Pollination is the transfer of pollen from the anther to the stigma of the same flower or of another flower on the same plant, or of another flower on a different plant of the same species. When the anthers are fully mature they shed off the pollens either through slits in their walls or through terminal pores. The pollen is generally carried to the help of various agencies like wind, water, insects, birds, etc.
After pollination petals and stamens fall off. Sometimes the sepals are persistent and remain on the fruit. The pollination is of two kinds:
1 Self Pollination
2 Cross Pollination
If mature pollen grains from the anther of flower are transferred to stigma same flower or to that of another flower of the same plant, e.g., in Pea, Cotton or “Tomato, the process is termed as Self-pollination or (auto – self, Gamos – marriage).
Self-pollination takes place in bisexual flowers only and never in unisexual flowers. Even in bisexual flowers there are often certain mechanisms which wholly prevent self-pollination. The progeny resulting from self-pollination gradually become weaker, and it is therefore, less frequent than the cross-pollination.
Cross-pollination is the rule in nature, and self-pollination is, in most cases, more or less incidental. Except in few specific cases, self-pollination takes place only when and cross-pollination fails.
Some plants have devices which help in self-pollination.
Types of Self Pollination
(A). Homogamy (homos – the same)
This is the condition in which the anthers and stigmas of bisexual flower mature at the same time.
Self-pollination in homogamous plants may occur in the following ways:
(i) A gentle breeze might shake the mature anthers which then shed their pollen grains onto any ripe stigma situated below them.
(ii) Insects may visit the flower for its nectar and carry mature pollen grains mature stigma of the same flower.
(iii) As the anther mature, the filaments of the anther recoil and bring the anthers close to the stigma. The anthers then burst and discharge their pollens right on the surface of the stigma, bringing about the self-pollination, as in Mirabilis jalapa (Gul-e-Abbasi). The reverse is also true in some cases, e.g., in some members of Compositae and Malvaceae. The stigmas themselves may bend over and come into contact with the anthers.
(iv) The anthers may initially be situated above the stigmas as in the case of tubular flowers (Gardenia) whose anthers are crowded together at the mouth of the corolla tube. The mature stigma pushes their way out of the corolla tube and they brush against the anthers and collect pollen grains.
(B). Cleistogamy (Kleistos – closed)
Some small, inconspicuous bisexual flowers never open at all. They are known as Cleistogamous or Closed Flowers. Ripe pollen grains are discharged onto the stigmas. The both anthers and carpels ripe at the same time. This condition is called Cleistogamy. Cleistogamy is generally accompanied by Geocarpy (underground fruit formation). In such cases the cleistogamous flowers arise on underground shocks.
Cleistogamous flowers are without color and they do not secrete nectar. Cliestogamy is found in some species of Imaptiens, Oxalis etc.
Advantages and Disadvantages of Self-Pollination
- Genetic uniformity in populations is maintained and parental characters are preserved over a considerable period of time.
- Seed production in ensured.
- As compared to cross-pollination, it is highly economical method as it tries to conserve energy which might have, otherwise, been utilized in the production of more pollens grains, nectar and accessory structures.
- It is one of the chief methods to obtain homozygous individuals particularly in plants where anther culture has failed to yield haploids.
- The resulting plants are pre-adapted to the prevailing environmental conditions.
- Additional characters cannot, by themselves, get incorporated in self-pollinated plants.
- With passage of time self-pollinated plants become more susceptible to disease. Also, seed production is also affected.
- It is not possible to get rid of any undesirable characters.