A plant is dioecious if male and female forms exist in separate individuals.
Indeed, the word translates roughly to "two houses", which makes a useful
mnemonic. About 6% of the world's flowering plants (
i.e. angiosperms) are dioecious. Dioecy is believed to convey an
evolutionary advantage to a
species because it helps prevent
inbreeding; a dioecious plant cannot
fertilize itself. Thus, a dioecious population can avoid
inbreeding depression, the exposure of
harmful mutations.
Despite this hypothesized advantage, dioecy is less frequent than one would be led to believe.
See also: gynodioecious, monoecious.
Reference: Heilbuth JC. 2000. American Naturalist 156: 221-241.