Monoecious is a term which is used to describe an organism which has both male and female sex organs present, as separate structures. The term is most commonly used in reference to plants, although some animal species are monoecious as well, in which case it is more commonly referred to as hermaphroditism. There are a number of advantages to being monoecious, making it a fairly common sexual configuration, although it might seem a bit unusual to humans.
In a plant which has monoecious characteristics, the plant produces both male and female flowers. Alders and corn are two examples of plants which are considered monoecious. The male flowers on the plant are capable of fertilizing the female flowers, and they can also cross pollinate with other plants in the vicinity. The male and female flowers usually look different, as one is designed to create pollen for distribution, while the other develops ova which can be fertilized, resulting in the development of seeds.
Self fertilization is a useful ability, as it can ensure that a plant species survives and spreads when no other representatives of the species are present. For that reason, many monoecious plants can fertilize themselves, although they may have adaptations such as staggered blooming times which are designed to make it difficult, because self fertilization can weaken the genetics of the species. The ability to cross pollinate when it is an option can be extremely useful for the long term genetic robustness of the species.
In a consecutive monoecious plant, flowers of one sex appear first, followed by flowers of another sex. The plant can be either protogynous or protoandrous, depending on which sex appears first. Plants can also be simultaneously monoecious, meaning that male and female flowers appear at the same time. Plants typically evolve one approach or the other in response to the environments they develop in, as there are advantages and disadvantages to both, and sometimes humans deliberately breed plants to develop a specific desired sexual characteristic, such as the ability to produce male and female flowers simultaneously.
By contrast, dioecious plants are either male or female. For these plants, the presence of a plant of the opposite sex is necessary for fertilization to occur, although one male can fertilize many females. Plants can also have so-called “perfect” or “bisexual” flowers, in which both of the sexual organs are present in every single flower. These types of plants are known as hermaphrodites; hermaphroditism in plants should not be confused with the version seen in animals.