Pollination

The essentials

Pollination is the transfer of pollen grains from the anther to the stigma of a pistil. Based on the source of pollen, there are three kinds. Autogamy is transfer within the same flower; it needs synchrony between pollen release and stigma receptivity. Cleistogamous flowers, which never open (as in Viola, Oxalis and Commelina), are fully autogamous. Geitonogamy is transfer to another flower of the same plant; it is functionally cross-pollination but genetically similar to self-pollination. Xenogamy is transfer to a flower on a genetically different plant and brings new genetic combinations.

Agents may be abiotic (wind, water) or biotic (insects, birds, bats and others). Wind-pollinated flowers have light, non-sticky pollen, well-exposed stamens and large feathery stigmas, often with a single ovule per ovary (grasses, maize). Water pollination is rare and seen in Vallisneria, Hydrilla and the marine seagrass Zostera. Most water plants such as water hyacinth and water lily are actually pollinated by insects or wind. Insect-pollinated flowers are large, colourful, scented and rich in nectar.

Key points

• Outbreeding devices: unisexuality, pollen release and stigma receptivity not synchronised, anther and stigma at different positions, and self-incompatibility.
• Dioecy (e.g. papaya) prevents both autogamy and geitonogamy; monoecy (e.g. castor, maize) prevents only autogamy.
• Pollen–pistil interaction: the pistil recognises compatible pollen and allows a pollen tube to grow; incompatible pollen is rejected.
• Artificial hybridisation uses emasculation (removing anthers) and bagging to prevent unwanted pollen.
• Plant–animal mutualism examples: Amorphophallus and its insects, Yucca and its moth.

Exam tips

Remember that geitonogamy needs a pollinating agent even though the pollen is genetically identical. Learn the examples for each agent.