We admire flower petals. We photograph them and make paintings of them, dry them for later use, make perfume from them--even eat them. For the plant, however, flower petals exist only to aid in fertilization. Their life cycle promotes pollination and fertilization of the flower, and it ends when fertilization has been achieved.
The immature flower appears as a bud on the plant. It is encased in sepals, small leaf-like structures that protect the immature flower. The bud will remain closed, slowly growing larger, until it is ready to open.
Flower petals slowly unfurl, exposing the reproductive organs of the flower. In most flowers, the cells on opposite sides of the petals expand at different rates, causing the bud to open. The petals, collectively called the corolla, are arranged in a whorl around the reproductive organs. Their primary function is to lure pollinators. They are also responsible for protecting the reproductive organs and conserving the plant's resources.
Some flowers are attractive to a variety of pollinators, and their petals are typically exposed for a long time. Other flowers are pollinated by few, or only one, species of insect or bird. Typically, these flowers bloom and expose their petals only during the days or weeks when the pollinator is likely to be present.
Many flowers remain fully open once they are mature. Others, however, close and reopen daily. This helps the plant conserve moisture, and it limits the amount of scent that is emitted. Typically, the petals are exposed when the preferred pollinator is likely to be present. Morning glories, for example, open early in the morning and close as the day progresses; four o'clocks are open in the afternoon; and flowering nicotiana opens in the evening.
The petals' color, shape and scent are designed to lure pollinators to the plant, and sometimes to guide pollinators to the sexual organs of the plant. Plants that are usually pollinated by bees have an area of low ultraviolet reflectance on their petals that guides the bees efficiently to the center of the flower. This ultraviolet pattern, though invisible to humans, is helpful to bees.
When the flower has been pollinated and fertilization has occurred, the petals are no longer needed. They slowly wither and die. Preventing fertilization delays withering, giving the plant a longer bloom time. Cutting the plant and displaying it indoors automatically prevents fertilization. Florists sometimes remove anthers and/or stigmas from flowers.
Petals are so essential to plant reproduction that other parts of the plant sometimes mimic them. Sepals may take on the color of petals. Bracts, which are below the sepals and which normally support flower petals, are large and brightly colored in some plants. Poinsettias are a good example of this. In other plants, two of the stamens have evolved to be infertile and fused together, forming a labellum that functions like a petal. Ginger is a good example of this kind of false petal.