The transfer of pollen grains from anther to the stigma a pistil is termed as pollination.
Pollination
The transfer of pollen grains from anther to the stigma a pistil is termed as pollination.
Depending upon sources of pollen grains, pollination can be divided into three types.
Autogamy: Transfer of pollen grains from the anthers to the stigma of the same flower. e.g., pea, rice, wheat, etc.
Geitonogamy: Transfer of pollen grains from the anthers to the stigma of another flower of the same plant. It is functionally cross-pollination and involving a pollinating agent. e.g., Cucurbita.
Xenogamy/allogamy: Transfer of pollen grains from the anthers to the stigma of a different plant of the same species. e.g., papaya, maize.
Contrivances or Devices for Self-pollination (Autogamy)
Cross-pollination can be prevented by exhibiting
Cleistogamous flowers: These are bisexual closed flowers which never open and the anthers dehisce inside these closed flowers, e.g., Commelina.
Homogamy: It is the condition of the maturity of anther and stigma at the same time, e.g., Catharanthus (Vinca).
Close association between anther and stigma, e.g., Mirabilis.
Contrivances or Devices for Cross-pollination (Xenogamy/Allogamy)
Self-pollination can be prevented by exhibiting
Unisexuality: Male and female flowers are present on different plants.
Dichogamy: The condition in which the stamens and stigma of a bisexual flower mature at different times.
Protandry: This is the condition where anthers mature earlier than the stigma and release pollens.
Protogyny: This is the condition where the stigma matures earlier than the anther.
Self-sterility or self-incompatibility: It is a genetic mechanism that prevents self-pollination.
Chasmogamous flowers: These are open flowers with exposed stamens and stigma which facilitate cross-pollination.