What is Pollination?
The transfer of pollen grains from the anther of a flower to the stigma of the same or a different flower of the same species or related species is called pollination. The male gametes or the pollen grains are produced in the anther and the female gamate or the eggs are borne within the embryo sac of the ovary. These pollen grains are transferred from the anther to the stigma by the process of pollination. When this occurs within the same flower, it is self- pollination, but if it occurs between two different flowers, it is cross- pollination. After pollination, the pollen grains are received by the stigma, the functional nucleus of the pollen grain produce the two male gametes, which descend down the style via the pollen tube and enters the ovule. In the meanwhile, the embryo sac develops within the nucleus of the ovule, and the maturation of the egg, fusion nucleus, formation of the synergids and the antipodals takes place.
Types of Pollination
There are two types of pollination depending upon the nature, they are self-pollination and cross-pollination.
Self-pollination or Autogamy :
The transfer of pollen grain from the anther of a flower to the stigma of the same flower or to the stigma of another flower of the same plant is called self-pollination. The first process mentioned above involving the transfer of pollen grain from anther to stigma of the same flower is called autogamy, while the second process involving the transfer of pollen grain from the anther of one bisexual flower to the stigma of another bisexual flower belonging to the same plant is called geitonogamy.
Characteristics of self-pollination are as follows:
i) They take place only in bisexual flowers.
ii) They occur between flowers belonging to same geitotical constitution.
iii) Usually the anther should be almost at similar height with the stigma
iv) The anther and stigma should mature at the same time
v) The flower may close on maturity.
Method of self-pollination
There are three different methods of self-pollination. Those are:
i) Natural: The stigma is at a lower level from that of the anther and on maturity, the anther dehisces and the pollen grains fall on the stigma instinctively, which is not helped by any agent.
ii) By wind: The pollen grains are carried by wind from one flower to another, both belonging to the same plant. They are granular and light weight in nature. In these flowers, the stigma is sticky in nature to trap the pollen grains.
iii) By insects: The flowers are colored, scented and provided with nectarines. The insects enter the flower in search of nectar and in the process; pollen grains are elated to the sticky stigma of another flower of the same plant.
Adaptations for self-pollination
i) Homogamy : The androecium and gynoecium mature at the same time, so self- pollination is probable. Example: Mirabilis, Gardenia.
Cleistogamy: The flowers remain closed even in matured condition, to ensure self- pollination and the occurrence is called cleistogamy. It is opposite to the normal phenomenon of chasmogamy, which means the flowers open on maturity. The flowers exhibiting cleistogamy are called cleistogamous flowers and there may be various degree of cleistogamy. When the flowers remain close even after pollination is called Normal cleistogamy. Example: Commelinabenghalensis. The flowers remain close till pollination is over and the case is called Partialcleistogamy; example: Some species of Commelina. And those plant contains both chasmogamous and cleistogamous flowers are called Chasmocleistogamy, paddy is the example.