Question

explain any two devices by which autogamy is preventd in flowering plants

Answer 1

1)Dichogamy: Sometimes the stamens and stigma of a bisexual flowermature at different time toprevent autogamy. 2) Self incompatibility being genetic mechanism prevent self-pollination.

Answer 2

Self pollination is common and more likely to happen in case of hermaphrodite flowers but a successive series of self pollination affects negatively and causes inbreeding depression. This also results in homozygous genes. 

Unisexual flower: If a flower is unisexual i.e., contain only one sex either female or male, cross pollination is the only choice.
Non-Synchronization: Timing is important for successful pollination. Pollen release and receptivity of stigma should happen simultaneously. Sometimes, pollen matures and releases before stigma is open which leads to loss of pollen vitality or vice-versa. This prevents self pollination; even though flower is hermaphrodite.
Self-incompatibility: Incompatibility within a flower (or plant) includes self-sterility; structural barrier .Self-sterility means even though pollination takes place it can’t proceed to fertilization due to further pollen growth failure. Structural barriers include height difference between gynoecium and androecium and some structures which hinder the stigma from receiving pollen. These are more or less genetic mechanism.