Pollination is the transfer of pollen from a flower's anther to a flower's stigma, stimulating germination as the pollen produces a tube which grows down the style to the egg in the ovule. While many flowers rely on bees, insects and wind for reproduction assistance, there are a few species that will self-pollinate, performing the act alone. This is possible if the flower has both female and male parts, and is not adapted to prevent self-pollination.
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Both hermaphrodite and monoecious species have the potential for self-pollination leading to self-fertilization unless there is a mechanism to avoid it. Eighty percent of all flowering plants are hermaphrodite, meaning they contain both sexes, while 5 percent of plant species are monoecious, or unisexual.
Arum lilies, tridax (part of the daisy family) and some orchids are self-pollinating flowers. Dates, box-elder and buffalo berry are self-pollinating flowering trees. There are quite a few vegetables that self-pollinate, such as tomatoes, okra, peas, snap peas, soybeans and lima beans. In some cases, like the soybean, the flower will be open to cross-pollination during the day, but will self-pollinate if needed before closing.
There are several advantages for self-pollinating flowers. If a given genotype is well-suited for an environment, self-pollination helps to keep this trait stable in the species. Not being dependent on pollinating agents allows self-pollination to occur when bees and wind are nowhere to be found. Self-pollination can be an advantage when the number of flowers are small or widely spaced.
The disadvantages of self-pollination come from a lack of variation that allows no adaptation to the changing environment or potential pathogen attack. Self-pollination can lead to inbreeding depression, or the reduced health of the species, due to the breeding of related specimens. This is why many flowers that could potentially self-pollinate have a built-in mechanism to avoid it, or make it second choice at best.
Self-pollinators are more likely to be found in areas that lack pollinating agents or mates. It is likely then that geographic location has influenced the development of many self-pollinating species.