1. Pollen release and stigma receptivity are not synchronised, ie. either the pollen is released before stigma become receptive or stigma becomes receptive much before the release of pollen.
2. In some plants the anther and stigma are placed at different positions so that pollen cannot come in contact with the stigma of the same flower.
3. Self pollination is prevented by self incompatibility which is a genetic mechanism that prevents the pollen of the same flower or flowers of the same plant from fertilizing the ovules.
4. Production of unisexual flowers also prevents autogamy but not geitonogamy. Dioceous plants prevents both autogamy and geitonogamy.
