Flower

I

Introduction

Flower, reproductive organ of most seed-bearing plants. Flowers carry out the multiple roles of sexual reproduction, seed development, and fruit production. Many plants produce highly visible flowers that have a distinctive size, color, or fragrance. Almost everyone is familiar with beautiful flowers such as the blossoms of roses, orchids, and tulips. But many plants—including oaks, beeches, maples, and grasses—have small, green or gray flowers that typically go unnoticed.

Whether eye-catching or inconspicuous, all flowers produce the male or female sex cells required for sexual reproduction. Flowers are also the site of fertilization, which is the union of a male and female sex cell to produce a fertilized egg. The fertilized egg then develops into an embryonic (immature) plant, which forms part of the developing seed. Neighboring structures of the flower enclose the seed and mature into a fruit.

Botanists estimate that there are more than 240,000 species of flowering plants. However, flowering plants are not the only seed-producing plants. Pines, firs, and cycads are among the few hundred plants that bear their seeds on the surface of cones, rather than within a fruit. Botanists call the cone-bearing plants gymnosperms, which means naked seeds; they refer to flowering plants as angiosperms, which means enclosed seeds.

Flowering plants are more widespread than any other group of plants. They bloom on every continent, from the bogs and marshes of the Arctic tundra to the barren soils of Antarctica. Deserts, grasslands, rainforests, and other biomes display distinctive flower species. Even streams, rivers, lakes, and swamps are home to many flowering plants.

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In their diverse environments, flowers have evolved to become irreplaceable participants in the complex, interdependent communities of organisms that make up ecosystems. The seeds or fruits that flowers produce are food sources for many animals, large and small. In addition, many insects, bats, hummingbirds, and small mammals feed on nectar, a sweet liquid produced by many flowers, or on flower products known as pollen grains. The animals that eat flowers, seeds, and fruits are prey for other animals—lizards, frogs, salamanders, and fish, for example—which in turn are devoured by yet other animals, such as owls and snakes. Thus, flowers provide a bountiful feast that sustains an intricate web of predators and prey (see Food Web).

Flowers play diverse roles in the lives of humans. Wildflowers of every hue brighten the landscape, and the attractive shapes and colors of cultivated flowers beautify homes, parks, and roadsides. The fleshy fruits that flowers produce, such as apples, grapes, strawberries, and oranges, are eaten worldwide, as are such hard-shelled fruits as pecans and other nuts. Flowers also produce wheat, rice, oats, and corn—the grains that are dietary mainstays throughout the world. People even eat unopened flowers, such as those of broccoli and cauliflower, which are popular vegetables. Natural dyes come from flowers, and fragrant flowers, such as jasmine and damask rose, are harvested for their oils and made into perfumes. Certain flowers, such as red clover blossoms, are collected for their medicinal properties, and edible flowers, such as nasturtiums, add color and flavor to a variety of dishes. Flowers also are used to symbolize emotions, as is evidenced by their use from ancient times in significant rituals, such as weddings and funerals.

II

Parts of a Flower

Flowers typically are composed of four parts, or whorls, arranged in concentric rings attached to the tip of the stem. From innermost to outermost, these whorls are the (1) pistil, (2) stamens, (3) petals, and (4) sepals.

A

Pistil

The innermost whorl, located in the center of the flower, is the female reproductive structure, or pistil. Often vase-shaped, the pistil consists of three parts: the stigma, the style, and the ovary. The stigma, a slightly flared and sticky structure at the top of the pistil, functions by trapping pollen grains, the structures that give rise to the sperm cells necessary for fertilization. The style is a narrow stalk that supports the stigma. The style rises from the ovary, a slightly swollen structure seated at the base of the flower. Depending on the species, the ovary contains one or more ovules, each of which holds one egg cell. After fertilization, the ovules develop into seeds, while the ovary enlarges into the fruit. If a flower has only one ovule, the fruit will contain one seed, as in a peach. The fruit of a flower with many ovules, such as a tomato, will have many seeds. An ovary that contains one or more ovules also is called a carpel, and a pistil may be composed of one to several carpels.

B

Stamens

The next whorl consists of the male reproductive structures, several to many stamens arranged around the pistil. A stamen consists of a slender stalk called the filament, which supports the anther, a tiny compartment where pollen forms. When a flower is still an immature, unopened bud, the filaments are short and serve to transport nutrients to the developing pollen. As the flower opens, the filaments lengthen and hold the anthers higher in the flower, where the pollen grains are more likely to be picked up by visiting animals, wind, or in the case of some aquatic plants, by water. The animals, wind, or water might then carry the pollen to the stigma of an appropriate flower. The placement of pollen on the stigma is called pollination. Pollination initiates the process of fertilization.