A flower is the reproductive part of a plant that produces seeds. Plants that produce flowers and fruit are called angiosperms. There are more than 300,000 species of angiosperms, and their flowers and fruits vary significantly. Flowers and fruits are among the most useful features for identifying plant species.
Many modern cultures consider flowers attractive, and scholars have been fascinated with flowers for thousands of years. Dioscorides, a first-century Greek physician, wrote the most influential early book on plants, De materia medica. This was the first text about the medicinal uses of plants, and it contained many diagrams of plants and their flowers. The book helped other physicians identify the species of plant to prescribe to their patients for a particular ailment. De materia medica remained an important reference on botany (the study of plants) for more than 1,500 years.
In the mid-1700s, Swedish botanist Carolus Linnaeus revolutionized the field of botany. He classified plant species according to the morphology (form and structure) of their flowers and fruits. Modern botanists continue to rely upon his classification system.
Up until the late 1700s, people believed that flowers with beautiful colors and sweet smells were created by God to please humans. However, German botanist Christian Konrad Sprengel disputed this view. He held that the characteristics of flowers (shape, color, smell) are related to their method of reproduction. Sprengel published his theory of flowers in 1793 in The Secret of Nature Revealed. Although not widely accepted in his own time, Sprengel's views were soon considered scientifically correct.
Angiosperm: Plant that produces flowers and seeds.
Anther: Top part of the stamen that produces pollen.
Filament: Stalk of the stamen that bears the anther.
Corolla: Layers of petals in a flower.
Morphology: Branch of biology dealing with the form and structure of living organisms.
Ovary: Base part of the pistil that bears ovules and develops into a fruit.
Ovule: Structure within the ovary that develops into a seed after fertilization.
Petal: Whorl of a flower just inside the sepals that is often colored.
Pistil: Female reproductive organ of flowers that is composed of the stigma, style, and ovary.
Pollen: Powdery grains that contain the male reproductive cells of angiosperms.
Pollination: Transfer of pollen from the male reproductive organs to the female reproductive organs of a plant.
Sepal: External whorl of a flower that is typically leaflike and green.
Stamen: Male reproductive organ of flowers that is composed of the anther and filament.
Stigma: Top part of the pistil upon which pollen lands and germinates.
Style: Stalk of the pistil that connects the stigma to the ovary.
There are considerable differences among the many species of flowers. Flowers can develop on different places on a plant. Terminal flowers, like a tulip, are single flowers that bloom at the apex or end of an upright stalk. Other flowers arise in an inflorescence, a branched cluster of individual flowers. Begonias are an example of this type. Those flowers that grow at the base of a leaf where it attaches to the stem of the plant are called axillary flowers. Snapdragons are an example of axillary flowers.
There are four concentric whorls (rings) of organs in a complete flower. From the center to the outside, they are the pistil, stamens, petals, and sepals. Fundamentally, these four parts are modified leaves.
The pistil, a long stalk arising in the center, is the female reproductive organ of a flower. It is composed of the stigma, style, and ovary. The stigma is the sticky knob at the outer end of the stalk. The style is the portion of the stalk connecting the stigma to the ovary. The ovary is the round base that contains one or more undeveloped seeds called ovules. In each ovule is an egg waiting to be fertilized by a sperm.
Stamens, the male reproductive organs, also arise from the center of the flower and encircle the pistil. The stamens are composed of a stalk, called a filament, topped by an anther. The anther produces many microscopic pollen grains. The male sex cell, a sperm, develops within each pollen grain.
Petals, the often-brightly colored portion surrounding the pistil and stamens, are a flower's showpiece. They attract the attention of passing insects, birds, and people. The layers of petals in a flower comprise the corolla.
Sepals lie below the petals and are usually green and leaflike in appearance. Sepals form a temporary, protective cover over an unopened flower. When the petals of a flower are ready to unfurl, the sepals fold back.
In some species, one or more of the four whorls of floral organs is missing, and the flower is referred to as an incomplete flower. A bisexual flower is one with both stamens and a pistil, whereas a unisexual flower is one that has either stamens or a pistil, but not both. All complete flowers are bisexual since they have all four floral whorls. All unisexual flowers are incomplete since they lack either stamens or a pistil.
In angiosperms, pollination is the transfer of pollen from an anther to a stigma. Pollen grains land on the sticky stigma, where they begin to germinate or grow. A pollen tube then forms down the style, sperm is delivered to the ovules, and fertilization takes place.
If the transfer of pollen occurs between an anther and the stigma of the same plant, it is known as self-pollination. Complete flowers are able to self-pollinate. When the transfer of pollen occurs between an anther and the stigma of different plants, it is known as cross-pollination. Of the two methods, cross-pollination produces stronger and healthier off-spring since it mixes up the genetic make-up of plants. Cross-pollination can be brought about by wind, rain, mammals, birds, and insects.
Pollination by wind. Many angiosperms are pollinated by wind. Wind-pollinated flowers, such as those of corn and all grasses, tend to have a simple structure lacking petals. The anthers dangle on long filaments, allowing the light pollen grains to be easily caught by the wind. The stigma are freely exposed to catch the airborne pollen. Large amounts of pollen are usually wasted because they do not reach female reproductive organs. For this reason, most wind-pollinated plants are found in temperate regions, where members of the same species often grow close together.
Pollination by animals. In general, pollination by insects and other animals is more efficient than pollination by wind. Many times flowers
offer "rewards" to attract these animals—sugary nectar, oil, solid food, a place to sleep, or even the pollen itself. Generally, plants use color and fragrances to lure their pollen-transporting agents.
The flowers of many species of plants are marked with special pigments that absorb ultraviolet light (light whose wavelengths are shorter than visible light). These pigments are invisible to humans and most animals. But the eyes of bees are sensitive enough to detect the patterns created by the pigments and so the bees are drawn to them.
Having been attracted to a flower, an insect or other small animal probes inside for its reward. In doing so, it brushes against the anthers and picks up dust pollen on its body. When the animal moves on to the next flower, it brushes past the stigma, depositing pollen. Many flowers are designed precisely to match the body forms of the animals participating in this pollen transfer. In this way, contact with both the anthers and the stigma is ensured. A few orchids use a combination of smell, color, and shape to mimic the female of certain species of bees and wasps. The male bees and wasps then try to mate with the flower. In the process, they either pick up or transfer pollen to that flower.