35.1: Pollination and Flower Structure

Flowers are the reproductive, seed-producing structures of angiosperms. Typically, flowers consist of sepals, petals, stamens, and carpels. Sepals and petals are the vegetative flower organs. Stamens and carpels are the reproductive organs.

Flowers must be pollinated to produce seeds. In angiosperms, pollination is the transfer of pollen from the anther of the stamen (the male structure) to the stigma of the carpel (the female structure). Flowers may be self-pollinated or cross-pollinated. However, most plants have developed mechanisms that prevent self-pollination.

Cross-pollination is the transfer of pollen among flowers of separate plants. Cross-pollination is often carried out by animals—most commonly insects—called pollinators. Pollinators carry pollen on their bodies from flower to flower.

Plants evolved to attract different pollinators, which accounts for much of the abundant variety of features found in flowers. For example, bees are most attracted to bright blue and yellow flowers with sweet fragrances, while flies are drawn to fleshy flowers that smell like rotting meat.

Many birds are also pollinators. While birds often have a weak sense of smell, many are attracted to bright red and yellow flowers with sweet nectar. Certain bat species also pollinate. The lesser long-nosed bat, for example, pollinates agave and cactus species as it eats their nectar and pollen.

Some plants are pollinated by wind or water, rather than animals; such flowers are often dull and lack nectar. Grasses, for example, often have green, odorless flowers that release many tiny, wind-dispersed pollen grains.

While many flowers have stamens and carpels, some flowers are unisexual—lacking either functional stamens or carpels. Sometimes, both types of unisexual flower are on the same plant. In other cases, flowers with stamens and flowers with carpels are found on different plants. Additionally, some plants can alternate between producing male flowers, female flowers, and both flower types.

Flowers are reproductive multitaskers. In most flowering species, flowers produce and release sperm, create and store eggs, and capture sperm for fertilization. Following fertilization, flowers develop into seeds, and often fruit. 

Flowers have up to four types of floral organs: sepals, petals, stamens, and carpels. 

Sepals encase and protect flower buds, and are typically green and leaflike. Petals protect other flower structures and are often brightly colored, which helps attract pollinators. 

Stamens and carpels are fertile flower organs. They produce spores, which eventually develop into sperm and eggs.

A stamen consists of a filament and anther. Cells in the anther create spores that develop into pollen grains, which give rise to sperm. 

A carpel consists of an ovary, style, and stigma. The ovary contains one or more ovules. In an ovule, an embryo sac—which includes an egg—is formed. The style extends from the ovary to the stigma. The sticky stigma traps pollen, allowing the mobile sperm to reach the stationary egg. Fertilized ovules develop into seeds. 

For fertilization and seed formation to occur, flowering plants must be pollinated. Pollination is the transfer of pollen—the sperm source—to the flower part containing ovules—the egg source. In flowering plants, pollen is transferred from anther to stigma. 

Most plants are pollinated by animals, which often eat and carry their nectar and pollen. The most common pollinators are insects—such as bees, butterflies, moths, and flies. Many bird and bat species also pollinate.

Plants can also be pollinated by wind and, less frequently, water. Grasses, for example, often release many tiny pollen grains that are dispersed by wind. 

After pollination, the pollen tube is formed, sperm are released into the ovule, and fertilization occurs. Fertilization triggers the flower’s development into fruit as its ovules become seeds. An unpollinated flower, however, typically fails to become fruit and dies.