<Li> Unisexual: having either functionally male or functionally female flowers . This condition is also called diclinous, incomplete or imperfect . </Li> <P> Outcrossing, cross-fertilization or allogamy, in which offspring are formed by the fusion of the gametes of two different plants, is the most common mode of reproduction among higher plants . About 55% of higher plant species reproduce in this way . An additional 7% are partially cross-fertilizing and partially self - fertilizing (autogamy). About 15% produce gametes but are principally self - fertilizing with significant out - crossing lacking . Only about 8% of higher plant species reproduce exclusively by non-sexual means . These include plants that reproduce vegetatively by runners or bulbils, or which produce seeds without embryo fertilization (apomixis). The selective advantage of outcrossing appears to be the masking of deleterious recessive mutations . </P> <P> The primary mechanism used by flowering plants to ensure outcrossing involves a genetic mechanism known as self - incompatibility . Various aspects of floral morphology promote allogamy . In plants with bisexual flowers, the anthers and carpels may mature at different times, plants being protandrous (with the anthers maturing first) or protogynous (with the carpels mature first). Monoecious species, with unisexual flowers on the same plant, may produce male and female flowers at different times . </P> <P> Dioecy, the condition of having unisexual flowers on different plants, necessarily results in outcrossing, and might thus be thought to have evolved for this purpose . However, "dioecy has proven difficult to explain simply as an outbreeding mechanism in plants that lack self - incompatibility". Resource - allocation constraints may be important in the evolution of dioecy, for example, with wind - pollination, separate male flowers arranged in a catkin that vibrates in the wind may provide better pollen dispersal . In climbing plants, rapid upward growth may be essential, and resource allocation to fruit production may be incompatible with rapid growth, thus giving an advantage to delayed production of female flowers . Dioecy has evolved separately in many different lineages, and monoecy in the plant lineage correlates with the evolution of dioecy, suggesting that dioecy can evolve more readily from plants that already produce separate male and female flowers . </P>

Different maturation time of androecium and gynoecium in the same flower is