Role of Nature to encourage cross pollination and avoid self pollination

In Flowering plants, Continued self  pollination result inbreeding depression. Therefore flowering plant has developed many devices to prevent self pollination and and enourage the cross pollination because cross pollination lead the genetic variabiliry  and it is an essential elements for the process of evolution. 

There are various devices that are given by the nature to the flowering plants to encourage the cross pollination. These devices are called as  outbreeding devices. 

Self incompatibility - it is a condition where pollen of flower  either do not grow at all or group very slowly if they fall on the stigma. This situation prevents the self  pollination.

Dichogamy-  The maturation of pollen and stigma of a flower are not synchronised. It means that time of stigma receptivity  and time of pollen release are different. This is called as Dichogamy. 

When the stigma become receptive  before the release of pollen. This condition is called protogyny. The Protogyny  condition can be seen in Magnolia, Aristolochia.

When the pollen are release before the stigma became receptive. this condition is called Protandry. This can be seen in Salvia, Helianthus.

Herkogamy- in some bisexual flower the structure anther and style is organised in such condition that  self pollination is not possible and you can say the anther and stigma are located at different position so pollen can  not come in contact with stigma. this condition is called Herkogamy.

In caryophyllaceae, the style is much longer than stamen so  the pollen is fail to reach the stigma. 

Similarly in Gloriosa, the anther release pollen at much distance from stigma. So there is no contact between pollen and stigma. 

Besides these, the production of unisexual  flowers is is also a device that prevent self pollination. Such plants are called  diocious plant. In these  plant male and female flowers are originated on the different plants  so there is no chances for self pollination.