Kinds of pollination and its agents

Both gametes of flowering plants are non-motile, so pollination, the transfer of pollen from an anther to a stigma, is needed before fertilisation.

Autogamy is pollination within the same flower. Complete autogamy is rare in flowers that open, because it needs pollen release and stigma receptivity to coincide and anther and stigma to lie close.

Viola (common pansy), Oxalis and Commelina make two kinds of flowers: chasmogamous flowers that open and expose anthers and stigma, and cleistogamous flowers that never open. Cleistogamous flowers are always autogamous and set seed even when no pollinator comes.

Geitonogamy moves pollen to another flower of the same plant. It needs a pollinator, so it works like cross-pollination, but genetically it is the same as autogamy because the pollen comes from the same plant.

Xenogamy moves pollen to the stigma of a different plant; it is the only kind that brings genetically different pollen to the stigma.

Abiotic agents are wind and water; biotic agents are animals, and most flowering plants use animals. Wind is the commoner abiotic agent. Wind-pollinated plants have light, non-sticky pollen, well-exposed stamens, large often feathery stigmas, and often one ovule per ovary in crowded inflorescences, as in the maize cob, whose silky threads are the stigmas and styles. Wind pollination is common in grasses.

Water pollination is rare, limited to about 30 genera, mostly monocots, such as Vallisneria and Hydrilla in fresh water and sea-grasses such as Zostera. In Vallisneria the female flower reaches the surface on a long stalk and pollen floats to it; in sea-grasses the female flowers stay submerged and long, ribbon-like pollen drifts in the water. A mucilage coat keeps such pollen from getting wet. Water hyacinth and water lily, though aquatic, are pollinated by insects or wind.

Wind- and water-pollinated flowers are not very colourful and do not make nectar.

Animal pollinators include bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds, humming birds) and bats; among animals, insects, especially bees, dominate. Lemurs, tree-dwelling rodents and lizards (gecko, garden lizard) pollinate some species.

Insect-pollinated flowers are mostly big, bright, scented and full of nectar; where flowers are small, many are grouped into an inflorescence so they stand out. Flowers served by flies and beetles smell foul. Nectar and pollen are the usual rewards, and the pollen of animal-pollinated flowers is generally sticky.

Some flowers reward pollinators with a safe place to lay eggs: Amorphophallus, whose flower is about 6 feet tall, and Yucca, where a moth lays eggs in the ovary locule and pollinates the flower, so neither can complete its life cycle without the other. Visitors that take nectar or pollen without pollinating are pollen/nectar robbers.