Pollination is the transfer of pollen grains from the anther to the stigma. As the pollen grains are immobile, they require an external agent to reach the stigma. The external agent may be water, wind, animal etc. Pollination is of two types -self-pollination and cross pollination.
Self-pollination involves the transfer of pollen grains from the anthers to the stigma of the same flower whereas cross pollination is the transfer of pollen grains from the anther of one flower to the stigma of another flower borne by the same plant or by another plant of same or allied species.
Self-pollination is the transfer of pollen grains from the anther of a flower to the stigma of either of the same or genetically similar flower. Self-pollination occurs in most of the species of cereals and grasses. Self-pollination is of two types – autogamy and geitonogamy.
It is the pollination of a flower by its own pollen. In this process, the pollen grains from the anther of a flower are deposited on the stigma of the same flower. Thus, autogamy involves only one flower. Examples are rice, wheat, pea etc. For autogamy, it is essential that both anther and stigma mature at the same time.
It involves two flowers which belong to the same parent plant. In this process the pollen grains from the anthers of one flower are deposited on the stigma of another flower borne on the same plant.
Adaptations or contrivances for self pollination:
The following are the major adaptations or contrivances which favour self-pollination.
In some plants, stamens and carpels of a flower mature at the same time. Therefore, pollen grains of a flower can easily pollinate the stigma of the same flower. The anther and stigma of an open flower are brought together by growth, bending and folding.
In Vinca, the growth of the style brings the stigma in contact with mature anthers present on the mouth of corolla tube. In Mirabilis (4’o clock plant), the bending of filaments brings the ripe anthers in contact with stigma.
Potato flowers show curling of style for carrying of stigma to ripe anthers.When cross pollination fails in sunflower, the bifid stigma curls back so as to the pick pollen sticking to the surface of style.
Homogamy is also seen in flowers like Gardenia and Ixora, the anthers lie at the mouth of the narrow corolla tube and when the stigma pushes through this tube ,it dashes with the mature anthers.The anthers shed the pollen grains over the stigma and bring about pollination.In case of Helianthus (sunflower) the stigma recoils and touches the anthers which on maturity bring pollination. Homogamy is also seen in potato, wheat, rice etc.
There are some bisexual flowers, which never open. They are usually very small and sometimes inconspicuous and remain closed. The pollen grains, therefore, may only pollinate on the stigma of the same flowers. Such a condition is called cleistogamy and the flowers are said to be cleistogamous flowers. Cleistogamous flowers usually have dull colours.
Generally, the flowers open after their maturation and expose their anthers and stigmas to the environment. Such flowers are called chasmogamous. The cleistogamous flowers do not open even at maturity and are forced to undergo self-pollination. Cleistogamy occurs lately in the flowering season in some plants, as in Camelina benghalensis, Oxalis, Viola, balsam.
Camelina bears normal flowers on its aerial parts in addition to underground cleistogamous flowers. Such plants which bear both cleistogamous and chasmogamous flowers are called chasmocleistogamous flowers. In rice, flowers remain till pollination is complete and then open afterwards.
The pollination taking place when the flower is in the bud condition is called bud pollination. The anthers and the stigmas of the flower ripen before the opening of the buds. Bud pollination is seen in pea, wheat, rice.
Advantages and disadvantages of self-pollination:
The advantages of self pollination are as follows:
- Self-pollination is not dependent on any external agency to deliver the pollen, hence, there are less chances of failure of pollination.
- It maintains the parental characters or purity of the race indefinitely.
- Self-pollination is used to maintain pure lines for hybridization experiments.
- Since there is almost no wastage of pollen grains, hence, plants need not to produce pollens in large quantities.
- Flowers do not have to be large, showy, attractive and scented to attract insect pollinators.
- Self-pollination ensures seed production.
- Self-pollination eliminates some bad recessive characters.
The disadvantages of self pollination are as follows:
- Continuous self-pollination results in loss of vigor and vitality of the race. Consequently, the progeny becomes weaker.
- New useful characters are seldom introduced.
- Successive generations show decreased resistance to diseases.
- Variability and hence, adaptability to changed environment are reduced.