in Flowering Plants
Solution: