Flower - biology. 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. 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. C Petals Petals, the next whorl, surround the stamens and collectively are termed the corolla. Many petals have bright colors, which attract animals that carry out pollination, collectively termed pollinators. Three groups of pigments--alone or in combination--produce a veritable rainbow of petal colors: anthocyanins yield shades of violet, blue, and red; betalains create reds; and carotenoids produce yellows and orange. Petal color can be modified in several ways. Texture, for example, can play a role in the overall effect--a smooth petal is shiny, while a rough one appears velvety. If cells inside the petal are filled with starch, they create a white layer that makes pigments appear brighter. Petals with flat air spaces between cells shimmer iridescently. In some flowers, the pigments form distinct patterns, invisible to humans but visible to bees, who can see ultraviolet light. Like the landing strips of an airport, these patterns, called nectar guides, direct bees to the nectar within the flower. Nectar is made in specialized glands located at or near the petal's base. Some flowers secrete copious amounts of nectar and attract big pollinators with large appetites, such as bats. Other flowers, particularly those that depend on wind or water to transport their pollen, may secrete little or no nectar. The petals of many species also are the source of the fragrances that attract pollinators. In these species, the petals house tiny glands that produce essential, or volatile, oils that vaporize easily, often releasing a distinctive aroma. One flower can make dozens of different essential oils, which mingle to yield the flower's unique fragrance. D Sepals The sepals, the outermost whorl, together are called the calyx. In the flower bud, the sepals tightly enclose and protect the petals, stamens, and pistil from rain or insects. The sepals unfurl as the flower opens and often resemble small green leaves at the flower's base. In some flowers, the sepals are colorful and work with the petals to attract pollinators. E Variations in Structure Like virtually all forms in nature, flowers display many variations in their structure. Most flowers have all four whorls--pistil, stamens, petals, and sepals. Botanists call these complete flowers. But some flowers are incomplete, meaning they lack one or more whorls. Incomplete flowers are most common in plants whose pollen is dispersed by the wind or water. Since these flowers do not need to attract pollinators, most have no petals, and some even lack sepals. Certain wind-pollinated flowers do have small sepals and petals that create eddies in the wind, directing pollen to swirl around and settle on the flower. In still other flowers, the petals and sepals are fused into structures called a floral tube. Flowers that lack either stamens or a pistil are said to be imperfect. The petal-like rays on the edge of a sunflower, for example, are actually tiny, imperfect flowers that lack stamens. Imperfect flowers can still function in sexual reproduction. A flower that lacks a pistil but has stamens produces pollen, and a flower with a pistil but no stamens provides ovules and can develop into fruits and seeds. Flowers that have only stamens are termed staminate, and flowers that have only a pistil are called pistillate. Although a single flower can be either staminate or pistillate, a plant species must have both to reproduce sexually. In some species with imperfect flowers, the staminate and pistillate flowers occur on the same plant. Such plants, known as monoecious species, include corn. The tassel at the top of the corn plant consists of hundreds of tiny staminate flowers, and the ears, which are located laterally on the stem, contain clusters of pistillate flowers. The silks of corn are very long styles leading to the ovaries, which, when ripe, form the kernels of corn. In dioecious species--such as date, willow, and hemp--staminate and pistillate flowers are found on different plants. A date tree, for example, will develop male or female flowers but not both. In dioecious species, at least two plants, one bearing staminate flowers and one bearing pistillate flowers, are needed for pollination and fertilization. Other variations are found in the types of stems that support flowers. In some species, flowers are attached to only one main stem, called the peduncle. In others, flowers are attached to smaller stems, called pedicels, that branch from the peduncle. The peduncle and pedicels orient a flower so that its pollinator can reach it. In the morning glory, for example, pedicels hold the flowers in a horizontal position. This enables their hummingbird pollinators to feed since they do not crawl into the flower as other pollinators do, but hover near the flower and lick the nectar with their long tongues. Scientists assign specific terms to the different flower and stem arrangements to assist in the precise identification of a flower. A plant with just one flower at the tip of the peduncle--a tulip, for example--is termed solitary. In a spike, such as sage, flowers are attached to the sides of the peduncle. Sometimes flowers are grouped together in a cluster called an inflorescence. In an indeterminate inflorescence, the lower flowers bloom first, and blooming proceeds over a period of days from the bottom to the top of the peduncle or pedicels. As long as light, water, temperature, and nutrients are favorable, the tip of the peduncle or pedicel continues to add new buds. There are several types of indeterminate inflorescences. These include the raceme, formed by a series of pedicels that emerge from the peduncle, as in snapdragons and lupines; and the panicle, in which the series of pedicels branches and rebranches, as in lilac. In determinate inflorescences, called cymes, the peduncle is capped by a flower bud, which prevents the stem from elongating and adding more flowers. However, new flower buds appear on side pedicels that form below the central flower, and the flowers bloom from the top to the bottom of the pedicels. Flowers that bloom in cymes include chickweed and phlox. III SEXUAL REPRODUCTION Sexual reproduction mixes the hereditary material from two parents, creating a population of genetically diverse offspring. Such a population can better withstand environmental changes. Unlike animals, flowers cannot move from place to place, yet sexual reproduction requires the union of the egg from one parent with the sperm from another parent. Flowers overcome their lack of mobility through the all-important process of pollination. Pollination occurs in several ways. In most flowers pollinated by insects and other animals, the pollen escapes through pores in the anthers. As pollinators forage for food, the pollen sticks to their body and then rubs off on the flower's stigma, or on the stigma of the next flower they visit. In plants that rely on wind for pollination, the anthers burst open, releasing a cloud of yellow, powdery pollen that drifts to other flowers. In a few aquatic plants, pollen is released into the water, where it floats to other flowers. Pollen consists of thousands of microscopic pollen grains. A tough pollen wall surrounds each grain. In most flowers, the pollen grains released from the anthers contain two cells. If a pollen grain lands on the stigma of the same species, the pollen grain germinates--one cell within the grain emerges through the pollen wall and contacts the surface of the stigma, where it begins to elongate. The lengthening cell grows through the stigma and style, forming a pollen tube that transports the other cell within the pollen down the style to the ovary. As the tube grows, the cell within it divides to produce two sperm cells, the male sex cells. In some species, the sperm are produced before the pollen is released from the anther. Independently of the pollen germination and pollen tube growth, developmental changes occur within the ovary. The ovule produces several specialized structures--among them, the egg, or female sex cell. The pollen tube grows into the ovary, crosses the ovule wall, and releases the two sperm cells into the ovule. One sperm unites with the egg, triggering hormonal changes that transform the ovule into a seed. The outer wall of the ovule develops into the seed coat, while the fertilized egg grows into an embryonic plant. The growing embryonic plant relies on a starchy, nutrient-rich food in the seed called endosperm. Endosperm develops from the union of the second sperm with the two polar nuclei, also known as the central cell nuclei, structures also produced by the ovary. As the seed grows, hormones are released that stimulate the walls of the ovary to expand, and it develops into the fruit. The mature fruit often is hundreds or even thousands of times larger than the tiny ovary from which it grew, and the seeds also are quite large compared to the miniscule ovules from which they originated. The fruits, which are unique to flowering plants, play an extremely important role in dispersing seeds. Animals eat fruits, such as berries and grains. The seeds pass through the digestive tract of the animal unharmed and are deposited in a wide variety of locations, where they germinate to produce the next generation of flowering plants, thus continuing the species. Other fruits are dispersed far and wide by wind or water; the fruit of maple trees, for example, has a winglike structure that catches the wind. IV FLOWERING AND THE LIFE CYCLE The life cycle of a flowering plant begins when the seed germinates. It progresses through the growth of roots, stems, and leaves; formation of flower buds; pollination and fertilization; and seed and fruit development. The life cycle ends with senescence, or old age, and death. Depending on the species, the life cycle of a plant may last one, two, or many years. Plants called annuals carry out their life cycle within one year. Biennial plants live for two years: The first year they produce leaves, and in the second year they produce flowers and fruits and then die. Perennial plants live for more than one year. Some perennials bloom every year, while others, like agave, live for years without flowering and then in a few weeks produce thousands of flowers, fruits, and seeds before dying. Whatever the life cycle, most plants flower in response to certain cues. A number of factors influence the timing of flowering. The age of the plant is critical--most plants must be at least one or two weeks old before they bloom; presumably they need this time to accumulate the energy reserves required for flowering. The number of hours of darkness is another factor that influences flowering. Many species bloom only when the night is just the right length--a phenomenon called photoperiodism. Poinsettias, for example, flower in winter when the nights are long, while spinach blooms when the nights are short--late spring through late summer. Temperature, light intensity, and moisture also affect the time of flowering. In the desert, for example, heavy rains that follow a long dry period often trigger flowers to bloom. V EVOLUTION OF FLOWERS Flowering plants are thought to have evolved around 135 million years ago from cone-bearing gymnosperms. Scientists had long proposed that the first flower most likely resembled today's magnolias or water lilies, two types of flowers that lack some of the specialized structures found in most modern flowers. But in the late 1990s scientists compared the genetic material deoxyribonucleic acid (DNA) of different plants to determine their evolutionary relationships. From these studies, scientists identified a small, cream-colored flower from the genus Amborella as the only living relative to the first flowering plant. This rare plant is found only on the South Pacific island of New Caledonia. The evolution of flowers dramatically changed the face of earth. On a planet where algae, ferns, and cycads tinged the earth with a monochromatic green hue, flowers emerged to paint the earth with vivid shades of red, pink, orange, yellow, blue, violet, and white. Flowering plants spread rapidly, in part because their fruits so effectively disperse seeds. Today, flowering plants occupy virtually all areas of the planet, with about 240,000 species known. Many flowers and pollinators coevolved--that is, they influenced each other's traits during the process of evolution. For example, any population of flowers displays a range of color, fragrance, size, and shape--hereditary traits that can be passed from one generation to the next. Certain traits or combinations of traits appeal more to pollinators, so pollinators are more likely to visit these attractive plants. The appealing plants have a greater chance of being pollinated than others and, thus, are likely to produce more seeds. The seeds develop into plants that display the inherited appealing traits. Similarly, in a population of pollinators, there are variations in hereditary traits, such as wing size and shape, length and shape of tongue, ability to detect fragrance, and so on. For example, pollinators whose bodies are small enough to reach inside certain flowers gather pollen and nectar more efficiently than larger-sized members of their species. These efficient, well-fed pollinators have more energy for reproduction. Their offspring inherit the traits that enable them to forage successfully in flowers, and from generation to generation, these traits are preserved. The pollinator preference seen today for certain flower colors, fragrances, and shapes often represents hundreds of thousands of years of coevolution. Coevolution often results in exquisite adaptations between flower and pollinator. These adaptations can minimize competition for nectar and pollen among pollinators and also can minimize competition among flowers for pollinators. Comet orchids, for example, have narrow flowers almost a foot and a half long. These flowers are pollinated only by a species of hawk moth that has a narrow tongue just the length of the flowers. The flower shape prevents other pollinators from consuming the nectar, guarantees the moths a meal, and ensures the likelihood of pollination and fertilization. Most flowers and pollinators, however, are not as precisely matched to each other, but adaptation still plays a significant role in their interactions. For example, hummingbirds are particularly attracted to the color red. Hummingbird-pollinated flowers typically are red, and they often are narrow, an adaptation that suits the long tongues of hummingbirds. Bats are large pollinators that require relatively more energy than other pollinators. They visit big flowers like those of saguaro cactus, which supply plenty of nectar or pollen. Bats avoid little flowers that do not offer enough reward. Other examples of coevolution are seen in the bromeliads and orchids that grow in dark forests. These plants often have bright red, purple, or white sepals or petals, which make them visible to pollinators. Night-flying pollinators, such as moths and bats, detect white flowers most easily, and flowers that bloom at sunset, such as yucca, datura, and cereus, usually are white. The often delightful and varied fragrances of flowers also reveal the hand of coevolution. In some cases, insects detect fragrance before color. They follow faint aromas to flowers that are too far away to be seen, recognizing petal shape and color only when they are very close to the flower. Some night-blooming flowers emit sweet fragrances that attract night-flying moths. At the other extreme, carrion flowers, flowers pollinated by flies, give off the odor of rotting meat to attract their pollinators. Flowers and their pollinators also coevolved to influence each other's life cycles. Among species that flower in response to a dark period, some measure the critical night length so accurately that all species of the region flower in the same week or two. This enables related plants to interbreed, and provides pollinators with enough pollen and nectar to live on so that they too can reproduce. The process of coevolution also has resulted in synchronization of floral and insect life cycles. Sometimes flowering occurs the week that insect pollinators hatch or emerge from dormancy, or bird pollinators return from winter migration, so that they feed on and pollinate the flowers. Flowering also is timed so that fruits and seeds are produced when animals are present to feed on the fruits and disperse the seeds. VI FLOWERS AND EXTINCTION Like the amphibians, reptiles, insects, birds, and mammals that are experiencing alarming extinction rates, a number of wildflower species also are endangered. The greatest threat lies in the furious pace at which land is cleared for new houses, industries, and shopping malls to accommodate rapid population growth. Such clearings are making the meadow, forest, and wetland homes of wildflowers ever more scarce. Among the flowers so endangered is the rosy periwinkle of Madagascar, a plant whose compounds have greatly reduced the death rates from childhood leukemia and Hodgkin's disease. Flowering plants, many with other medicinal properties, also are threatened by global warming from increased combustion of fossil fuels; increased ultraviolet light from ozone layer breakdown; and acid rain from industrial emissions. Flowering plants native to a certain region also may be threatened by introduced species. Yellow toadflax, for example, a garden plant brought to the United States and Canada from Europe, has become a notorious weed, spreading to many habitats and preventing the growth of native species. In some cases, unusual wildflowers such as orchids are placed at risk when they are collected extensively to be sold. Many of the threats that endanger flowering plants also place their pollinators at risk. When a species of flower or pollinator is threatened, the coevolution of pollinators and flowers may prove to be disadvantageous. If a flower species dies out, its pollinators will lack food and may also die out, and the predators that depend on the pollinators also become threatened. In cases where pollinators are adapted to only one or a few types of flowers, the loss of those plants can disrupt an entire ecosystem. Likewise, if pollinators are damaged by ecological changes, plants that depend on them will not be pollinated, seeds will not be formed, and new generations of plants cannot grow. The fruits that these flowers produce may become scarce, affecting the food supply of humans and other animals that depend on them. Worldwide, more than 300 species of flowering plants are endangered, or at immediate risk of extinction. Another two dozen or so are considered threatened, or likely to become extinct in the near future. Of these species, fewer than 50 were the focus of preservation plans in the late 1990s. Various regional, national, and international organizations have marshaled their resources in response to the critical need for protecting flowering plants and their habitats. In the United States, native plant societies work to conserve regional plants in every state. The United States Fish and Wildlife Endangered Species Program protects habitats for threatened and endangered species throughout the United States, as do the Canadian Wildlife Service in Canada, the Ministry for Social Development in Mexico, and similar agencies in other countries. At the international level, the International Plant Conservation Programme at Cambridge, England, collects information and provides education worldwide on plant species at risk, and the United Nations Environmental Programme supports a variety of efforts that address the worldwide crisis of endangered species. Contributed By: James David Mauseth Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
