The transference of the pollen grains from the anthers to the stigma is known as pollination.
Importance of Pollination- In flowering plants, it is essential that the pollen must be transferred to the stigma if sexual reproduction must take place. In other words, pollination is the essential prerequisite to sexual reproduction and the subsequent setting of the seeds and fruits.
Types of Pollination- There are two main types of pollination . (1) Self-Pollination and (2) Cross-Pollination.
Self Pollination- When pollen grains are transferred from the anthers to the stigma of the same flower, it is called self pollination or autogamy.(pollination involving only one flower is called self-pollination or autogamy).
Autogamy is possible only in bisexual (hermaphrodite)flowers. It is less common in nature, and is observed in Mirabilis, Solanum etc.
Geitonogamy: it is a type of self-pollination between two flowers present on the same parent plant. When pollen grains are transferred from the anthers of a flower to the stigma of another flower present on the same parent plant, it is described as geitonogamy.e,g, Maize.
Some prefer to consider geitonogamy as a type of cross-pollination because it takes place between two flowers.
Advantages of self –pollination.
(1) Pollen grains are easily available and there is no distance barrier beteween the anthers and stigmas, hence pollination rarely fails.
(2) Flowers do not have to depend on external agencies like wind, water, animals. etc.
(3) Minimum wastage of pollen.
(4) Pure, homozygous characters can be obtained and retained through repeated self-pollination.
Disadvantages of self-pollination.
(1) Progeny becomes weaker due to continued self-pollination generation after generation.
(2) There is no chance of either introduction of new, desirable character or the elimination of some undesirable character through self-pollination.
Cross-Pollination: When pollen grain are transferred from the anthers of one flower to the stigma of another flower, it is called cross-pollination or allogamy.(when pollination takes place between two flowers it is called cross-pollination).
Cross-pollination is very common in nature and takes place in great majority of plants, e.g. sunflower, papaya, castor , maize, etc. it can take place in bisexual as well as unisexual flowers. In fact, in unisexual flowers, only cross-pollination is possible.
Advantages of cross-pollination
(1) Results in new combinations of characters.
(2) The progeny shows enhanced vigour.
(3) Offsprings are more viable and resistant.
(4) There is always a chance for introduction of new desirable characters and /or elimnation of the undesired characters through cross-pollination.
Disadvantages of cross-pollination.
(1) It can fail to take place because of the distance barrier and also as the flowers have to depend totally on the external agencies for pollination.
(2) Greater wastage of pollen.
(3) It can introduce some undesirable characters in the offsprings.
Agencies of pollination.
Abiotic agencies/agents. The various physical factors which help in transferring the pollen grains during pollination are described as abiotic agents of pollination. The most common abiotic agents are wind, and water. The pollination is described accordingly as anemophily(wind pollination) and hydrophily (water pollination)
Biotic Agencies/agents: various kinds of animals which bring about the transfer of pollen during pollination are the biotic agents of pollination. The pollination through the agency of animals is described as zoophily.
Depending on the type of animals involved in pollination, zoophily is further classified in to various sub-types. Some of the more common sub-types are as follows.
(a) Entomophily: Pollination through insects.
(b) Ornithophily: Pollination through birds.
(c) Chiropterophily: Pollination through bats.