Pollination is the transfer of pollen grains from the anther to the stigma of a flower of the same species. It must happen before fertilisation, because the male gametes inside the pollen cannot move on their own to reach the egg.Self-pollination is the transfer of pollen to the stigma of the same flower, or of another flower on the same plant. It is reliable but gives less genetic variation. Cross-pollination is the transfer of pollen to a flower on a different plant of the same species. It increases genetic variation, because the gametes come from two different parents.Many plants have features that promote cross-pollination, such as anthers and stigmas maturing at different times, separate male and female flowers or plants, or genetic self-incompatibility.Pollen is carried mainly by insects or by wind, and flowers are adapted to their method:
• Insect-pollinated flowers: large, brightly coloured petals, scent and nectar to attract insects; anthers and stigma inside the flower, where insects brush against them; smaller amounts of large, sticky or spiky pollen that clings to insects; a sticky stigma.
• Wind-pollinated flowers (such as grasses): small, dull or no petals, no scent or nectar; anthers hanging outside the flower on long filaments; very large amounts of small, light, smooth pollen; large, feathery stigmas hanging outside the flower to catch pollen from the air.
Wind pollination is wasteful because most pollen never reaches a stigma, which is why so much is produced.After a compatible pollen grain lands on the stigma, it grows a pollen tube down the style towards the ovule, leading to fertilisation.
• Insect-pollinated flowers: large, brightly coloured petals, scent and nectar to attract insects; anthers and stigma inside the flower, where insects brush against them; smaller amounts of large, sticky or spiky pollen that clings to insects; a sticky stigma.
• Wind-pollinated flowers (such as grasses): small, dull or no petals, no scent or nectar; anthers hanging outside the flower on long filaments; very large amounts of small, light, smooth pollen; large, feathery stigmas hanging outside the flower to catch pollen from the air.
Wind pollination is wasteful because most pollen never reaches a stigma, which is why so much is produced.After a compatible pollen grain lands on the stigma, it grows a pollen tube down the style towards the ovule, leading to fertilisation.