Question

Describe three devices by which cross-pollination is encouraged in angiosperms by avoiding self- pollination.

Answer 1

Genetic diversity is an essential factor for evolution by natural selection. Continued self-pollination results in inbreeding depression. Thus, plants have developed many devices to encourage cross-pollination. Examples of outbreeding devices are as follows:

1. Unisexuality:
   In this, 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, anthers and stigmas mature at different times in a bisexual flower due to which self-pollination is prevented. It can be further divided into two types:
   1. Protandry:
      In this type, anthers mature first, but the stigma of the same flower is not receptive at that time.
      e.g. in the disc florets of sunflower.
   2. Protogyny:
      In this type, stigma of carpel matures earlier than anthers of the same flower. e.g. Gloriosa.
3. Prepotency:
   In this, pollen grains of other flowers germinate rapidly over the stigma than the pollen grains from the same flower, e.g. Apple.
4. Heterostyly (heteromorphy):
   Plants like Primula (Primrose) produce two or three types of flowers in which stigmas and anthers are placed at different levels (heterostyly and heteroanthy).
   This prevents the pollen from reaching the stigma and pollinating it. In heteromorphic flowers, pollen grains produced from anther pollinate stigmas produced at the same level.
   Thus self-pollination is not possible in such cases.
5. Herkogamy:
   It is a mechanical device to prevent self-pollination in a bisexual flower. In plants, a natural physical barrier is present between two sex organs and avoid contact of pollen with the stigma of the same flower, in e.g. Calotropis, pentangular stigma is positioned above the level of anthers (pollinia).
6. Self-incompatibility (self-sterility):
   This is a genetic mechanism due to which the germination of pollen on the stigma of the same flower is inhibited, e.g. Tobacco, Thea.