Complete vs. Incomplete Flower Types
There are different flower types according to the presence of the different parts of a flower. A flower is referred to as complete if all four floral organs (sepal, petal, stamen and pistil) are present in the same flower structure. A commonly illustrated complete flower is that of the gumamela or China rose (Hibiscus rosa-sinensis). An incomplete flower lacks any one or more of these parts. Grass flowers are mainly wind-pollinated and are incomplete, lacking both sepals and petals. Relying on wind to accomplish pollination, there is no need for these organs to attract pollinators.
Perfect vs. Imperfect Flower Types
Flowers that contain both sexual flower parts (stamen and pistil) are called perfect or bisexual while those that contain either stamen or pistil only are called imperfect or unisexual flowers, regardless of whether they lack sepals or petals. The separation of the male and female sexual organs increases the possibility of outcrossing or cross pollination.
To interrelate, all complete flowers are perfect because they necessarily have both the stamen and pistil. All imperfect flowers are incomplete because either the stamen or pistil is lacking. But not all incomplete flowers are imperfect because both the stamen and pistil may be present and what makes the flower incomplete is the absence of either sepal or pistil or both.
Imperfect flowers that bear stamens only are called staminate flowers (male flower) and those that bear pistils only are called pistillate flowers (female flower). In corn, the miniature ear that is borne on the leaf axil is a pistillate flower, and the “silks” that protrude at the tip are elongated styles. The corn tassel, which arises at the apical part of the plant shoot, is an inflorescence bearing staminate flowers.
The presence of perfect and/or imperfect flowers in plants determines these plants’ sex. A plant bearing only perfect (or bisexual) flowers is called a hermaphrodite or hermaphroditic plant. Plants that bear only imperfect (or unisexual) flowers are either monoecious or dioecious. In general, plants that bear perfect and imperfect flowers on the same individual are called polygamous. However, different forms of polygamy can occur. Likewise, individuals within certain species can exhibit different plant sex.
Monoecious plants are those in which both the staminate and pistillate flowers are borne on the same plant (e.g. corn, strawberry, squash and other cucurbits). Species in which the two imperfect flowers are borne separately in different plants are called dioecious. Dioecy occurs in date palm, spinach, asparagus, pili nut, and rambutan.
Papaya or pawpaw (Carica papaya) is a polygamous plant. However, different individuals exhibit several variations in bearing perfect and imperfect flowers. Through controlled pollination, seeds can be obtained possessing desirable proportions of male, female, and hermaphroditic plant characteristics.
Flower Types Affecting Pollination
As to synchrony in the maturation of the male (stamen) and female (pistil) flower parts, flowers are called dichogamous when the two organs mature at different time. Dichogamy, as in the avocado, is a natural mechanism by which plants avoid self pollination within the same flower (autogamy) thus reducing inbreeding and, conversely, promoting outcrossing.
Dichogamous flowers are either protandrous or protogynous. Protandrous flowers (n. protandry), as in carrots, are those in which the stamens mature ahead of the pistils so that by the time that the stigma becomes receptive, the pollen grains within the same floral structure are already shed. Protogyny is the reverse of protandry. The stigma of protogynous flowers, as in the water lilies (Nymphaea spp.), becomes receptive while the pollen grains are still immature.
Flower types also differ depending on whether they open or remain close during the pollination period (pollination is the transfer of pollen grains from an anther to a stigma). Chasmogamous flowers (n. chasmogamy) are open during the period of pollination and are generally cross-pollinated. In the cleistogamous flowers (n. cleistogamy) of tomato, lettuce and some rice species, the flowers do not open during the period of pollination and thus these plants are largely self pollinated.
(Ben G. Bareja May 2011)