Flowers

Flowers are the typically showy reproductive organs of angiosperms (flowering plants). Their diverse blooms generate countless horticultural products; functionally flowers are essential for sexual reproduction. Flowers may occur individually on the plant or together with other flowers forming inflorescences. Their flowers and inflorescences are: (1) highly adapted to the plant's ecology and pollination strategy; (2) highly variable among the flowering plants; (3) critically important to plant identification; and (4) subject to their own rich and specialized terminology.

Morphologically and evolutionarily, the flower is regarded as a terminal shoot with variably leaflike lateral appendages . The stem of the flower, known as a pedicel, is connected to the other floral parts via the receptacle. From the outside rim of the receptacle, the first appendages are typically green, leaflike sepals , collectively called the calyx. The next whorl is composed of variously colored and often complex petals, collectively called the corolla.

The androecium forms the next whorl, formed of stamens, each made of a filament and anther. Anther sacs are filled with pollen at maturity. Anthers release the pollen timed to match the receptivity of the female of either the same flower or others of the same species.

The center of the flower is occupied by the gynoecium, which is formed of one or more carpels. Carpels may fuse to form a compound pistil or may remain separate and unfused. The carpel is divided into three regions, from the tip: the stigma, a receptive surface for pollen; the style, a specialized region for pollen tube elongation; and the ovary, a location where immature seeds called ovules occur, containing the female gametophyte or embryo sac. Meiosis occurs deep within the anther to produce many haploid male gametophytes (which become pollen) and deep within the ovule to produce the female gametophyte (or embryo sac).

The group of angiosperms known as monocotyledons typically have floral organs in multiples of three, whereas dicotyledons have floral organs in multiples of four or five. Fusion of floral organs often occurs obscuring these parts and increasing the diversity of floral form. Similar organs fuse in the example of the corolla tube (as in the morning glory), but dissimilar organs fuse in the example of apple flowers.

The largest flowers are species of Rafflesia, a carrion flower measuring up to 2 meters (6 feet) wide. This flower, native to the tropical rain forests of Sumatra, has no leaves or aboveground organs except for the flower. Living up to its name, Rafflesia attracts and is pollinated by carrion flies.

The smallest flower is a duckweed. The smallest duckweeds are members of the genus Wolffia, with the whole plant measuring less than 1 millimeter (.039 inch) across. The flower of this plant is reduced to one pistil and one anther, which are borne on different plants. Other floral parts are vestigial or fail to form.

Organs may also be modified to perform different functions. For example, roses have five petals (as do most dicots), but some or all of the numerous stamens form petal-like staminodes, sterile petal-like stamens that give horticultural roses the appearance of having many more petals. Symmetry of individual flowers may be radial (forming mirror images around the center), or bilateral (forming mirror images along only one plane).

The flower is the focus of considerable metabolic activity for the plant, producing reproductive organs with high energy content. Flowers attract insects and other animal pollinators to visit the flower. In return, pollinators are often provided with a food reward of pollen or nectar to assure this continued relationship. After the flower is fertilized, floral organs, including petals, stigma, and style, die back while seeds form inside the carpels. The tissues surrounding the ovary are dramatically modified to become the fruit.

see also Angiosperms; Fruits; Pollination and Fertilization; Seeds

Scott D. Russell

Bibliography

Uno, Gordon E., Richard Storey, and Randy Moore. Principles of Botany. Boston: Mc-Graw-Hill, 2001.

Young, Paul G., and Jacquelyn Giuffre. The Botany Coloring Book. New York: Harper Perennial, 1982.

167. Flowers

See also 54. BOTANY ; 319. PLANTS

anthesis

full bloom of a flower.

anthoecology

the branch of ecology that studies the relationship of flowers to their environment.

anthography

Botany. the description of flowers.

anthomania

Rare. an extreme love for flowers.

anthophagy

the habit, as of larvae, of feeding on flowers. —anthophagous, adj.

anthophobia

an abnormal fear of flowers.

cleistogamy

the state of bearing small flowers as well as fully developed ones, as in the pansy, in which the small ones do not open but are pollinated by their own anthers. —cleistogamous, adj.

conservatory

a greenhouse, especially one used to grow delicate, rare, and exotic flowers and plants for decorative purposes. See also 284. MUSIC

dichogamy

the condition, in some flowering plants, in which the pistils and stamens mature at different times, thus preventing self-pollination. —dichogamous, adj.

diclinism

the condition of having the stamens and pistils in separate flowers. —diclonous, adj.

efflorescence

the process of flowering or blooming. —efflorescent, adj.

epanody

peloria.

estivation, aestivation

the arrangement of petals in a flower before it opens; prefloration. Also aestivation.

florescence

1 . the state or condition of being in flower or blooming

2 . the period during which this occurs.

3 . a period of great development. —florescent, adj.

floretum

a garden specifically used for the growth and scientific study of flowers.

floribunda

one of several varieties of rosé characterized by their long blooming period and their large flowers, often in clusters.

floriculture

the cultivation of flowers, especially of decorative flowering plants, usually on a commercial scale. —floriculturist , n. —floricultural, adj.

florimania

a mania for plants and flowers.

homogony

the condition of similarity in length and location of all the pistils and stamens in flowers of the same species. —homogonous, adj.

ikebana

the Japanese art of flower arrangement, especially for the home.

peloria

the phenomenon of a regular structure appearing as an abnormality in flowers which are usually irregular. Also called epanody . —peloric, pelorian, adj.

perigyny

the state of having the pistils, stamens, petals, etc., arranged around a cuplike receptacle. —perigynous, adj.

phyllody

the process by which floral organs turn into foliage. Also phyllomorphy.

rosarium

a rose garden.

staminody

the metamorphosis of various flower organs, as petals or sepals, into stamens.

synanthy

whole or partial union of several flowers that are usually separate and distinct. —synanthous adj.

tulipomania

a mania for planting and growing tulips, especially such a mania in Holland in the 1630s, when a sum equivalent to $5200 was paid for a single bulb. —tulipomaniac , n.

Flowers

An enormous diversity of size, shape, and complexity exists among the flowers of the quarter-million species of angiosperms . Flower size varies over a thousandfold, with Rafflesia (Rafflesiaceae) flowers as large as 1 meter in diameter dwarfing the minuscule flowers of Wolffia (Lemnaceae), which measure less than 1 millimeter across. The number of floral organs also varies, with the complex flowers of the Tambourissa (Monimiaceae) species having more than one thousand organs while the simple flowers of the Chloranthaceae may consist of just a few. The coevolution of angiosperms with their animal pollinators is a driving force in the generation of flower diversity. The end product of pollination is the formation of a viable seed, therefore ensuring that the species will be perpetuated. Exclusive pollinator-flower relationships ensure that pollen will not be wasted by delivery to flowers of a different species.

Definition and Flower Parts

Despite the enormous diversity in the number, size, and shape of floral organs within the angiosperms, they all are built of four basic organ types (sepals , petals, stamens, and carpels ) whose relative positions are invariant. The flower is an assemblage of sterile and fertile (reproductive) parts borne on a shoot or axis called the receptacle. The sterile parts include the sepals (collectively called the calyx) and the petals (collectively called the corolla). The sepals and petals together constitute the perianth. In a typical flower the sepals are green, and they enclose and protect the young flower before it opens. The petals, whose function is to attract pollinators, exhibit an assortment of colors, shapes, and sizes. In flowers in which the sepals and petals are indistinguishable from each other, such as tulips (Liliaceae), the perianth parts are called tepals .

The reproductive parts can be divided into the androecium and gynoecium . The androecium is composed of stamens, the male floral organs. Stamens usually have an apical anther, in which pollen develops, and a basal filament connecting the anther to the receptacle. One or more carpels constitute the gynoecium. Carpels are made of several functional tissues that facilitate pollination and protect developing ovules and seeds. The stigma on which the pollen germinates, and the style, through which the pollen tube grows toward the ovules, are examples of tissues that are intimately associated with pollination. The ovary, which houses the ovules, provides protection for both the developing ovules and seeds. In addition, the ovary often develops into a fruit that facilitates seed dispersal. The formation of a protected chamber in which the ovules and seeds develop is one of the defining features of angiosperms. The term angiosperm is derived from the Greek angio (a capsule-like covering) and sperm (seed).

Function of Flowers

The function of the flower is to facilitate the reproduction of the organism. Cells within the pollen and embryo sac are haploid and are derived from the diploid cells that develop within the anthers and ovules, respectively. In angiosperms, pollination results in double fertilization. The egg cell nucleus fuses with a sperm nucleus to produce the zygote while the other sperm cell nucleus fuses with the two polar nuclei to form the triploid endosperm . The endosperm acts as a food supply for the developing embryo. After fertilization, the ovule with the developing embryo becomes a seed and the ovary becomes the fruit that houses the seed(s).

Diversity of Flowers

Among the quarter-million species of angiosperms, there are many variations of the generalized flower yielding an immense diversity of floral patterns. The diversity is due to variations in the number, symmetry, size, and fusion of floral parts. While most flowers contain both stamens and carpels (and are referred to as hermaphroditic), other flowers are unisexual. These may be either staminate flowers (missing the carpels) or carpellate flowers (missing the stamens). Species bearing both carpellate and staminate flowers on a single plant are referred to as monoecious ("one house"; maize [Gramineae] and oak trees [Fagaceae], for example). In contrast, dioecious ("two houses") species bear staminate and carpellate flowers on different plants (willow [Salicaceae], for example). Monoecism and dioecism, both of which have evolved multiple times within the angiosperms, provide a mechanism to promote outbreeding. Many other species have evolved more subtle mechanisms to promote cross-pollination. For example, plants may be self-incompatible; that is, they discriminate between self and nonself pollen and consequently reject their own pollen. Alternatively, a difference in the timing of maturation of the androecium and gynoecium in hermaphroditic flowers favors outbreeding.

Two conspicuous characters that contribute to floral diversity are the number and size of floral organs. Angiosperms are often divided into two groups based on their cotyledon number: monocotyledons (monocots, meaning one cotyledon) and dicotyledons (dicots, meaning two cotyledons). One of the characters that distinguish monocots from dicots is the number of appendages within a whorl . Monocots usually have flowers with floral parts in multiples of three, whereas the dicots often have floral parts in multiples of four or five. Organ size can also vary enormously. For example, in the species Lepidium (Brassicaeae), the petals are microscopic, but in the genus Camellia (Theaceae), some species have petals 15 centimeters long.

Variations in fusion, arrangement, and symmetry of floral organs provide further diversity. Fusion between organs of the same whorl (coalescence) and fusion of organs from separate whorls (adnation) is common among many families of angiosperms. For instance, coalescence is seen in snapdragon (Antirrhinum, Scrophulariaceae) with the petals fused at the base, and adnation in tobacco (Nicotiana, Solanaceae) with stamens fused to the petals. Position of organ attachment to the receptacle also influences flower architecture. If the corolla and stamens attach to the receptacle below the ovary, the flower is referred to as having a superior ovary (e.g., Liriodendron tulipifera, Magnoliaceae). In contrast, having the corolla and stamens attach to the receptacle above the ovary produces a flower with an inferior ovary (e.g., Iris, Iridaceae). In radially symmetric flowers, termed actinomorphic, all organs within any particular whorl are identical and positioned equidistant from other organs within the whorl (e.g., California poppy Eschscholzia californica, Papaveraceae). Flowers in which organs in a particular whorl differ from other organs in the same whorl are referred to as zygomorphic (e.g., most orchids, Orchidaceae).

While all of the mechanisms generating diversity contribute to their interactions with pollinators, the fusion and asymmetry of floral organs has allowed the evolution of fascinating and often bizarre plant-insect interactions. For example, some species of orchids attract potential pollinators with insect pheromones , luring the insect into a maze constructed of fused petals and stamens in which there is one entrance and one exit. In navigating the maze, the insect both delivers to the stigma of the gynoecium pollen from another flower and picks up a load of pollen to be distributed to another flower.

Evolution of Flowers

While angiosperms are prevalent in the fossil record from the mid-Cretaceous (approximately one hundred million years ago), it is thought that they may have evolved substantially earlier, perhaps as far back as two hundred million years ago. The closest extant relatives of the angiosperms are the gymnosperms, of which conifers are members. Conifers do not have flowers but rather produce female and male cones consisting of scales bearing exposed ovules and pollen sacs, respectively. It's intriguing to consider what evolutionary processes occurred to produce the complex assemblage of floral organs of extant angiosperms. Charles Darwin, upon thinking about this question, stated that flowers are an "abominable mystery." The answer to this question, however, may be found in comparative genetic studies. For example, some of the important regulatory genes promoting floral organ development are also found in conifers. Understanding their function in the cones of conifers may allow scientists to model evolutionary changes that occurred resulting in the formation of early flowers.

It is not clear which features were present in the flowers of the earliest angiosperms. Although by the mid-Cretaceous period angiosperm flowers were already quite diverse, a number of key features of extant flowers had not yet appeared. Fossil flowers from the Cretaceous often have organs that are spirally arranged, a perianth that does not have a distinct calyx and corolla, relatively few stamens, and multiple carpels that are not fused together. The attractiveness of the mainly fly- and beetle-pollinated flowers was due to the androceium, which was composed of anthers attached to showy, leaflike structures. The stigma and style of the early individually fused carpels ran down along the side of the ovary instead of being at the top, as seen in most extant carpels.

There are a number of major evolutionary trends when comparing these Cretaceous flowers to their modern counterparts. For example, in many modern flowers the perianth is differentiated into a distinct calyx and corolla. The evolution of the corolla facilitated the reduction in stature of the androecium such that the stamens are composed of anthers attached to slender filaments rather than large, leaflike appendages. In addition, fusion of organs occurred along with the establishment of zygomorphic flowers, creating flowers with deep, open, funnel-shaped flowers, both innovations allowing the evolution of elaborate pollination strategies. Another evolutionary trend is the formation of a gynoecium made up of multiple fused carpels that have only one stigma and style situated at the apical end of the ovary. This has been hypothesized to provide selection at the level of the male gametophyte , which must grow through these structures to effect fertilization.

Coevolution with Pollinators

The early seed-producing plants (such as conifers) utilize wind to move pollen from the staminate cones to the female ovule-bearing cones. To ensure that enough viable seeds are generated, copious quantities of pollen need to be produced. This process requires the expenditure of large amounts of stored resources and is not very efficient. Utilizing insects to transfer pollen to other flowers enables angiosperms to produce less pollen and still maintain a high fecundity in comparison with wind-pollinated plants. In contrast to flowers of early angiosperms, extant flowers have evolved highly attractive characters to ensure that a specific pollinator continues to visit flowers from a specific species of plant. Flowers provide special sources of food for their pollinators to induce them to visit similar flowers. In addition to pollen and other edible floral parts, nectaries provide nectar, a high-energy food source for animals that can sometimes contain amino acids, proteins, lipids, antioxidants , and alkaloids.

Many of the modifications that have evolved in angiosperm flowers are adaptations to promote constancy in pollinator visitation. However, not all angiosperm flowers require animal pollinators. The grasses, which evolved from insect-pollinated flowers, are wind-pollinated. The flowers of grasses are small with reduced or absent petals, and they produce large amounts of pollen.

The Development of Flowers

A basic floral ground plan exists that defines the relationship between organ type and position in all angiosperm species. Because of the constancy in the relative positions of floral organ types, it is hypothesized that a common genetic program to specify floral organ identity is utilized during the development of all flowers. Floral organs are ultimately derived from primordia that arise from the flanks of the flower meristem . It is thought that cells within the flower meristem assess their position relative to other cells and differentiate into the appropriate floral organ based on this positional information.

To clarify how the identity of flower organ primordia is specified, researchers have taken a genetic approach. Mutations affecting flowers and their organs provide a powerful means for studying the genetic interactions involved in their development. Differences in the development of mutant versus normal (wild-type) plants reveal the function of the mutated gene. To carry out this work, researchers use two model plant systems, Arabidopsis (Brassicaceae) and Antirrhinum (Scrophulariaceae), and screen plants that have induced mutations in their genome . Studies have focused on a particular set of homeotic mutations. In homeotic mutants, normal organs develop in the positions where organs of another type are typically found. Specifically, the Arabidopsis floral homeotic mutations result in transformations of one floral organ type into another floral organ type.

It is of interest to note that several homeotic mutants exist in commonly cultivated garden plants. For example, a wild rose flower has five petals, and yet some of the hybrid tea roses have many times that number. Similar situations exist for camellias and carnations. These hybrid varieties represent changes in the genetic constitution of the plant to yield alterations in the floral architecture, which some people find attractive.

see also Angiosperms; Genetic Mechanisms and Development; Identification of Plants; Inflorescence; Interactions, Plant-Insect; Pollination Biology.

Stuart F. Baum

John L. Bowman

Bibliography

Bentley, Barbara, and Thomas Elias, eds. The Biology of Nectaries. New York: Columbia University Press, 1983.

Campbell, Neil A. Biology, 3rd ed. Redwood City, CA: The Benjamin/Cummings Publishing Company, Inc., 1993.

Crepet, W. L., and E. M. Friis. "The Evolution of Insect Pollination in Angiosperms."In The Origins of Angiosperms and Their Biological Consequences, eds. Else Marie Friis, William G. Chaloner, and Peter R. Crane. Cambridge: Cambridge University Press, 1987.

Heywood, V. H. Flowering Plants of the World. New York: Oxford University Press,1993.

Meyerowitz, Elliot M. "The Genetics of Flower Development." Scientific American 271 (1994): 56-57, 60-65.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. New York:Worth Publishers, Inc., 1986.

Rost, Thomas L., Michael G. Barbour, C. Ralph Stocking, and Terence M. Murphy. Plant Biology. Belmont, CA: Wadsworth Publishing Co., 1998.

273. Flowers

1. Anthea epithet of Hera, meaning “flowery.” [Gk. Myth.: Zimmerman, 121]
2. Anthesteria ancient Athenian festival, celebrating flowers and new wine. [Gk. Hist.: Misc.]
3. Black Tulip, The Dumas romance involved with tulipomania of 17th-century Holland. [Fr. Lit.: Benét, 111]
4. Chloris goddess of flowers. [Gk. Myth.: Kravitz, 59]
5. Tournament of Roses New Year’s Day flower festival and parade in Pasadena. [Am. Cult.: WB, C:45]
6. Zephyr and Flora wedded pair, scatter flowers from cornucopia. [Rom. Myth.: Hall, 125]

flowers no flowers by request an intimation that no flowers are desired at a funeral; the writer and humorist Alfred Ainger (1837–1904) used the phrase at a dinner for contributors in 1897 to sum up the principles on which the Dictionary of National Biography was being compiled.

see also hearts and flowers.

Flowers •has • Sayers •Algiers, cheers, Pamirs, Pears, Piers, Sears, Spears •Teniers •Blackfriars, Briers, pliers •Greyfriars •Bowers, Flowers, ours, Powers, Towers •bejabers • Chambers • Sobers •Scriptures • weight-watchers •glanders, Landers, Randers, sanders •alexanders, Flanders •Enders • Childers • flinders •Saunders • Bermudas • butterfingers •Tigers • Rodgers • starkers •Chequers • Snickers • camiknickers •bonkers • bluchers • Moluccas •Sellers • binoculars • Bahamas •Summers • Marianas • Connors •champers, Pampers •jeepers • jodhpurs • Messrs • Masters •Peters • squitters • Winters •headquarters, hindquarters, Waters •Klosters • Butters •Smithers, withers •Carothers, druthers •Travers • Havers • cleavers • Rivers •vivers • estovers • Marquesas