This video segment adapted from NOVA explains how flowers play a central role in the reproductive cycle of plants. Their striking array of colors, patterns, fragrances, and nectar all require lots of energy to produce. But it is these features that attract insects and other animals, which, in turn, carry genetic material from flower to flower. The video describes various parts of the reproductive system, including the stamens and pistil, which generate seeds and ensure the survival of an enormous variety of plant species.
Flower Reproductive Parts (Image)
Over millions of years, plants have evolved a variety of adaptations in both form and function to succeed in their struggle for survival. For the types of plant known as angiosperms, flowers are an integral part of the plant's reproductive scheme. Flowers form on a part of a branch called the receptacle. They consist of several distinct yet tightly arranged components that emerge from a single node.
[If you would like to see a diagram of the reproductive components of a flower, refer to Flower Reproductive Parts Image.]
The sepals and petals are both sterile parts of a flower. The leaf-like sepals surround the flower, protecting it before it blooms. Petals are colorful and often patterned features that are more likely to catch one's eye. Flowers have both male and female reproductive parts. When these occur in the same structural plan, the flower is described as being "perfect." Flowers that contain only male or female parts are called "imperfect." The male reproductive parts, called the stamens, produce pollen. Each stamen consists of an anther, which contains the pollen, and a filament, the stalk-like structure that supports the anther. The female reproductive parts, or carpels, produce the ovules, or eggs. In most flowers, the carpels are fused together to form a pistil, which consists of a stigma, an ovary, and a style—a long tube that connects the stigma to the ovary.
Sexual reproduction in plants occurs when the pollen from a stamen is transferred to a pistil, where it may fertilize the eggs stored inside the ovary. Plants that grow in isolated colonies, including tomatoes and lettuces, can pollinate themselves. Pollen simply fertilizes the eggs on the same flower. However, plants that inhabit a broader area exhibit strategies that prevent self-pollination. For example, some species produce pollen either before or after the stigma is receptive to it. These plants conduct an alternate reproductive strategy, called cross-pollination, through which pollen is carried to a flower of the same species on a different individual plant, often with an animal assisting in the process.
Insects and other animals do not knowingly participate in plant reproduction. Instead, they visit flowers for an entirely different reason, typically food. For example, as a honeybee collects the sweet nectar it seeks from a bright and fragrant flower, it also unwittingly collects tiny pollen grains, which attach to its legs and body. When it lands on its next bloom, it leaves behind some of the pollen.
When pollen latches on to the stigma of a receiving flower, a series of chemical reactions ensues. The pollen produces a pollen tube, which wends its way through the style to reach the flower's ovary. The pollen's sperm cells travel through the pollen tube to fertilize the eggs. Fertilized eggs develop into seeds, and the ovary develops into a fruit—the technical term for a structure that encloses and protects a seed or seeds.
Not all flowers are colorful or fragrant. In fact, some flowers lack petals entirely. Instead of animals, these flowers typically rely on the wind or water for pollination. While these other methods leave much to chance, the plants benefit from not having to devote so much energy to producing such "showy" enticements. Instead they generate a larger volume of pollen to improve their reproductive success rates.
Extract DNA from bananas in this NOVA classroom activity.
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