Flower

Study of flowers throughout history

Parts of the flower

Evolution of flowers

Induction of flowering

Resources

A flower is the reproductive structure of an angiosperm plant. Flowers have ovaries with ovules that develop into fruits with seeds. There are over 300,000 species of angiosperms, and their flowers and fruits vary significantly. Flowers and fruits are among the most useful features for the identification of plant species and determination of their evolutionary relationships.

Study of flowers throughout history

The hunter-gatherer ancestors of modern humans surely noticed that flowers gave rise to fruits that could be eaten. Because flowers signaled an anticipated harvest, it has been suggested that these early humans instinctively considered flowers attractive, an instinct that modern humans may also have. Many modern cultures consider flowers attractive, and scholars have been fascinated with flowers for millennia.

Dioscorides, a Greek physician in Emperor Nero’s army (first century AD), wrote the most influential early book on plants, De materia medica. This was the first book about the medicinal uses of plants, referred to as an herbal. Dioscorides’s book had diagrams of many plants and their flowers, and this helped other physicians to identify the species of plant to prescribe to their patients for a particular ailment.

De materia medica remained an important reference on plants for more than 1,500 years. However, early scholars lacked the printing press, so all copies had to be scripted by hand. Over time, the pictures of plants and their flowers in these hand-copied herbals became more romanticized and less accurate.

The 1500s were the golden age of herbals, when European scholars published their own books with illustrations were based on observations of living plants, rather than upon Dioscorides’s diagrams and descriptions. With the invention of the movable type printing press, these herbals became the first published scholarly works in botany and were widely read.

Carolus Linnaeus (1707–1778) of Sweden revolutionized botany in the mid-1700s. He classified plant species according to the morphology of their flowers and fruits. Modern botanists continue to rely upon flowers for identification as well as the determination of evolutionary relationships.

In Linnaeus’s time, many people argued about the doctrine of “divine beneficence,” which held that all things on Earth were created to please humans. Thus, people believed that flowers with beautiful colors and sweet smells were created by God to please humans. Christian Konrad Sprengel (1750–1816) of Germany disputed this view in the late 1700s. He held that the characteristics of flowers are related to their method of reproduction. Sprengel published his theory of flowers in his book The Secret of Nature Revealed (1793).

Sprengel’s ideas were not widely accepted in his own time. In 1841, however, the English botanist Robert Brown gave Charles Darwin (1809–1882) a copy of Sprengel’s book. This book influenced Darwin’s development of his theory of evolution by natural selection, which culminated in the publication of On the Origin of Species (1859). Sprengel’s work also stimulated Darwin’s subsequent study of orchids, and he wrote The Various Contrivances by Which Orchids Are Fertilized by Insects (1862). Darwin’s important studies of flowers and pollination supported Sprengel’s view that there is a relationship between the characteristics of a flower and its method of pollination. Moreover, Darwin demonstrated that some of the highly specialized characteristics of flowers had evolved by natural selection to facilitate their pollination.

Parts of the flower

There are considerable differences among the flowers of the 300,000 species of angiosperms. Botanists rely upon a large vocabulary of specialized terms to describe the parts of these various flowers. The most important morphological features of flowers are considered below.

Flowers can arise from different places on a plant, depending on the species. Some flowers are terminal, meaning that a single flower blooms at the apex of a stem. Some flowers are axial, in that they are borne on the axes of branches along a stem. Some flowers arise in an inflorescence, a branched cluster of individual flowers.

There are four whorls of organs in a complete flower. From the outside to the inside, one encounters sepals, petals, stamens, and carpels. The sepals are leaflike organs, which are often green, but can sometimes be brown or brightly colored, depending on the species. The petals are also leaflike and are brightly colored in most animal-pollinated species but dull in color or even absent in wind-pollinated plants.

The stamens and carpels, the reproductive organs, are the most important parts of a flower. The stamens are the male, pollen-producing organs. A stamen typically consists of an anther attached to a filament (stalk). The anther produces many microscopic pollen grains. The male sex cell, a sperm, develops within each pollen grain.

Carpels are female ovule-producing organs; they typically consist of an ovary, style, and stigma. The stigma is the tip of the carpel upon which pollen grains land and germinate. The style is a stalk that connects the stigma and ovary. After the pollen grain has germinated, its pollen tube grows down the style into the ovary. The ovary typically contains one or more ovules, structures that develop into seeds upon fertilization by the sperm. As the ovules develop into seeds, the ovary develops into a fruit, whose characteristics depend on the species.

In some species, one or more of the four whorls of floral organs is missing, and the flower is referred to as an incomplete flower. A bisexual flower is one with both stamens and carpels, whereas a unisexual flower is one that has either stamens or carpels, but not both. All complete flowers are bisexual since they have all four floral whorls. All unisexual flowers are incomplete since they lack either stamens or carpels. Bisexual flowers, with stamens and carpels, can be complete or incomplete, since they may lack sepals and/or petals.

Evolution of flowers

The flower originated as a structure adapted to protect ovules, which are born naked and unprotected in the gymnosperms, ancestors of the angiosperms. Botanists are uncertain about which group of gymnosperms is most closely related to the angiosperms. Recently, examination of sexual fertilization in the different groups of gymnosperms suggests that angiosperms may be most closely related to the Gnetophyta, a small phylum with three genera (Ephedra, Gnetum, and Welwitschia ) and about 70 species.

The angiosperms first appeared in the fossil record in the early Cretaceous period (about 130 million years ago) and rapidly increased in diversity. Once the flowering plants had evolved, natural selection for efficient pollination by insects and other animals was important in their diversification. By the mid-Cretaceous, species with flowers of many different designs had evolved. These varying designs evolved as a consequence of the close association of the flowers and their animal pollinators, a process referred to as coevolution. In addition, many flowers became self-incompatible, in that they relied upon cross-pollination, that is, pollination by another individual of the same species. Cross-pollination increases the genetic diversity of the offspring, in general making them more fit.

Today, flowering plants are the dominant terrestrial plants in the world. There are more than 300 different families of flowering plants. The Asteraceae, with over 15,000 species, is one of the largest and most diverse families of angiosperms, and their flowers are highly evolved. The dandelion, daisy, and sunflower are familiar species of the Asteraceae. In these species, many small individual flowers are packed closely together into a dense inflorescence called a head, which appears rather superficially like a single large flower. Each individual flower in the head has a single ovule, which produces a single seed upon fertilization. The flowers of many species of the Asteraceae, such as the dandelion, evolved highly specialized sepals, which are scalelike and are referred to as a pappus. In summer, the pappus of the dandelion expands into the furry white structure which aids the tiny attached seeds in their dispersal by the wind.

Induction of flowering

Many environmental cues signal a plant to produce flowers, and light is one of the most important of these. In many species, flowering is a photoperiodic response, in that it is controlled by the length of the light and dark periods to which the plant is exposed.

Some plants, such as Maryland mammoth tobacco, soybean, and hemp, are short-day plants: they flower in the spring and autumn when the days are shorter. Other plants, for example, spinach, mouse ear cress, and fuchsia, are long-day plants, in that they flower in the

KEY TERMS

Anther —The part of the stamen that produces pollen.

Carpel —Female reproductive organ of flowers which is composed of the stigma, style, and ovary.

Filament —Stalk of the stamen which bears the anther.

Ovary —Basal part of the carpel which bears ovules and develops into a fruit.

Ovule —Structure within the ovary which develops into a seed after fertilization.

Petal —Whorl of a flower just inside the sepals that is often colored.

Sepal —External whorl of a flower which is typically leaflike and green.

Stamen —Male reproductive organ of flowers that produces pollen.

Stigma —The part of the female organs of a plant flower (the pistil) upon which pollen lands in the first stage of fertilization.

Style —A stalk that joins the pollen-receptive surface of the stigma, to the ovary of the female organ of a plant (i.e., the pistil).

summer when the days are longer. Some plants, such as cucumber, corn, and garden peas, are day-neutral plants, in that they flower regardless of the day length. Often, different varieties of the same species have different light requirements for flowering.

The dark period is as crucial as the light period for induction of flowering. In particular, when a short-day plant is exposed to short days, but given a pulse of light during the dark period, flowering is inhibited. When a long-day plant is exposed to short days but given a pulse of light during the dark period, flowering is promoted. Phytochromes are the photo-receptive plant pigments that detect these light pulses. Phytochromes also control other stages of plant growth and development, and phytochrome genes have been cloned and sequenced in many plant species.

Regrettably, plant physiologists have made little additional progress in understanding the mechanism of flower induction in recent years. Little is known about the biochemical reactions that follow from activation of phytochrome or how plants measure photo-period. This is an area in which future botanists may make great progress.

See also Sexual reproduction.

Resources

BOOKS

American Horticultural Society. American Horticultural Society Encyclopedia of Plants and Flowers. New York: DK Publishing, 2002.

Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg, Michael J. Donoghue, and Peter Stevens. Plant Systematics: A Phylogenetic Approach. 2nd ed. with CD-ROM. Suderland, MD: Sinauer, 2002.

Kaufman, P.B., et al. Plants: Their Biology and Importance. New York: Harper College Press, 1990.

PERIODICALS

Adams, K.L., et al. “Repeated, Recent and Diverse Transfers of a Mitochondrial Gene to the Nucleus in Flowering Plants.” Nature 408 (2000): 354-357.

OTHER

National Wildlife Federation: eNature. “Wildflowers: All Families” <http://www.enature.com/fieldguides/view_default.asp?curGroupID=11> (accessed November 24, 2006).

Peter A. Ensminger

Flower

A flower is the reproductive structure of an Angiosperm plant. Flowers have ovaries with ovules that develop into fruits with seeds . There are over 300,000 species of Angiosperms, and their flowers and fruits vary significantly. Flowers and fruits are among the most useful features for the identification of plant species and determination of their evolutionary relationships.

Study of flowers throughout history

The hunter-gatherer ancestors of modern humans surely noticed that flowers gave rise to fruits which could be eaten. Because flowers signaled an anticipated harvest, it has been suggested that these early humans instinctively considered flowers attractive, an instinct that modern humans may also have. Many modern cultures consider flowers attractive, and scholars have been fascinated with flowers for millennia.

Dioscorides, a Greek physician in Emperor Nero's army (first century a.d.), wrote the most influential early book on plants, De Materia Medica. This was the first book about the medicinal uses of plants, referred to as an herbal. Dioscorides's book had diagrams of many plants and their flowers, and this helped other physicians to identify the species of plant to prescribe to their patients for a particular ailment.

De Materia Medica remained an important reference on plants for more than 1,500 years. However, early scholars lacked the printing press, so all copies had to be scripted by hand. Over time , the pictures of plants and their flowers in these hand-copied herbals became more romanticized and less accurate.

The 1500s were the "golden age" of herbals, when European scholars published their own books whose illustrations were based on observations of living plants, rather than upon Dioscorides's diagrams and descriptions. With the invention of the movable type printing press, these herbals became the first published scholarly works in botany and were widely read.

Carolus Linnaeus of Sweden revolutionized botany in the mid-1700s. He classified plant species according to the morphology of their flowers and fruits. Modern botanists continue to rely upon flowers for identification as well as the determination of evolutionary relationships.

In Linnaeus's time, many people argued in the doctrine of "Divine Beneficence," which held that all things on Earth were created to please humans. Thus, people believed that flowers with beautiful colors and sweet smells were created by God to please humans. Christian Konrad Sprengel of Germany disputed this view in the late 1700s. He held that the characteristics of flowers are related to their method of reproduction. Sprengel published his theory of flowers in his book The Secret of Nature Revealed (1793).

Sprengel's ideas were not widely accepted in his own time. However, in 1841 the English botanist Robert Brown gave Charles Darwin a copy of Sprengel's book. This book influenced Darwin's development of his theory of evolution by natural selection , which culminated in the publication of The Origin of Species (1859). Sprengel's work also stimulated Darwin's subsequent study of orchids, and he wrote The Various Contrivances by Which Orchids Are Fertilized by Insects (1862). Dar win's important studies of flowers and pollination supported Sprengel's view that there is a relationship between the characteristics of a flower and its method of pollination. Moreover, Darwin demonstrated that some of the highly specialized characteristics of flowers had evolved by natural selection to facilitate their pollination.

Parts of the flower

There are considerable differences among the flowers of the 300,000 species of Angiosperms. Botanists rely upon a large vocabulary of specialized terms to describe the parts of these various flowers. The most important morphological features of flowers are considered below.

Flowers can arise from different places on a plant, depending on the species. Some flowers are terminal, meaning that a single flower blooms at the apex of a stem. Some flowers are axial, in that they are borne on the axes of branches along a stem. Some flowers arise in an inflorescence, a branched cluster of individual flowers.

There are four whorls of organs in a complete flower. From the outside to the inside, one encounters sepals, petals, stamens, and carpels. The sepals are leaf-like organs, which are often green, but can sometimes be brown or brightly colored, depending on the species. The petals are also leaf-like and are brightly colored in most animal-pollinated species but dull in color or even absent in wind-pollinated plants.

The stamens and carpels, the reproductive organs, are the most important parts of a flower. The stamens are the male, pollen-producing organs. A stamen typically consists of an anther attached to a filament (stalk). The anther produces many microscopic pollen grains. The male sex cell , a sperm, develops within each pollen grain.

The carpels are the female ovule-producing organs. A carpel typically consists of an ovary, style, and stigma. The stigma is the tip of the carpel upon which pollen grains land and germinate. The style is a stalk that connects the stigma and ovary. After the pollen grain has germinated, its pollen tube grows down the style into the ovary. The ovary typically contains one or more ovules, structures which develop into seeds upon fertilization by the sperm. As the ovules develop into seeds, the ovary develops into a fruit, whose characteristics depend on the species.

In some species, one or more of the four whorls of floral organs is missing, and the flower is referred to as an incomplete flower. A bisexual flower is one with both stamens and carpels, whereas a unisexual flower is one which has either stamens or carpels, but not both. All complete flowers are bisexual since they have all four floral whorls. All unisexual flowers are incomplete since they lack either stamens or carpels. Bisexual flowers, with stamens and carpels, can be complete or incomplete, since they may lack sepals and/or petals.

Evolution of flowers

The flower originated as a structure adapted to protect ovules, which are borne naked and unprotected in the Gymnosperms, ancestors of the Angiosperms. Botanists are uncertain about which group of Gymnosperms is most closely related to the Angiosperms. Recently, examination of sexual fertilization in the different groups of Gymnosperms suggests that Angiosperms may be most closely related to the Gnetophyta, a small phylum with three genera (Ephedra, Gnetum, and Welwitschia) and about 70 species.

The Angiosperms first appeared in the fossil record in the early Cretaceous period (about 130 million years ago) and rapidly increased in diversity. Once the flowering plants had evolved, natural selection for efficient pollination by insects and other animals was important in their diversification. By the mid-Cretaceous, species with flowers of many different designs had evolved. These varying designs evolved as a consequence of the close association of the flowers and their animal pollinators, a process referred to as coevolution. In addition, many flowers became self-incompatible, in that they relied upon cross-pollination, that is, pollination by another individual of the same species. Cross-pollination increases the genetic diversity of the offspring, in general making them more fit.

Today, flowering plants are the dominant terrestrial plants in the world. There are more than 300 different families of flowering plants. The Asteraceae, with over 15,000 species, is one of the largest and most diverse families of Angiosperms, and their flowers are highly evolved. The dandelion, daisy, and sunflower are familiar species of the Asteraceae. In these species, many small individual flowers are packed closely together into a dense inflorescence called a head, which appears rather superficially like a single large flower. Each individual flower in the head has a single ovule, which produces a single seed upon fertilization. The flowers of many species of the Asteraceae, such as the dandelion, evolved highly specialized sepals, which are scale-like and are referred to as a pappus. In summer, the pappus of the dandelion expands into the furry white structure which aids the tiny attached seeds in their dispersal by the wind .

Induction of flowering

Many environmental cues signal a plant to produce flowers, and light is one of the most important of these. In many species, flowering is a photoperiodic response, in that it is controlled by the length of the light and dark periods to which the plant is exposed.

Some plants, such as Maryland mammoth tobacco, soybean , and hemp , are short-day plants, in that they flower in the spring and autumn when the days are shorter. Other plants, for example, spinach , mouse ear cress, and fuchsia, are long-day plants, in that they flower in the summer when the days are longer. Some plants, such as cucumber, corn, and garden peas, are day-neutral plants, in that they flower regardless of the daylength. Often, different varieties of the same species have different light requirements for flowering.

The dark period is as crucial as the light period for induction of flowering. In particular, when a short-day plant is exposed to short days, but given a pulse of light during the dark period, flowering is inhibited. When a long-day plant is exposed to short days but given a pulse of light during the dark period, flowering is promoted. Phytochromes are the photoreceptive plant pigments which detect these light pulses. Phytochromes also control other stages of plant growth and development, and phytochrome genes have been cloned and sequenced in many plant species.

Regrettably, plant physiologists have made little additional progress in understanding the mechanism of flower induction in recent years. Little is known about the biochemical reactions that follow from activation of phytochrome or how plants measure photoperiod. This is an area in which future botanists may make great progress.

See also Sexual reproduction.

Resources

books

The American Horticultural Society. The American Horticultural Society Encyclopedia of Plants and Flowers. New York: DK Publishing, 2002.

Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg, Michael J. Donoghue, and Peter Stevens. Plant Systematics:A Phylogenetic Approach. 2nd ed. with CD-ROM. Suderland, MD: Sinauer, 2002.

Kaufman, P.B., et al. Plants: Their Biology and Importance. New York: Harper College Press, 1990.

periodicals

Adams, K.L., et al. "Repeated, Recent and Diverse Transfers of a Mitochondrial Gene to the Nucleus in Flowering Plants." Nature 408 (2000): 354-357.

Peter A. Ensminger

KEY TERMS

Anther

—The part of the stamen that produces pollen.

Carpel

—Female reproductive organ of flowers which is composed of the stigma, style, and ovary.

Filament

—Stalk of the stamen which bears the anther.

Ovary

—Basal part of the carpel which bears ovules and develops into a fruit.

Ovule

—Structure within the ovary which develops into a seed after fertilization.

Petal

—Whorl of a flower just inside the sepals that is often colored.

Sepal

—External whorl of a flower which is typically leaflike and green.

Stamen

—Male reproductive organ of flowers that produces pollen.

Stigma

—The part of the female organs of a plant flower (the pistil) upon which pollen lands in the first stage of fertilization.

Style

—A stalk that joins the pollen-receptive surface of the stigma, to the ovary of the female organ of a plant (i.e., the pistil).

Flower

A flower is the reproductive part of a plant that produces seeds. Plants that produce flowers and fruit are called angiosperms. There are more than 300,000 species of angiosperms, and their flowers and fruits vary significantly. Flowers and fruits are among the most useful features for identifying plant species.

Study of flowers throughout history

Many modern cultures consider flowers attractive, and scholars have been fascinated with flowers for thousands of years. Dioscorides, a first-century Greek physician, wrote the most influential early book on plants, De materia medica. This was the first text about the medicinal uses of plants, and it contained many diagrams of plants and their flowers. The book helped other physicians identify the species of plant to prescribe to their patients for a particular ailment. De materia medica remained an important reference on botany (the study of plants) for more than 1,500 years.

In the mid-1700s, Swedish botanist Carolus Linnaeus revolutionized the field of botany. He classified plant species according to the morphology (form and structure) of their flowers and fruits. Modern botanists continue to rely upon his classification system.

Up until the late 1700s, people believed that flowers with beautiful colors and sweet smells were created by God to please humans. However, German botanist Christian Konrad Sprengel disputed this view. He held that the characteristics of flowers (shape, color, smell) are related to their method of reproduction. Sprengel published his theory of flowers in 1793 in The Secret of Nature Revealed. Although not widely accepted in his own time, Sprengel's views were soon considered scientifically correct.

Words to Know

Angiosperm: Plant that produces flowers and seeds.

Anther: Top part of the stamen that produces pollen.

Filament: Stalk of the stamen that bears the anther.

Corolla: Layers of petals in a flower.

Morphology: Branch of biology dealing with the form and structure of living organisms.

Ovary: Base part of the pistil that bears ovules and develops into a fruit.

Ovule: Structure within the ovary that develops into a seed after fertilization.

Petal: Whorl of a flower just inside the sepals that is often colored.

Pistil: Female reproductive organ of flowers that is composed of the stigma, style, and ovary.

Pollen: Powdery grains that contain the male reproductive cells of angiosperms.

Pollination: Transfer of pollen from the male reproductive organs to the female reproductive organs of a plant.

Sepal: External whorl of a flower that is typically leaflike and green.

Stamen: Male reproductive organ of flowers that is composed of the anther and filament.

Stigma: Top part of the pistil upon which pollen lands and germinates.

Style: Stalk of the pistil that connects the stigma to the ovary.

Parts of the flower

There are considerable differences among the many species of flowers. Flowers can develop on different places on a plant. Terminal flowers, like a tulip, are single flowers that bloom at the apex or end of an upright stalk. Other flowers arise in an inflorescence, a branched cluster of individual flowers. Begonias are an example of this type. Those flowers that grow at the base of a leaf where it attaches to the stem of the plant are called axillary flowers. Snapdragons are an example of axillary flowers.

There are four concentric whorls (rings) of organs in a complete flower. From the center to the outside, they are the pistil, stamens, petals, and sepals. Fundamentally, these four parts are modified leaves.

The pistil, a long stalk arising in the center, is the female reproductive organ of a flower. It is composed of the stigma, style, and ovary. The stigma is the sticky knob at the outer end of the stalk. The style is the portion of the stalk connecting the stigma to the ovary. The ovary is the round base that contains one or more undeveloped seeds called ovules. In each ovule is an egg waiting to be fertilized by a sperm.

Stamens, the male reproductive organs, also arise from the center of the flower and encircle the pistil. The stamens are composed of a stalk, called a filament, topped by an anther. The anther produces many microscopic pollen grains. The male sex cell, a sperm, develops within each pollen grain.

Petals, the often-brightly colored portion surrounding the pistil and stamens, are a flower's showpiece. They attract the attention of passing insects, birds, and people. The layers of petals in a flower comprise the corolla.

Sepals lie below the petals and are usually green and leaflike in appearance. Sepals form a temporary, protective cover over an unopened flower. When the petals of a flower are ready to unfurl, the sepals fold back.

In some species, one or more of the four whorls of floral organs is missing, and the flower is referred to as an incomplete flower. A bisexual flower is one with both stamens and a pistil, whereas a unisexual flower is one that has either stamens or a pistil, but not both. All complete flowers are bisexual since they have all four floral whorls. All unisexual flowers are incomplete since they lack either stamens or a pistil.

Pollination

In angiosperms, pollination is the transfer of pollen from an anther to a stigma. Pollen grains land on the sticky stigma, where they begin to germinate or grow. A pollen tube then forms down the style, sperm is delivered to the ovules, and fertilization takes place.

If the transfer of pollen occurs between an anther and the stigma of the same plant, it is known as self-pollination. Complete flowers are able to self-pollinate. When the transfer of pollen occurs between an anther and the stigma of different plants, it is known as cross-pollination. Of the two methods, cross-pollination produces stronger and healthier off-spring since it mixes up the genetic make-up of plants. Cross-pollination can be brought about by wind, rain, mammals, birds, and insects.

Pollination by wind. Many angiosperms are pollinated by wind. Wind-pollinated flowers, such as those of corn and all grasses, tend to have a simple structure lacking petals. The anthers dangle on long filaments, allowing the light pollen grains to be easily caught by the wind. The stigma are freely exposed to catch the airborne pollen. Large amounts of pollen are usually wasted because they do not reach female reproductive organs. For this reason, most wind-pollinated plants are found in temperate regions, where members of the same species often grow close together.

Pollination by animals. In general, pollination by insects and other animals is more efficient than pollination by wind. Many times flowers

offer "rewards" to attract these animals—sugary nectar, oil, solid food, a place to sleep, or even the pollen itself. Generally, plants use color and fragrances to lure their pollen-transporting agents.

The flowers of many species of plants are marked with special pigments that absorb ultraviolet light (light whose wavelengths are shorter than visible light). These pigments are invisible to humans and most animals. But the eyes of bees are sensitive enough to detect the patterns created by the pigments and so the bees are drawn to them.

Having been attracted to a flower, an insect or other small animal probes inside for its reward. In doing so, it brushes against the anthers and picks up dust pollen on its body. When the animal moves on to the next flower, it brushes past the stigma, depositing pollen. Many flowers are designed precisely to match the body forms of the animals participating in this pollen transfer. In this way, contact with both the anthers and the stigma is ensured. A few orchids use a combination of smell, color, and shape to mimic the female of certain species of bees and wasps. The male bees and wasps then try to mate with the flower. In the process, they either pick up or transfer pollen to that flower.

Flower

A flower is a plant structure that contains the organs needed for sexual reproduction. The function of a flower is to make seeds. Flowers and the seeds they produce are also a major source of nutrients for almost all animals.

THE STRUCTURE OF FLOWERS

Most plants have flowers, and any plant that produces some sort of flower, even a small, colorless one, is a flowering plant. Grasses and oak trees are flowering plants just like roses and cherry trees. As many as 200,000 types of flowers have been classified, from tiny pondweed flowers to the bathtub-size flower of the tropical Giant Rafflesia. Despite the enormous variety in size, shape, color, and fragrance, all flowers have similar structure. Every flower has four basic organs or parts: the sepals, the petals, the stamen, and the pistil.

The Sepals and Petals. The sepals are the outermost part of the flower (where the flower emerges from the stem), and resemble green flaps that protect the flower when it is still a bud. The petals are usually the flower's most distinctive part and signal to animal pollinators (animals who transfer pollen, containing male sex cells to the pistil containing female sex cells) with bright colors and strong scents. Sometimes a flower's petals resemble a female insect, attracting males who pollinate as they land on their "mate." However, flowers that use the wind as a pollinator rather than animals usually have small petals or none at all.

The Stamen. The stamen is the male reproductive organ of a flower and lies inside the petals. Each is a slender stalk or a ribbon-like thread called a filament with an enlarged tip or head called an anther. The anther is like a small sac and contains the pollen, which are dustlike particles that contain the plant's male sex cells.

The Pistil. The pistil is the female reproductive organ of a flower and is usually located in the center of the flower. It is here that the seed is actually produced. Each pistil has three parts: the stigma, the style, and the

ovary. The stigma is the sticky top of the style, which is a slender stalk or tube that connects with the ovary below. The ovary contains the ovules that store the female sex cells. In terms of functions, the stigma catches or collects the pollen, the style is the tube down which the pollen travels to the ovary, and the ovary is where the pollen fertilizes the ovules. It is the ovule that develops into the seed after fertilization, and the ovary that becomes the plant's fruit.

There are several other names for a flower's different subparts. For example, the protective petals around the stamens are collectively called the corolla. The sepals are also collectively called the calyx. In flowers that have two or more pistils (called compound pistils), the individual pistil is called a carpel. Flowers that have all these parts are termed perfect or complete, while those missing one or more parts are called incomplete or imperfect flowers.

REPRODUCTION IS THE KEY TO CLASSIFICATION

The flowering plants of the world have been classified into roughly three hundred families according to these flower parts. Thus, even though certain plants may grow in very different climates and soils and have varying shapes and colors, they may still be part of the same family because of how their reproductive organs look and function. Since reproduction is the key in classifying flowers, the lily family includes not only the tulip and hyacinth but also the onion, garlic, aloe, and yucca plant. All are pollinated by insects, contain three sepals and petals that closely resemble each other, six stamens, one pistil, long sheathlike leaves with parallel veins, and fruit that contains many seeds within one capsule. Further, the pea family includes not only beans, lentils, and peanuts but trees like the locust, vines like the wisteria, and herbs like licorice.

Flowers are critically important as the key to a flowering plant's ability to reproduce more of its own kind. Exactly when a flower blooms is also very important, since plants need to flower and form fruits and seeds before the cold season resumes. The length of daylight and darkness, as well as consistent temperature change, are mechanisms that signal a plant it should begin to blossom.

Besides acting as a reproductive agent for plants, flowers also beautify the world for people. Since ancient times, humans have prized flowers for their shapes, colors, and fragrances. They also have used flowers as medicines and given them symbolic meanings.

[See alsoBotany; Plant Anatomy; Plant Reproduction ]

flow·er / ˈflou(-ə)r/ • n. Bot. the seed-bearing part of a plant, consisting of reproductive organs (stamens and carpels) that are typically surrounded by a brightly colored corolla (petals) and a green calyx (sepals). ∎  a brightly colored and conspicuous example of such a part of a plant together with its stalk, typically used with others as a decoration or gift: I stopped to buy Bridget some flowers. ∎  the state or period in which a plant's flowers have developed and opened: the roses were just coming into flower.• v. [intr.] (of a plant) produce flowers; bloom: these daisies can flower as late as October. ∎ fig. be in or reach an optimum stage of development; develop fully and richly: it is there that the theory of deconstruction has flowered most extravagantly | [as n.] (flowering) the flowering of Viennese intellectual life. ∎  [tr.] induce (a plant) to produce flowers.PHRASES: the flower of1. the finest individuals out of a number of people or things: the flower of college track athletes.2. the period of optimum development: a young policeman in the flower of his life gunned down.DERIVATIVES: flow·er·less adj.flow·er·like / -ˌlīk/ adj.