Cross-pollination
Cross-pollination, the transfer of pollen from one plant to another plant. The opposite of cross-pollination is self-pollination, in which a plant pollinates itself. Seed obtained from cross-pollinated plants is usually stronger and more vigorous than seed from self-pollinated plants. (There are a few exceptions. Tobacco is an example.) The principle of cross-pollination is used in the production of hybrid corn and many types of hybrid flowers.
In dioecious plants, the male and female flowers are on different plants and the only way that seed can be produced is through cross-pollination. Examples are the willow and the ailanthus (tree of heaven). Monoecious plants have the male and female flowers on the same plant or even have the male and female structures in the same flower In these cases, self-pollination may be prevented by any of several ways. In the case of the geranium, for example, the pollen-producing structures mature at a different time from the ovules. In self-sterile plants, such as the Chinese primrose, these structures are located in such a way that the plant cannot pollinate itself. An artificial method of preventing self-pollination is to tie bags over the male flowers, or to remove them. This is done in the case of corn.
In nature, cross-pollination is usually accomplished by the wind or by insects. Many trees and grasses have light pollen that is distributed by the wind. This method is common in species that grow closely together in large groups. Bees are the main pollinating insects. As they gather nectar from flowers, pollen brushes off on their bodies and is transferred from one blossom to another. Other pollinating insects include butterflies, moths, and flies. Certain tropical flowers are pollinated by hummingbirds, bats, snails, rodents, marsupials, or lemurs. A number of water plants are pollinated by water. Many hybrid plants must be pollinated by man.