Pollination and Flower Structure

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.