Pollination

Before a sperm can fertilize the egg contained in an embryo sac, pollination must occur. In flowering plants, pollination occurs when pollen grains are transferred from an anther to a stigma. Pollination that involves just one flower, flowers on the same plant, or flowers from two genetically identical plants is called self-pollination. In contrast, pollination that involves two genetically different plants is called cross-pollination.

The pollination of flowering plants occurs in several ways. Flower structure promotes self-pollination in plants, such as peas and beans, that have flowers with petals that completely enclose both the male and female flower parts. Some aquatic plants, such as sea grasses, have pollen that is dispersed by water. Many plants, such as oak trees and grasses, release their pollen into the air. The flowers of such wind-pollinated angiosperms are small and lack showy petals and sepals. Successful wind pollination depends on four conditions: the release of large amounts of pollen, the ample circulation of air to carry pollen, the relative proximity of other plants for the pollen to be transferred to, and dry weather to ensure that pollen is not washed from the air by rain.

Most plants that have colorful or fragrant flowers are pollinated by animals. Bright petals and distinctive odors attract animals that feed on pollen and nectar, a nourishing solution of sugars. Animal pollinators include bats, bees, beetles, moths, butterflies, mosquitoes, monkeys, and hummingbirds. When these animals gather nectar, pollen sticks to their bodies. As they collect more nectar, the animals deposit some of the pollen on other flowers. For example, as the hummingbird in Figure 30-8 collects nectar from a flower, pollen from the anthers is deposited on the hummingbird's beak and head. When the hummingbird moves on to another flower, the pollen may be transferred to the stigma of the second flower.

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