In botany, dioecious is a term used to describe the sexuality of certain flowering plants. Dioecious plants are species in which each individual plant has only male or only female flowers, but normally not both. Unlike the majority of flowering plants, these species cannot self-fertilize.
Dioecious plants make up about six or seven percent of plant species. The other main types of plant sexuality are described as monoecious and hermaphroditic. In monoecious plants, each individual plant has separate male and female flowers. In hermaphroditism, the most common type of plant sexuality, occurring in over 80% of species, male and female parts are found in each individual flower.
Most flowers are hermaphroditic. They can therefore produce both female reproductive cells, called megaspores or seeds, and male reproductive cells, called microspores or pollen. However, this is not the case in dioecious flowers, which produce only one type of spore. Male, or androecious, flowers produce pollen, and female, or gynoecious, flowers produce seeds.
Sometimes a monoecious plant will occur in a population of a plant species that is normally considered dioecious. When this occurs, a plant will have both male and female flowers even though that is not normally the case for the species in question. Such species are sometimes referred to as subdioecious species.
A well-known example of a dioecious plant species is the European holly, Ilex aquifolium. Both male and female holly plants have white flowers, but they are very different in terms of their sexual structure. The male plant has flowers with stamens, the male structures, which produce pollen, and small, sterile female stigma. The female plant has normal stigma and small, sterile stamens which cannot produce any pollen.
Dioecy tends to be more common in trees and shrubs than in other types of flowering plants. These include trees of the willow family, birch trees, and some types of fig tree. Some urban tree planters favor female trees because they do not produce pollen, and therefore reduce the incidence of severe tree pollen allergies.
The biological advantage of dioecy is often believed to be the fact that cross-pollination is assured in dioecious species. This means that genetic diversity of offspring will be increased. However, dioecious plants run the risk of being unable to reproduce if another plant of the opposite sex is not located in the immediate vicinity.