Flowers and trees have one important thing in common with the birds and the bees -- sexuality. In many plants, it takes pollen containing sperm from the male form of a plant to fertilize an egg in the plant's female structure. This pollination produces a seed that grows into a plant [source: Davis].
Cross-pollination refers to the transfer of pollen from the female flower on one plant to the male flower on another plant. Monoecious plants bear both female and male flowers; the male flower of corn is in the tassel, the female where the stem meets the leaves [source: Cox]. In some plants, the same flower produces both the female and male parts. Since the same plant produces the male and female structures, both self-pollination and cross-pollination are possible.
As is the case with humans, inbreeding can produce weak offspring. So it's an evolutionary advantage for plants to avoid self-pollination. There are natural and artificial ways to prevent self-pollination in monoecious plants. In plants such as geraniums, the pollen-producing structures and the ovules mature at different times preventing self-pollination. In plants such as the Chinese primrose, the structures involved in pollination are located in such a way that self-pollination cannot take place. A grower can prevent self-pollination artificially by covering the male flowers with a bag or by removing the male flowers.
Dioecious plants, which have male and female flowers on different plants, can only produce seeds through cross-pollination between the two separate parent plants. Examples of dioecious plants include the ailanthus tree, sometimes referred to as the "tree of heaven," and the willow tree. There are approximately 1,400 plants grown as crops around the world produce food and industrial products; nearly 80 percent require a bee, a bird or another creature to pollinate them [source: U.S. Forest Service].
Grape flowers can be female, male or hermaphroditic, meaning that the flower has both male and female structures (also known as a "perfect" flower) [source: Reisch and Stewart]. The hermaphroditic petunia has genes that prevent a plant from fertilizing itself [source: Science Daily].
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