Define the term pollination. Differentiate between self-pollination and cross-pollination. What is the significance of pollination?

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Hint: Pollination is a type of sexual reproduction where male gametophyte develops from pollen grains and female gametophyte develops from the megaspore mother cell. The growth of the male gamete completes over the female reproductive organ.

Complete answer:
Pollination is the process of transfer of pollen grains from the anther to micropylar canals from stigma. Depending upon the genetic similarity between anther and stigma pollination are of two types self-pollination and cross-pollination.
Self-pollination takes place in the same plant therefore it does not require external agency. Transfer of pollen takes place between the same flower or genetically similar flower, and due to this, it cannot produce variety. Flowers are intersexual or hermaphrodite, in which anther and stigma mature at the same time. Flowers can be opened and closed and it does not require a special structure. It introduces homozygosity by forming blue lines. It cannot introduce a new trait and eliminate harmful traits. It shows a decrease in yield with time due to a fall in adaptability and a high risk of disease.
Cross-pollination takes place between the flowers of the different plants therefore it requires an external agency to perform pollination. Transfer of pollen takes place from anther to the stigma of a genetically different flower. It produces new varieties and flowers are unisexual. Another stigma matures at different times and flowers are always open. This lot of wastage occurs if pollen does not meet the compatible stigma. It maintains heterozygosity and produces variation by introducing new traits. It shows high adaptability with the changing environment. Yields are due to maintaining immunity toward disease.
Significance of pollination is that it brings me and female gamete for fertilization through both biotic and abiotic agency, it prevents abscission of the premature ovary, it led to the production of hormones from pollen grain which stimulate the growth of ovary, it helps in gene recombination and helps in the development of hybrid seeds.

Note: Cross-pollination preferred over self-pollination because it does not allow harmful or lethal recessive alleles to become homozygous, whereas self-pollination is unable to eliminate harmful traits and because of this they become lethal.