There are four basic parts that flowers may have: sepals, which protect the bud of an unopened flower; petals, which may help to attract insects and animals; the androecium, the male reproductive part of a flower; and the gynoecium, the female part of the bloom. If a flower has all four, it is considered both complete and perfect, perfect because it has all the parts for reproduction. Flowers that don't possess both male and female parts, then, are incomplete and imperfect. All imperfect flowers are unisexual.
Imperfect flowers are by nature unisexual as opposed to perfect flowers, which are bisexual (hermaphroditic and androgynous). Lacking both male and female parts in one flower, a plant might still produce male and female parts on separate flowers. The male flowers are staminate, having stamens to produce pollen, and the female flowers are pistillate with a pistil to receive the pollen. When a plant does this, it is monoecious. "Monoecious" comes from Greek roots and means one house. A begonia is monoecious. So is corn and the walnut tree.
Some plants produce neither androgynous flowers nor have both male and female flowers on one plant. Instead, a plant is either male or female with either all pistillate (female) flowers on a plant or all staminate (male) flowers. To get fruit or seed, both male and female plants must live near one another, since the male plant is needed to produce sperm-containing pollen to pollinate female flowers so that they can produce fruit and seed. "Dioecious" means double house. Holly, willow and poplar trees are dioecious.
Changeable Flowers and Plants
Not all imperfect flowers and plants remain male or female. Some start off as one sex, as if they are dioecious plants, then begin producing bisexual flowers. The cucumber produces flowers this way. First it grows male flowers, then becomes bisexual. After this phase, the cucumber produces female flowers. Finally, some cucumbers produce flowers that create fruit even without pollination. Such flowers are parthenocarpic. "Parthenocarpic" means virgin fruit. Cornell University has developed a parthenocarpic summer squash line. The importance of parthenocarpic flowers lies in the possibility of getting fruit even when pollinators like bees aren't around.