explain any two devices by which autogamy is preventd in flowering plants
Autogamy is prevented by formation of Unisexual flowers. Two conditions are their:
Monoecious Plants : Male and female flowers are produced on the same plant. It prevents autogamy but not geitonogamy. Example : Maize
Dioecious Plants : Male and female flowers are produced on two separate plants. It prevents autogamy as well as geitonogamy. Example: Papaya
In some species the anther and stigma are placed at different position, so that pollan cannot come in contact with the stigma of the same flower.