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Self-pollination is a form of pollination that can occur when a flower has both stamen and a carpel (pistil) in which the cultivar or species is self fertile and the stamens and the sticky stigma of the carpel contact each other in order to accomplish pollination. The term is inaccurately used in many cases where an outside pollinator is actually required; such plants are merely self-fertile, or self pollenizing.
Few plants actually self-pollinate. The mechanism is seen most often in some legumes such as peanuts. In another legume, soybeans, the flowers open and remain receptive to insect cross pollination during the day. If this is not accomplished, the flowers self-pollinate as they are closing. Among other plants that can self-pollinate are many kinds of orchids, peas, sunflowers and tridax. Most of the self-pollinating plants have small, relatively inconspicuous flowers that shed pollen directly onto the stigma, sometimes even before the bud opens. Self-pollinated plants expend less energy in the production of pollinator attractants and can grow in areas where the kinds of insects or other animals that might visit them are absent or very scarce—as in the Arctic or at high elevations.
Self-pollination limits the variety of progeny and may depress plant vigor. However, self-pollination can be advantageous, allowing plants to spread beyond the range of suitable pollinators or produce offspring in areas where pollinator populations have been greatly reduced or are naturally variable.
Pollination can also be accomplished by cross-pollination. Cross-pollination is the transfer of pollen, either by wind or insect, from the anther to the stigma between different flowers.
Types of self pollination 
There are two types of self-pollination:
In Type I self pollination, pollen is transferred from the anther to the stigma of the same flower. Such flowers are hermaphrodites, which have both sexes.
In Type II pollen is transferred from the anther of one flower to the stigma of another flower from the same plant.
Types of Flowers That Self Pollinate 
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.
Advantages of self-pollination 
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.
Disadvantages of self-pollination 
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.
See also 
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