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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. 80% of all flowering plants are hermaphroditic, meaning they contain both sexes in the same flower, while 5 percent of plant species are monoecious.
Self-incompatibility (SI) is a general name for several genetic mechanisms that prevent self-fertilization in sexually reproducing organisms, and thus encourage outcrossing and allogamy. It is contrasted with separation of sexes among individuals ( dioecy ), and their various modes of spatial ( herkogamy ) and temporal ( dichogamy ) separation.
Self-fertilization, where pollen from a plant will fertilise reproductive cells or ovules of the same plant; Cross-pollination, where pollen from one plant can only fertilize a different plant; Asexual propagation (e.g. runners from strawberry plants) where the new plant is genetically identical to its parent
Plants adapted for cross-pollination have several mechanisms to prevent self-pollination; the reproductive organs may be arranged in such a way that self-fertilisation is unlikely, or the stamens and carpels may mature at different times.
The main characteristics which facilitate cross-pollination are: Herkogamy: Flowers possess some mechanical barrier on their stigmatic surface to avoid self-pollination, e.g. presence of gynostegium and pollinia in Calotropis. Dichogamy: Pollen and stigma of the flower mature at different times to avoid self-pollination.
This arrangement causes floral visitors to first contact the anthers before the stigma. For this reason, reverse herkogamy is believed to facilitate greater pollen export than approach herkogamy. This type of sex-organ arrangement is typically associated with Lepidopteran (moth or butterfly) pollination.
There are a large number of other mechanisms that enhance cross-pollination and prevent self-pollination. The forces that led to the evolution of such systems as bearing male and female flowers on separate kinds of plants is still unclear.
Pollination bags are designed to fit well over the inflorescence or individual flowers of a plant type. The size, shape and strength of bag should ensure that there is no contact with flowers to avoid development of diseases and physical hindrances in seed development.