1. Unisexuality. The Plants bear unisexual flowers i.e. stamens and carpels occur in different flowers. They may be present on same plant  e.g., in maize or on different plants e.g. in Papaya. The first condition is called monoecious and second is called dioecious. Since the male and female flowers are different the chances of self pollination are less.

2. Dichogamy. When stamens and carpels of bisexual flowers mature at different times, it is called as dichogamy. If stamens mature first it is called protandrous condition, or if carpels mature first it is called protogynous condition. since the stamens and the carpels mature at different time they cannot self pollinate.