Flowering plants have developed many devices to discourage self-pollination and to encourage cross-pollination.

First device: In some species, pollen release and stigma receptivity are not synchronised. Either the pollen is released before the stigma becomes receptive or stigma becomes receptive much before the release of pollen. This condition is called dichogamy in which stigma and anther matures at different time.

Second device: In some species, the anther and stigma are placed at different positions so that the pollen cannot come in contact with the stigma of the same flower. This condition is called heterostyly.

Herkogamy: Non-transfer of pollen from anther to stigma of the same flower due to a mechanical barrier is present between anther and stigma.

E.g.: Calotropis (Asclepiadaceae), Aristolochia, Gloriosa superba.

The third device to prevent inbreeding is self-incompatibility or self-sterility. This is a genetic mechanism and prevents self-pollen (from the same flower or other flowers of the same plant) from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil.