Flowers offer a huge diversity of structure and color, but all their parts, inside and out, are tailored towards pollination--the transfer of pollen from one plant to another--which is necessary for a flowering plant to reproduce. As such, although flowers may look different, many of them have essentially the same basic structure.
Sepals and Petals
Sepals are green, life-like structures found on the outside of a flower. Their purpose is to protect the petals and the internal parts of a flower, especially when it is in bud, and may drop off when a flower blooms. Once a flower blooms, the petals also protect the internal reproductive parts, but their main purpose is to attract insects and, in some cases, birds or animals, that transfer pollen from one plant to another. It is for this reason that petals are brightly colored and scented. Even insects with compound eyes have difficulty seeing low-frequency red light, so red flowers are relatively uncommon in the natural world.
Stamens are the male, pollen-producing parts of a flower. They consist of an anther, which is the part responsible for pollen production, atop a stalk known as a filament, and are typically arranged in a circle around the female parts of the plant, inside the petals. Pollen consists of microscopic grains, often yellow, which are resistant to chemical breakdown and decomposition for hundreds, thousands or even millions of years. Some flowers produce a sweet, sugar-rich liquid called nectar, which is secreted from small repositories, known as nectaries, at the base of the stamens, and which serves as a reward for a pollinator.
Carpels are the female parts of a flower, consisting of a stigma connected to an ovary—which contains the female egg cells, or ovules—by a stalk, known as a style. The stigma is covered with a sticky substance to which pollen grains adhere if carried to the flower by a pollinator. The ovary protects the ovules and, following fertilisation of the flower by a visiting pollinator, becomes part of the fruit. One, two or more carpels are found in the centre of a flower. Carpels can be different shapes and can be variously described as round, if their two halves are almost circular, cordate, if the two halves are heart shaped, or mucronate, if they come to a sharp point.