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flower, name for the specialized part of a plant containing the reproductive organs, applied to angiosperms only. A flower may be thought of as a modified, short, compact branch bearing lateral appendages. Like twigs, flowers develop from buds, and the basic floral parts (sepal, petal, stamen, and carpel) are in actual fact greatly modified leaves. A typical flower is a concentric arrangement of these parts attached at their base to the receptacle, the tip of the stem. Outermost is a whorl of leaflike green sepals (the calyx) encircling a whorl of usually showy, colored petals (the corolla). Within the corolla the stamens, bearing anther sacs full of pollen, surround the central carpels (ovary). Inside the ovary at the base of the pistil are the ovules, containing the female sex cells; after fertilization of the egg, the ovule becomes the seed and the ovary becomes the fruit. The ovary and stamens are termed essential flower parts, the petals and sepals accessory parts. The number and arrangement of the floral organs vary considerably among the many families and orders of plants and are used in the classification of plants; they also indicate the degree of evolution of the plant. In general, the higher a plant is on the evolutionary scale, the greater is the flower's complexity. The basic number of parts differs from class to class and from family to family; in monocotyledonous plants the parts generally occur in groups of three or in multiples of three, and in dicotyledons more often in groups of two, four, and five. Flowers may be staminate (lack carpels), carpellate, or both; staminate and carpellate flowers may appear on the same plant, on separate plants, or in the same inflorescence. One type of inflorescence, characteristic of the parsley family, is the umbel, in which the tiny florets are borne on separate stalks radiating out from the stem tip. Sometimes the parts serve unusual purposes: the true flowers of the dogwood and the poinsettia are inconspicuous, and the showy “petals” are really modified leaves called bracts. In the jack-in-the-pulpit the florets are clustered on a spike canopied by a large bract, the spathe; the hood of the lady's-slipper, on the other hand, is a modified sterile stamen. Grass inflorescences are tiny spikelets sheathed by protective scales called glumes (the chaff or grain). Flowers have been cultivated and bred for their beauty and their perfume from earliest times and have accumulated a vast and intricate treasury of symbolic associations derived from legend and folklore. Individual flowers have been celebrated in heraldry (rose), in religion (lotus), and in politics (violet) and have become emblems for many countries, including Switzerland (edelweiss), France (fleur-de-lis), Scotland (thistle), the Netherlands (tulip), and the United States (see state flowers).
See S. Buchmann, The Reason for Flowers (2015).
A higher plant's sexual apparatus in the aggregate, including the parts that produce sex cells and closely associated attractive and protective parts (Fig. 1). “Flower” as used in this article will be limited, as is usual, to the angiosperms, plants with enclosed seeds and the unique reproductive process called double fertilization. In its most familiar form a flower is made up of four kinds of units arranged concentrically. The green sepals (collectively termed the calyx) are outermost, showy petals (the corolla) next, then the pollen-bearing units (stamens, androecium), and finally the centrally placed seed-bearing units (carpels, gynoecium). This is the “complete” flower of early botanists, but it is only one of an almost overwhelming array of floral forms. One or more kinds of units may be lacking or hard to recognize depending on the species, and evolutionary modification has been so great in some groups of angiosperms that a flower cluster (inflorescence) can took like a single flower.
Most botanical terms are descriptive, and a botanist must have a large store of them to impart the multiformity of flowers. The examples that follow are only a smattering. An extra series of appendages alternating with the sepals, as in purple loosestrife, is an epicalyx. A petal with a broad distal region and a narrow proximal region is said to have a blade and claw: the crape myrtle has such petals. The term perianth, which embraces calyx and corolla and avoids the need to distinguish between them, is especially useful for a flower like the tulip, where the perianth parts are in two series but are alike in size, shape, and color. The members of such an undifferentiated perianth are tepals. When the perianth has only one series of parts, however, they are customarily called sepals even if they are petallike, as in the windflower.
A stamen commonly consists of a slender filament topped by a four-lobed anther, each lobe housing a pollen sac. In some plants one or more of the androecial parts are sterile rudiments called staminodes: a foxglove flower has four fertile stamens and a staminode. Carpellode is the corresponding term for an imperfectly formed gynoecial unit.
A gynoecium is apocarpous if the carpels are separate (magnolia, blackberry) and syncarpous if they are connate (tulip, poppy). Or the gynoecium may regularly consist of only one carpel (bean, cherry). A solitary carpel or a syncarpous gynoecium can often be divided into three regions: a terminal, pollen-receptive stigma; a swollen basal ovary enclosing the undeveloped seeds (ovules); and a constricted, elongate style between the two. The gynoecium can be apocarpous above and syncarpous below; that is, there can be separate styles and stigmas on one ovary (wood sorrel).
Every flower cited so far has a superior ovary: perianth and androecium diverge beneath it (hypogyny). If perianth and androecium diverge from the ovary's summit, the ovary is inferior and the flower is epigynous (apple, banana, pumpkin). A flower is perigynous if the ovary is superior within a cup and the other floral parts diverge from the cup's rim (cherry). A syncarpous ovary is unilocular if it has only one seed chamber, plurilocular if septa divide it into more than one. The ovules of a plurilocular ovary are usually attached to the angles where the septa meet; this is axile placentation, a placenta being a region of ovular attachment. There are other ways in which the ovules can be attached—apically, basally, parietally, or on a free-standing central placenta—each characteristic of certain plant groups (Fig. 2).
The term bract can be applied to any leaflike part associated with one or more flowers but not part of a flower. Floral bracts are frequently small, even scalelike, but the flowering dogwood has four big petallike bracts below each flower cluster. The broad end of a flower stalk where the floral parts are attached is the receptacle. The same term is used, rather inconsistently, for the broad base bearing the many individual flowers (florets) that make up a composite flower like a dandelion or a sunflower.
A plant species is diclinous if its stamens and carpels are in separate flowers. A diclinous species is monoecious if each plant bears staminate and carpellate (pistillate) flowers, dioecious if the staminate and carpellate flowers are on different plants. The corn plant, with staminate inflorescences (tassels) on top and carpellate inflorescences (ears) along the stalk, is monoecious. Hemp is a well-known dioecious plant.
Flowers pollinated by insects or other animals commonly have one or more nectaries, regions that secrete a sugar solution. A nectary can be nothing more than a layer of tissue lining part of a floral tube or cup (cherry), or it can be as conspicuous as the secretory spur of a nasturtium or a larkspur. It can be a cushionlike outgrowth at the base of a superior ovary (orange blossom) or atop an inferior ovary (parsley family). Gladiolus and a number of other monocotyledons have septal nectaries, deep secretory crevices where the carpels come together. Substances that give off floral odors—essential oils for the most part—ordinarily originate close to the nectar-producing region but are not coincident with it. Production by the epidermis of perianth parts is most common, but in some species the odor emanates from a more restricted region and may even come from a special flap or brush. Most insect-pollinated plants have visual cues, some of them outside the human spectral range, as well as odor to bring the pollinators to the flowers and guide them to the nectar. See Secretory structures (plant)
Inflorescence structure, the way the flowers are clustered or arranged on a flowering branch, is almost as diverse as floral structure. To appreciate this, one need only contrast the drooping inflorescences (catkins) of a birch tree with the coiled flowers of a forget-me-not or with the solitary flower of a tulip. In some cases one kind of inflorescence characterizes a whole plant family. Queen Anne's lace and other members of the parsley family (Umbelliferae) have umbrellalike inflorescences with the flower stalks radiating from almost the same point in a cluster. The stalkless flowers (florets) of the grass family are grouped into clusters called spikelets, and these in turn are variously arranged in different grasses.
Flowers of the arum family (calla lily, jack-in-the-pulpit), also stalkless, are crowded on a thick, fleshy, elongate axis. In the composite family, florets are joined in a tight head at the end of the axis; the heads of some composites contain two kinds, centrally placed florets with small tubular corollas and peripheral ray florets with showy, strap-shaped corollas (the “petals” one plucks from a daisy). See Inflorescence
Some of the general anatomical features of leaves can be found in the floral appendages. A cuticle-covered epidermis overlies a core of parenchyma cells in which there are branching vascular bundles (solitary bundles in most stamens). Sepal parenchyma and petal parenchyma are often spongy, but palisade parenchyma occurs only rarely in flowers and then only in sepals. As in other parts of the plant, color comes mostly from plastids in the cytoplasm and from flavonoids in the cell sap. Cells of the petal epidermis may have folded side walls that interlock so as to strengthen the tissue. In some species the outer walls of the epidermis are raised as papillae; apparently, this is part of the means of attracting pollinators, for the papillae are light reflectors.
As a stamen develops, periclinal divisions in the second cell layer of each of its four lobes start a sequence that will end with the shedding of pollen. The first division makes two cell layers. The outer daughter cells give rise to the wall of the pollen sac, and the inner ones are destined to become pollen after further divisions. When mature, a pollen sac typically has a prominent cell layer just below a less distinctive epidermis. The inner wall and the side walls of an endothecium cell carry marked thickenings, but the outer wall does not. Splitting of the ripe anther is due partly to the way in which these differentially thickened walls react to drying and shrinking and partly to the smaller size of the cells along the line of splitting. See Pollen
Like other floral parts, a carpel is made up of epidermis, parenchyma, and vascular tissue. In addition, a carpel commonly has a special tissue system on which pollen germinates and through which, or along which, pollen tubes are transmitted to the ovules. Most angiosperms have solid styles, and the transmitting tissue is a column of elongate cells whose softened walls are the medium for tubal growth. The epidermis at the stigmatic end of a carpel usually changes to a dense covering of papillae or hairs; the hairs can be unicellular or pluricellular, branched or unbranched. In taxa with hollow styles, the transmitting tissue is a modified epidermis running down the stylar canal. There are two kinds of receptive surfaces, and they are distributed among the monocotyledons and the dicotyledons with taxonomic regularity. One kind has a fluid medium for germinating the pollen, and the other has a dry proteinaceous layer over the cuticle. The proteins of the dry stigmas have a role in the incompatibility reactions that encourage outbreeding.
Ovule development usually takes place as the gynoecium forms, but it may be retarded when there is a long interval between pollination and fertilization (oaks, orchids). A typical ovule has a stalk (funiculus), a central bulbous body (nucellus), and one or two integuments (precursors of seed coats), which cover the nucellus except for a terminal pore (micropyle). Orientation of the ovule varies from group to group. It can be erect on its stalk or bent one way or another to differing degrees. There are also taxonomic differences in the extent to which the ovule is vascularized by branches from the gynoecial vascular system. See Fruit, Reproduction (plant)
a short, limited-in-growth reproductive shoot that performs the functions of microsporogenesis, megasporogenesis, pollination (effected by pollen carrier-agents), fertilization, embryonic development, and fruit formation. The flower is borne at the end of a main or lateral stem. The peduncle, the part of the stem that bears the flower, is usually leafless. Often it is very short and barely visible; in this case the flower is said to be sessile, or attached. The peduncle becomes the short axis of the flower, which is called the flower receptacle, or torus. The variously shaped receptacle is usually broader than the peduncle and is characterized by barely noticeable internodes. Its nodes bear all the various parts of the flower: the sterile sepals and petals and the fertile stamens and carpels.
The sepals, which together form the calyx, usually serve as protective organs of the flower, especially before blooming; they often also function as supplementary organs of photosynthesis. The sepals often become petallike, taking on the functions of petals (for example, in Clematis). In some instances, through the process of evolution, the sepals have concresced into a synsepalous calyx. The petals, which serve to attract pollinators, are collectively known as the corolla. The calyx and the corolla together make up the perianth. When the flower has both a calyx and a corolla, the perianth is said to be double. A simple perianth is one in which the petals are absent or in which there are no marked differences between the calyx and the corolla.
In the center of the flower are located the stamens and carpels. The stamens are microsporophylls both in function and in origin. The most primitive ones, as seen in a number of representatives of the families Magnoliaceae and Winteraceae, consist of a broad ribbonlike membrane equipped with four linear microsporangia that are approximate in pairs and that are more or less embedded in the tissue of the membrane. In the course of evolution the stamens gradually became differentiated into the filament and the anther, which consists of two halves joined by a continuation of the filament. The stamens are known collectively as the androecium. The filaments often concresce together or with elements of the perianth or carpels. Sometimes the stamens concresce together by both the filaments and anthers or by the anthers only.
The carpels consist of highly modified and specialized megasporophylls. A single carpel is often called a simple pistil (the term “pistil” is used in various senses, often creating confusion). The most primitive type of carpel is retained in the genera Degeneria (family Degeneriaceae) and Tasmania (family Winteraceae). It consists of a short stalk and a relatively thin adaxially folded (conduplicate) membrane; externally it strongly resembles the conduplicate leaves folded into the bud. Along the carpel pass three conducting bundles (veins), of which the middle or dorsal one branches profusely. Between the middle and lateral veins are numerous ovules. In the early stages of development the margins of the carpels of Degeneria almost do not touch each other; protection of the ovules during flowering is ensured by the proximity of the broad lateral parts of the carpel, which are located between the carpel’s margins and placentas. These approximate internal surfaces concresce only after flowering, that is, during fruit development. The most diverse stages of concrescence are observed in other representatives of Winteraceae.
In all flowering plants the carpel has a special glandular tissue, called the stigma, that serves to capture pollen. In Degeneria and Tasmania the stigma is primitive and extends the entire length of the carpel. The upper part of the carpel usually extends into a slender, sterile columnar process, which supports the stigma above the ovary—the fertile part of the carpel—and serves as a passageway to the pollen tube. This specialized process is called the style or the stylodium (in which case the term “style” is only used in reference to concresced stylodia). In the early stages of evolution, the style is characterized by a descending stigma; in later stages the stigma gradually becomes localized at the top.
The aggregate of carpels in the flower is called the gynoecium. The most primitive types of gynoecia with free carpels are called apocarpous; such gynoecia characterize Magnolia, Ranunculus, and Paeonia. In the process of evolution the carpels gradually concresced among themselves, and from the apocarpous gynoecium there emerged the coenocarpous gynoecium, in which the stylodia are free, or concresced to form the style. The evolution of both the coenocarpous and apocarpous gynoecia was usually accompanied by a decrease in the number of carpels and, in most cases, in the number of ovules. An extreme form is the pseudomonomerous gynoecium, in which a single carpel is fertile and completely developed (as seen in elm, mulberry, nettle, and hemp).
Most flowers have nectaries—special excretory organs for the production of nectar. Nectaries originated in the most diverse forms in the evolution of flowering plants and on the most diverse morphological bases, that is, on the most diverse parts of the flower.
Primitive types of flowers are characterized by a relatively long receptacle, but in some cases, for example, in Myosurus and some Magnolia, there is a greatly elongated receptacle of secondary origin. In the process of evolution there has been a gradual shortening of the receptacle, as a result of which the flower parts have grown closer together and the initial spiral formation has changed to a cyclic one. The parts of a cyclic flower are usually arranged in whorls; less commonly, they are opposite. Cyclic flowers are found more frequently than spiral ones. In an intermediate position are spirocyclic flowers, such as those of Ranunculus.
Primitive flowers are actinomorphic (regular); that is, they can be divided by a vertical plane passing through the symmetrical axis into equal halves in no less than two directions. Owing to the biology of pollination, actinomorphic flowers gave rise to zygomorphic (irregular) ones, which can be divided into two halves only in a single direction (for example, the blossom of the sweet pea or snapdragon). In contrast to actinomorphic and zygomorphic flowers, which are both symmetrical, there are also asymmetrical flowers, which cannot be divided into two equal halves by a plane of symmetry (for example, cannas).
Flowers may be bisexual or unisexual. Unisexual flowers originated from bisexual ones as a result of the reduction of the stamens in some cases and of the carpels in others. Unisexual flowers often retain the rudiments of stamens and carpels. It is conjectured that the flower is descended from the reproductive shoots of the supposed ancestors of flowering plants—seed ferns. As a result of shortening, the shoot axis was converted into a receptacle. Simplification of the segmented microsporophylls and megasporophylls resulted in the simple unbranched stamens and carpels of flowering plants. (The hypothesis that the flower is descended from the strobile of Cycadaceae, Gnetaceae, or Bennettitaceae is improbable.)
The evolution of the flower, like the evolution of the strobiles of various groups of gymnosperms, set off in various directions. Thus, current theories are based on the proposition that the fundamental parts of the flower—the stamens and carpels—are descended from the sporebearing organs of the ancestors of flowering plants. It is difficult to say with certainty whether these primitive spore cases were somewhat rounded or flat or whether they were achlorophyllous or green. It is most probable that they consisted of flat and more or less simplified, possibly lobular, sporophylls, which, especially if they were megasporophylls, probably partially performed the function of photosynthesis.
REFERENCESKerner von Marilaun, A. Zhizn’ rastenii, vol. 2. St. Petersburg, 1900. (Translated from German.)
Golenkin, M. I. Kurs vysshikh rastenii. Moscow-Leningrad, 1937.
Eames, A. Morfologiia tsvetkovykh rastenii. Moscow, 1964. (Translated from English.)
Takhtadzhian, A. L. Osnovy evoliutsionnoi morfologii pokrytose-mennykh. Moscow-Leningrad, 1964.
Pervukhina, N. V. Problemy morfologii i biologii tsvetka. Leningrad, 1970.
Esau, K. Anatomiia rastenii. Moscow, 1969. (Translated from English.)
Fedorov, A. A., and Z. T. Artiushenko. Atlas po opisatel’noi morfologii vysshikh rastenii. [Vol. 3:] Tsvetok. Leningrad, 1975.
A. L. TAKHTADZHIAN
What does it mean when you dream about a flower?
Flowers are natural symbols of beauty, delicacy, harmlessness, and attraction (e.g., the attraction of bees to nectar). Flowers are also symbols of the deep self. In both southern Asian yoga systems and the Western esoteric tradition, flowers represent the psychic centers referred to as chakras (“wheels”). The expression “spiritual unfoldment” is a flower-related image. Finally, the dreaming mind often literalizes common verbal expressions—such as “wallflower” and “flower power”—in an effort to convey something to the conscious mind.