Pollination

Pollination is the transfer of pollen from an anther to a stigma. It must happen before fertilisation, because the male and female gametes are in different places. There are three kinds, by where the pollen comes from:

• Autogamy: to the stigma of the same flower. Some plants have flowers that never open (cleistogamous flowers), and these can only self-pollinate; they set seed even without pollinators. Most flowers stay open (chasmogamous).
• Geitonogamy: to another flower on the same plant. It needs a pollinator, but the pollen is genetically the same as the plant's own, so it is like self-pollination.
• Xenogamy: to a flower on a different plant. This is the only kind that brings in genetically different pollen.

Pollen travels in different ways:

• By wind: wind-pollinated flowers make lots of light, non-sticky pollen; their stamens hang out in the open, and their stigmas are large and feathery to catch pollen from the air. Flowers are often small and plain, sometimes many together, as in maize tassels and grasses.
• By water: this is rare, found in a few aquatic plants such as Vallisneria, Hydrilla and the sea grass Zostera. In Vallisneria, the female flowers reach the surface on long stalks, and male flowers or pollen float to them. Note that many water plants, such as the water hyacinth and water lily, are pollinated by insects or wind, not water.
• By animals: mostly insects (bees, butterflies, moths), but also birds and bats. Such flowers are usually large, colourful, scented and full of nectar, the reward. Some plants are so tied to one insect that neither can survive without the other: the Yucca plant and its moth.

Self-pollination over many generations causes inbreeding depression: weaker offspring. So flowering plants have outbreeding devices that discourage it:

• The pollen is released and the stigma becomes ready at different times.
• The anther and stigma are placed apart, so pollen does not easily fall on the flower's own stigma.
• Self-incompatibility: a genetic system that stops the flower's own pollen from growing a tube.
• Unisexual flowers. If male and female flowers are on the same plant (monoecious, like castor and maize), autogamy is prevented but geitonogamy is not. If they are on separate plants (dioecious, like papaya), both are prevented.