Flower

There are considerable differences among the flowers of the 300,000 species of Angiosperms. Botanists rely upon a large vocabulary of specialized terms to describe the parts of these various flowers. The most important morphological features of flowers are considered below.

Flowers can arise from different places on a plant, depending on the species. Some flowers are terminal, meaning that a single flower blooms at the apex of a stem. Some flowers are axial, in that they are borne on the axes of branches along a stem. Some flowers arise in an inflorescence, a branched cluster of individual flowers.

There are four whorls of organs in a complete flower. From the outside to the inside, one encounters sepals, petals, stamens, and carpels. The sepals are leaf-like organs, which are often green, but can sometimes be brown or brightly colored, depending on the species. The petals are also leaf-like and are brightly colored in most animal-pollinated species but dull in color or even absent in wind-pollinated plants.

The stamens and carpels, the reproductive organs, are the most important parts of a flower. The stamens are the male, pollen-producing organs. A stamen typically consists of an anther attached to a filament (stalk). The anther produces many microscopic pollen grains. The male sex cell, a sperm, develops within each pollen grain.

The carpels are the female ovule-producing organs. A carpel typically consists of an ovary, style, and stigma. The stigma is the tip of the carpel upon which pollen grains land and germinate. The style is a stalk that connects the stigma and ovary. After the pollen grain has germinated, its pollen tube grows down the style into the ovary. The ovary typically contains one or more ovules, structures which develop into seeds upon fertilization by the sperm. As the ovules develop into seeds, the ovary develops into a fruit, whose characteristics depend on the species.

In some species, one or more of the four whorls of floral organs is missing, and the flower is referred to as an incomplete flower. A bisexual flower is one with both stamens and carpels, whereas a unisexual flower is one which has either stamens or carpels, but not both. All complete flowers are bisexual since they have all four floral whorls. All unisexual flowers are incomplete since they lack either stamens or carpels. Bisexual flowers, with stamens and carpels, can be complete or incomplete, since they may lack sepals and/or petals.