In the majority of angiosperms both male and female parts develop to form perfect (hermaphrodite) flowers but, in others, only one organ develops to maturity to give staminate (male) or carpellate (female) flowers. It has been estimated that unisexual plants represent over 10% of all plant species, distributed among 75% of plant families. Male and female flowers occur on the same plant in monoecious species and on different plants in dioecious species. Unisexuality is usually caused by the reduction or abortion of sex organ primordia (which can occur at any stage of development) and unisexual flowers often pass through a bisexual stage in which all floral organs are initiated (e.g. maize, cucumber). Thus sex determination is traditionally considered to be the selective abortion of the gynoecium or androecium of initially hermaphroditic floral primordia. Only in a few species (e.g., Mercurialis, Cannabis and Spinacia) do the floral primordia lack any vestiges of inappropriate sex organs; however, even in these plants, sexuality can be reversed under certain conditions indicating that the floral primordia are sexually bipotent (Dellaporta and Calderon-Urrea, 1993).
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