Cross-pollination means pollen from one plant is transferred to another plant. External agents bring about cross-pollination -- they're really just mechanisms for pollen to be carried somewhere else. They key agents of cross-pollination are wind, insects, water and animals.
Some plants grow close together, in large groups, and they have light pollen grains. In such plants, wind is the primary way pollen is picked up and carried to other plants. Plants that use wind as their pollinating agent frequently have long stamens and pistils (the male and female parts, respectively, of flowering plants), and they can feel free to look as drab as they want: They don't need to attract animals or insects in order to spread their pollen, so they don't have to show off for Mother Nature.
Insects, meanwhile, also make good pollen haulers. Such cross-pollination agents include bees, butterflies, moths and flies. Among the insects, bees, which visit flowers to gather nectar, are the main pollinating agent. During this process, the pollen brushes off on the bee's body and onto the next flower the bee visits.
Water, for its part, is the agent of cross-pollination in aquatic plants.
Animals such as hummingbirds, bats, snails, rodents, marsupials and lemurs pollinate tropical flowers. Unlike plants that use wind as their primary pollinating agent, flowers that need to attract animals for pollination are frequently colored brightly to draw attention to themselves. They may even be scented, for much the same reason.
When insects and animals pollinate plants, it's not exactly a goal, some task they're driven to perform. Instead, they pollinate plants by accident. When they eat from a plant, they come in close contact with it and some of the plant's pollen will rub against the animal and stick. When the critters move on to the next plant, the pollen that hitched a ride on the animal or insect can make a new home on a new plant by disembarking at the appropriate arrival gate [source: Missouri Botanical Garden].
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