Flowers
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.
Figure 5.1: The Parts of the Flower
Calyx
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.
Corolla
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.
Androecium
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.
Gynoecium
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.
