Flowers, the reproductive structures of angiosperms, are adaptations designed to attract insects and other pollen-bearing animals to the plant to aid in pollen dispersal. For this reason, flowers are most often colorful and showy; not surprisingly, plants that rely on wind (instead of insects) for pollen dispersal have flowers that are more likely to be small and drab. The flower is composed of four whorls of modified leaves, the calyx, corolla, androecium, and gynoecium. Each of these whorls contains one of the flower organs, the sepals, petals, stamens, or pistils, respectively. Sepals and petals are not directly involved in reproduction, while the stamens and pistils are the male and female reproductive organs. In addition, each flower possesses an ovary (at the base of the pistil) formed from modified leaves called carpels (note that the pistil itself is sometimes referred to as the carpel). This ovary, an exclusive feature of angiosperms, encloses the ovules and develops into a fruit after fertilization. For a discussion of angiosperm reproduction, see also Plant Classification, Angiosperms.
The calyx is made up of sepals, green leaf-like structures that enclose the unopened bud. They serve a protective role for the flower before it opens, and afterward extend from the base of the flower.
The corolla is made up of the petals of the flower, which are usually brightly colored in order to attract insects. Together, the corolla and calyx make up the perianth, the nonreproductive portion of the flower.
The androecium is composed of the male reproductive organs, the stamens. Each stamen consists of a long, slender filament topped by a pollen-producing anther. The anther contains numerous sporangia, which give rise to microspores. These microspores develop, in turn, into pollen grains, which carry sperm cells to the female reproductive organs.
The gynoecium, composed of a pistil or pistils (or carpels, as they are also sometimes called), lies in the very middle of the flower. The top of the pistil, where pollen grains land, is called the stigma and the shaft leading down into the ovary is called the style. The ovary, containing ovules and egg cells, makes up the very bottom of the pistil.