Spatial separation of the sexes onto different flowers or different plants may also promote outcrossing. An individual flower may have only stamens (male) or only pistils (female), or both (hermaphroditic) in the same flower, and plants and populations may have various combinations of flowers. Monoecious ("one house") populations have both sexes of unisexual flowers on each plant (e.g., corn has tassels of male flowers and an ear of female flowers on the same plant). Gynodioecious populations, consisting of female and hermaphroditic plants, are also possible. Dioecious ("two-house") populations consist of male-only and female-only plants (e.g., marijuana, or Cannabis). Dioecy has arisen independently in the flowering plants many times. About 6 percent of flowering plant species are dioecious, and the incidence of dioecy is particularly high in the Hawaiian Islands (14.7 percent) and in New Zealand (12 to 13 percent). Flowering plants also have more complicated patterns of sex expression, and some plants are even capable of switching sex through time.
Two major theories have been proposed to explain the evolution of dioecy. One theory suggests that dioecy has evolved as a mechanism to avoid inbreeding depression and enforce outcrossing between unisexual plants. The other theory suggests that patterns of resource allocation between male and female function (sex allocation) are critical. According to this theory, dioecy should evolve from hermaphroditism when greater investment of resources in flowers of one sex yields a disproportionate gain in reproductive heterostylous having styles (female flower parts) of different lengths, to aid cross-pollination
pistil the female reproduction organ
success. In such cases, it would be advantageous to separate the sexes to allow more efficient resource allocation.
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