Pollination - Outbreeding Devices

Outbreeding devices (contrivances):

Introduction: 

• Plants have evolved various sexual strategies to promote cross pollination and increase genetic diversity, as self-pollination can lead to inbreeding depression.
• To counteract this, plants have developed devices like outbreeding devices, which discourage or prevent self-pollination, thereby promoting genetic diversity and natural selection.
• Thus, plants have developed many devices to encourage cross pollination. The examples of outbreeding devices are as follows: 

1) Unisexuality-  

In this case, the plant bears either male or female flowers. It is also called as dioecism. 

As flowers are unisexual, self pollination is  not possible. Plants may be monoecious, e.g. Maize or dioecious, e.g. Mulberry, Papaya. 

2) Dichogamy-  

In this device, anthers and stigmas mature at different times in a bisexual flower so as to prevent self pollination.   

Types of Dichogamy 

1. I. Dichogamy- In this type, androecium matures earlier than the gynoecium, g. in the disc florets of sunflower.
2. Protogyny- In this type, gynoecium matures earlier than the androecium, e.g. Gloriosa.

3) Prepotency- Pollen grains of other flowers germinate rapidly over the stigma than the pollen grains from the same flower, e.g. Apple. 

4) Heterostyly (heteromorphy)-  

In some plants like Primula (Primrose) plants have two or three flower forms with different stigmas and anthers (heterostyly and heteroanthy), preventing pollens from reaching the stigma. In heteromorphic flowers, pollen grains from anther pollinate stigmas produced at the same level. 

5) Herkogamy-  

It is a mechanical device prevents self-pollination in bisexual flowers by preventing contact between pollen and the stigma of the same flower, such as the Calotropis pentangular stigma, which is positioned above the anthers level. 

6) Self-incompatibility (self-sterility)- This is a genetic mechanism due to which the germination of pollen on stigma of the same flower is inhibited, e.g. Tobacco, Thea.