Unisexuality of flowers prevents:

1. Autogamy: Self-pollination within the same flower. This requires bisexual flowers. If a flower is unisexual (only male or only female), autogamy is impossible.2. Geitonogamy: Pollination between two different flowers on the same plant. Even if flowers are unisexual, if both male and female flowers are on the same plant (monoecious, like maize), geitonogamy can still occur.3. Xenogamy: Cross-pollination between flowers of different plants.Therefore, unisexuality prevents autogamy because a single flower cannot pollinate itself, but it does not necessarily prevent geitonogamy if the plant is monoecious.