Transfer of pollen grains in stamens , where they form, to the pistil .
Pollination is required for fertilization and the production of seeds . On the surface of the pistil the pollen grains germinate (see germination ) and form pollen tubes that grow downward toward the ovules. During fertilization, a sperm cell in a pollen tube fuses with the egg cell of an ovule, giving rise to the plant embryo. The ovule then grows into a seed. Since the pollen-bearing parts of the stamens are rarely in direct contact with the pistil, plants commonly rely on external agents for pollen transport. Insects (especially {{link=bee">bees ) and wind are the most important pollinators; other agents include birds and a few mammals (notably certain bats). Water transport of pollen is rare. An egg may be fertilized by self-pollination (when the sperm comes from pollen produced by the same flower or by another flower on the same plant) or by cross-pollination (when the sperm comes from the pollen of a different plant).