Pollination occurs in several different ways, depending on the structure of the flower:
Plants that self-pollinate are called selfers. For example, the reproductive structures of pea flowers are enclosed between two fused petals, called a keel, which ensures that self-fertilization takes place. Wind-pollinated plants, such as trees and grasses, generally produce large amounts of small, light pollen that is not sticky. The flowers are not usually colorful, do not produce nectar, and have no odor. The flowers form before the leaves do and the stamens and stigmas are exposed to the wind.
Insect pollinators pick up pollen grains as they feed on nectar from one plant and deposit it on subsequent plants that they visit. The flowers pollinated by insects tend to have sticky pollen that adheres to the hairs on the legs of the insects. There are approximately 20,000 species of bees attracted to sweet-scented flowers that produce nectar and 16 families of beetles that are lured to flowers with fruity, spicy, or pungent odors. Flies also pollinate flowers as they feed. Butterflies have a long, thin proboscis to suck nectar from tubular flowers or flowers with a long style, but the pollen tends not to stick to their bodies and so they are not efficient pollinators (Figure 3.2).
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