Flowers are mainly responsible for bringing about fertilisation and seed formation. To facilitate fertilisation, it is necessary that the male gametes inside the pollen grains and the female gamete inside the ovule are brought together. However, since the gametes are immotile, pollen grains have to be transferred to the stigma. This process of transfer of pollen to the stigma of a flower, brought about by agents such as insects, wind and water, is called pollination. Pollination can be of two types: self-pollination and cross-pollination. When pollination takes place within the same flower or between flowers of the same plant it is known as self-pollination. In plants, self-pollination can occur either through autogamy or geitonogamy.
In autogamy, pollen is transferred from the anther to the stigma of the same flower and is seen in cleistogamous flowers or flowers which do not open at all. Autogamy, however, is rare in chasmogamous flowers, which open and expose their anthers and stigma, as it requires synchrony between pollen release and stigma receptivity. In geitonogamy, the second type of self-pollination, pollen is transferred from the anther of one flower to the stigma of another flower of the same plant.
Apart from self-pollination; cross-pollination is also commonly seen in plants. This type of pollination involves the transfer of pollen grains from the flower of one plant to the stigma of the flower of another plant of the same type. This type of pollination is also called xenogamy or allogamy.
Honeybees as well as other insects also pollinate a large number of aquatic flowering plants such as the fresh-water lily. However, in certain fresh-water plants such as Vallisneria and Hydrilla, and marine sea-grasses such as Zostera, pollination occurs by water. This is also known as hydrophily. The pollen, released in water, is carried by water currents towards the submerged stigma and thus pollination is completed. Surprisingly, in aquatic plants, wind pollination, also called anemophily, is more widespread than water pollination. Wind-pollination also occurs in terrestrial plants such as grass, bamboo, coconut and maize. These plants possess a compact inflorescence with well-exposed stamens that allow easy dispersal of pollen and a large and feathery pistil, which makes it easy to trap pollen. The pollen, which is light and non-sticky, is produced in large quantities. A single flower of Cannabis, for instance, produces, 5,00,000 pollen grains to compensate the loss of pollen associated with wind pollination. Thus, wind, water as well as biotic pollinating agents bring about pollination, an important process that ultimately leads to fertilisation and the production of seeds in plants.