Flowering plants first appeared about 130 million years ago with a new feature, the gynoecium, a tough double layer that encloses the ovule, the place where seeds are formed, protecting them. Other layers, petals and usually green sepals, developed from leaves to add both protection and a mechanism for attracting pollinators. These traits allowed the flowering plants, the angiosperms, to become the most successful group of plants in the world.
Petals And Petaloid Structures
Petals are often brightly colored to attract pollinators and may be marked with lines known as nectar guides to encourage the pollinators to move toward the center of the plant, where the reproductive structures lie. Some plants, such as the flowering dogwoods, have the petals missing or much reduced, and the sepals enlarge to take over their function. In flowers of the daisy family, many tiny blossoms are crowded together with the ones at the outer edge modified to bear one large petal each, the whole group appearing to be one flower but with the capacity to develop many seeds.
The gynoecium is made up of one or more carpels, a structure made up of an ovary that encloses the egg, a stalk-like style with a sticky stigma at its end. When a pollen grain lands on the stigma, it sprouts a tube that goes down the style and into the ovary to deliver sperm that fertilize the egg that is protected within the tough covering of the gynoecium. As the egg becomes a seed, the ovary develops into a fruit, a hardened structure that further protects the egg from damage. Fruit may also develop flavors and colors that attract animals to help in seed dispersal.
Stamens usually grow in a ring around the carpels, topped with anthers where pollen is produced. When the pollen is ripe, it is released through pores or slits to the surface. Each grain of pollen has a hard coating that is sculptured with patterns of furrows, pores, spines or warts that may help the pollen adhere to a pollinator.
The necessity for getting the pollen to the stigma has encouraged many adaptations designed to attract certain insects or animals to carry it. Petals may be fused into tubes for hummingbirds, or modified into a platform for bees with a hood above it, as in snapdragons. Certain colors or smells may be used. Magnolias, for instance, date from the earliest stages of angiosperm evolution, when bees and butterflies were absent and pollination was effected by beetles. Their heavy scent, thick petals and white color all favor these insects.
Flowers plants have developed many ways to spread their seeds. Some have fruits that develop into capsules that break open with a snap that sends the seeds flying. Other use the aid of wind, birds and animals. The tiny parachutes of dandelion seeds are an example of wind dispersal. Some plants have flavorful fruit that attract animals who eat them and spread the seeds that pass through their digestive tract.