Pollination
The transfer of pollen grains from the anther of the stamen to the receptive stigma of the carpel/pistil is called pollination.
Classification
Depending upon the source of pollen grains, pollination is of following three types.
- Autogamy
- Geitonogamy
- Xenogamy/Allogamy
- Autogamy (Self-pollination) It is the kind of pollination achieved within the same flower. The pollens from the anthers of a flower are transferred to the stigma of the same flower, e.g., wheat, rice, pea, etc.
- (i) Pure breed of parents is maintained.
- (ii) Large numbers of pollen grains are not required for pollination.
- (iii) Seed production is ensured.
- (iv) Plants do not need to develop mechanisms for attracting pollinators.
- (i) Variations are not introduced in the progeny.
- (ii) Vitality of the breed decreases with time.
- (iii) Adaptability to changing environment is reduced.
Plants show following adaptations for autogamy.
- Cleistogamous flowers These are closed flowers and their anthers and stigma lie close to each other, e.g., Viola (common pansy), Mirabilis, Camelina and Oxalis. Almost all the cleistogamous flowers are invariably autogenous as there is no chance of cross-pollination.
- Chasmogamous flowers These are opened flowers with exposed sex organs. On these, pollens from another flower can land on the stigma as well, e.g., Catharanthus and Mirabilis.
- Geitonogamy It is the kind of self-pollination where the pollen grains from the anther of a flower are transferred to the stigma of another flower borne on the same plant, but at different branches. It usually occurs in plants which show monoecious condition, e.g., Cucurbita.
- Xenogamy (Cross-pollination) It involves the transfer of pollen grains from the anther of one plant to the stigma of another plant. This is the only type of pollination which brings genetically different types of pollen grains to the stigma during pollination, e.g., papaya, maize, etc.
Advantages of cross-pollination:
- (i) Variation in the progeny is introduced through recombination.
- (ii) The progenies have better adaptability to changing environment.
- (iii) Plants develop new traits like disease resistance.
- (iv) The defective characters are eliminated over the generations.
- (i) Large numbers of pollen grains need to produce of which most are lost during the process of pollination.
- (ii) There is always a chance that pollination may not occur.
- (iii) It is less economical.
Agents of Pollination
Pollination in plants occurs via various abiotic and biotic agents as follows.
Abiotic agents of pollination are wind (anemophily) and water (hydrophily).
Wind pollinated flowers have.
- Light and non-sticky pollen grains.
- Generally, a single ovule in each ovary and numerous flowers packed into an inflorescence, e.g., corn cob.
- Well-exposed stamens and large feathery stigma to trap air-borne pollen grains.
- Wind pollination is common in grass, maize, date palm, etc.
Water pollination occurs in mostly monocotyledons (in about 30 genera). Some examples of water pollinated plants are Hydrilla, Vallisneria and Zostera.
- In majority of aquatic plants, the flowers emerge above the level of water and are pollinated by insects or wind, e.g., water hyacinth and water lily.
- The female flower reaches the surface of water by the long stalk and pollen grains are released onto the surface of water, e.g., Vallisneria.
- In most of the water pollinated species, pollen grains are protected from wetting by mucilaginous covering.
Biotic agents of pollination are insects (entomophily), bats (chiropterophily), snails (malacophily), birds (ornithophily) etc.
- Insect pollinated flowers are generally large, colorful, fragrant and rich in nectar.
- The pollen grains and stigmatic surface are sticky.
- The animal pollinated flowers offer certain rewards to the pollinators. Some of them are.
- These offer nectar and (edible) pollen grains.
- Some species provide safe place for laying eggs, e.g., Amorphophallus and Yucca. A relationship exists between a species of moth and the plant Yucca, where both the species cannot complete their life cycles without each other. The moth deposits its eggs in the locule of the ovary and the flower, in turn, gets pollinated by the moth. The larvae of the moth come out of the egg as the seeds start developing.
- Many insects may consume pollen or the nectar without bringing about pollination. Such floral visitors are referred to as pollen/nectar robbers.
FAQ
What is pollination?
Pollination is the process by which pollen is transferred from the male reproductive organs (anthers) to the female reproductive organs (stigma) of a flower, resulting in fertilization and the production of seeds.
How is pollination classified?
Pollination can be classified into two main types: self-pollination and cross-pollination. Self-pollination occurs when pollen from the same flower or plant fertilizes the ovules within the same flower. Cross-pollination, on the other hand, involves the transfer of pollen from the anther of one flower to the stigma of another flower on a different plant of the same species.
