The flower is the reproductive structure of angiosperms, or flowering plants. Compared to the reproductive structures of other plants, the flower is unique in several ways. It consists of four kinds of modified leaves, two of which (stamens and carpels, the latter sometimes called pistils) bear POLLEN and seeds. Several nonflowering plants also produce pollen and seeds on modified leaves, but in angiosperms the modified leaf called the carpel forms an ovary that completely encloses the ovule, which becomes the seed. In the gynosperms, ovules are borne on open modified leaves, such as the scale of a pinecone. The term angiosperm, derived from the Greek, means "seed in a vessel." Gymnosperm means "naked seed." According to the fossil record, flowering plants appeared only about 140 million years ago, although some recently found fossil evidence suggests that they appeared 80 million years before that. (The earliest land plants, blue-green algae, appeared perhaps 1.2 billion years ago.) The angiosperms now dominate the world's vegetation. Only the gymnosperms offer any substantial competition. There may be more than 250,000 angiosperm species, compared to fewer than 1,000 gymnosperm species and fewer than about 40,000 other types of vascular plants (ferns and their relatives) and bryophytes (liverworts, mosses, hornworts). There are fewer than 15,000 species of algae, and perhaps more than 100,000 species of fungi and bacteria.

In modern classification systems, FUNGI, are not considered plants but form a kingdom of their own, and the blue-green ALGAE (cyanobacteria; 1,500 species) and the BACTERIA (1,500 species) also form a separate kingdom. Except for the mosses, ferns, and conifers, most of the plants encountered on land are angiosperms. They predominate in the vegetation of the grasslands, deciduous forests, tropical rain forests, shrubby chaparral, deserts, and tundras.

More than any other of the major plant groups, flowering plants are ecologically related to animals. Modern animals, including humans, and flowering plants are equally dependent upon each other. Most flowering species rely on animals for reproduction. Insects carry pollen from the stamens to the carpels; bats and birds participate in POLLINATION of some species. The dispersal and growth of the seeds are further ensured by animals attracted to their colorful and aromatic flowers and tasty fruits. Many fruits and seeds (the exclusive products of angiosperms) are also collected and consumed by humans, and the seeds are planted in extensive systems of agriculture. Almost all plants used in agriculture are angiosperms. (Mushrooms, fern fiddleheads, and pine nuts are exceptions.) In another relationship between plants and animals, only the special growing cells at the base of a grass (angiosperm) leaf seem well-adapted to animal grazing.

STRUCTURE OF FLOWERS

Four kinds of modified leaves make up a complete flower: carpels and stamens (primary reproductive structures) and petals and sepals (secondary structures). The carpel is the female reproductive structure. It has a stigma, where the pollen becomes attached and germinates; a style, through which the pollen tube grows; and an ovary with one or more ovules. The egg cell that will unite with the sperm cell (delivered by the pollen tube) forms in the ovule. The stamen is the male structure; its filament supports an anther, in which the pollen is formed. The often brightly colored petals are important in attracting pollinators, and the often leaflike sepals enclose the bud before the flower opens. The many species of flowering plants are usually distinguished from one another by the way these four basic flower parts are modified, although closely related species within a genus may have quite similar flowers.

Some flowers have only one carpel, others have two or a few, and still others have many. Several carpels in a single flower may be separate or fused together. If fused, they may be joined only at the ovaries or along their entire length. The ovary may contain one to many ovules, and these may be arranged in various ways. Frequently, the ovaries are attached to the receptacle (the end of the stem, or peduncle, that supports the flower parts) at the same level as the other flower parts, in which case the ovary is said to be hypogynous (or superior). In some cases the other flower parts are attached above the ovary, which is then said to be epigynous (inferior). In the rose family and some of its relatives, the stamens, petals, and sepals are attached around the ring of a cup with the ovaries at the bottom of the cup (perigynous).

Stamens also vary in several ways, although not as markedly as ovaries. Classification schemes often depend on the number of stamens in a given flower and whether they are attached oppositely or alternately with the petals.

The petals together form the corolla, with numerous and often beautiful forms. Besides the number of petals, two other important variations occur. First, petals may be separately attached to the receptacle, or they may be united along their edges to form a tube. Second, the corolla may be radially symmetrical, with petals radiating out in all directions from the center of the flower (as in a buttercup, geranium, lily, or rose), or some petals may have shapes different from others, so that the flower has dorsiventral symmetry--in which a vertical plane divides the flower into two equal, mirror-image halves (as in snapdragon, honeysuckle, or orchid).

Many flower petals have patterns of pigment that absorb only in the ultraviolet part of the spectrum. Insects, which have eyes that are sensitive to ultraviolet light, see patterns on the flower that are not visible to humans. These patterns frequently consist of radiating lines that lead the insect to the nectar. A few flowers (for example, clematis) have no true petals but do have colorful sepals.

If a flower lacks any of the four basic parts, it is called incomplete. If it lacks one of the essential reproductive parts (stamens or carpels), it is called imperfect. Thus, flowers that have both stamens and carpels but lack petals or sepals are perfect incomplete flowers. Imperfect flowers can be male or female. If male and female flowers occur on the same plant, the plant is called monoecious; if male and female flowers are on separate plants, it is dioecious. Maize (corn) is a monoecious plant, with its tassels (stamens) at the top and its ears (carpels) on the stem below. Cottonwoods are dioecious-- the male trees produce pollen, and female trees produce seeds.

In most angiosperms, pollen is transferred by insects. Insect- pollinated flowers often have rather showy corollas, which are often modified to ensure the dusting of pollen onto the insects as they penetrate the flowers in search of nectar. The dusted insects transfer the pollen to the stigma of the next flower they enter. Flowers pollinated by moths, hummingbirds, or bats may have specialized corollas that match the appropriate organs of the animals seeking the nectar.

In some major groups, pollen is transferred by the wind. Some species of wind-pollinated flowers are not at all showy, with the anthers suspended on long filaments so that the pollen dusts freely into the wind. The pollen grains may be winged, which allows them to be carried more easily on the breezes. Styles and stigmas may also extend some distance from the flower, to catch the blowing pollen. Sepals and petals may be either absent or quite small. Grasses, which are some of the most successful plants, are wind-pollinated, as are many trees- -for example, maples, oaks, and walnuts.

A few species, including such important crops as wheat, rice, barley, oats, and peas, are self-pollinated. The pollen is transferred directly from stamens to carpels. Such species naturally maintain their genetic purity. To produce new hybrids, cross-pollination must be carried out manually. Some flowers (dandelion, hawkweed, certain grasses) do not require pollination to produce seed. Certain cells in the ovule other than the egg cell develop into seeds in the process called apomixis.

FLOWER ARRANGEMENT ON THE PLANT

A group of flowers on a plant is called an inflorescence. A great variety of inflorescences occur among the angiosperms. The simplest is a single, solitary flower at the end of a stem, with leaves at the base. It is rare for an entire plant to have a single flower, as is true of the tulip; but a solitary, terminal flower at the end of the main stem, with axillary flowers in the angles between leaves and stems, is common.

A number of flowers radiating along a single stem, usually with modified leaves (bracts) at the base of the peduncles, is a raceme. Most racemes are indeterminate, meaning that the younger flowers are at the tip of the stem in the center of the raceme. A spike occurs when the flowers in the raceme are attached closely to the main stem. For example, a head of wheat is spiked, as are virtually all grass flowers. A compound raceme with several branching stems, each forming a raceme, is called a panicle. In a small number of species, the oldest flowers may occur near the stem tips of a raceme or panicle; this determinate structure is a cyme. When all the peduncles of several flowers in an inflorescence radiate from the same point, they form a flattopped, or sometimes rounded, umbel.

Flowers densely packed together on short peduncles and a short main axis form a head; clover is an example of this formation. The most common flower heads occur in the large aster or sunflower (composite) family. In a sunflower or daisy, two kinds of flowers occur in the head: ray or strap flowers, which consist of one long petal with an ovary and sometimes stamens; and disk flowers, which consist of five greatly reduced, radially symmetrical petals at the tips of a corolla tube, plus an ovary and, usually, stamens. The sepals in a composite flower head may have been modified to form filaments, such as the parachute on a dandelion seed. Two other special inflorescences are the catkins, rather loose, hanging spikes of flowers occurring on birch and other trees; and the spadices, which are spikes of male flowers above female flowers surrounded by large, sometimes colored leaves called spathes, as on calla lily.

THE SEED AND THE FRUIT

The products of the flower are the seed and the fruit. The seed is the mature ovule. It includes a minute embryo plant and, almost always, stored food that will supply the seedling when it begins to grow after sprouting, or germination. Important seeds that humans eat include the cereals, such as wheat, rice, maize; legumes, such as peas, beans, lentils, peanuts; and nuts. Many seeds are rich in fats (including oils), a concentrated form of energy, and are of great commercial importance--for example, soybeans, cottonseed, coconuts, peanuts, rapeseed, sunflower, and linseed (flax). The cereals store mostly carbohydrate, and many legumes store much protein along with carbohydrate and, often, fat.

In a restricted botanical sense, the fruit is the mature ovary wall, but often food is stored in accessory tissues besides the ovary wall.

FLOWERING TIME

Some plants, called annuals, germinate from seed and then flower and die within one year. Winter annuals may germinate in late autumn, live through the winter as slow-growing seedlings under the snow, and grow and flower in spring or early summer. Many cereals are winter annuals, but often a single species has winter-annual and spring-annual varieties or cultivars (agricultural varieties), as is the case with barley, rye, and wheat. Biennials typically germinate in the spring, grow as a rosette--a circle of leaves close to the ground, as in beets or dandelion--during the first summer, and send up a flowering shoot during the second season. Perennials, which grow and flower for several seasons, are called polycarpic. Monocarpic plants are those that flower only once and then die. These include annuals and biennials but also a few species such as bamboo and the century plant (Agave) that grow for several years, flower once, and then die.

The variety of flowering plants is enormous. Some angiosperm trees challenge the great conifers in size, while some floating flowering plants (Lemna) are smaller than a fingernail. Orchids grow suspended on the trunks of trees in tropical rain forests, and the sausage tree has a huge hanging flower pollinated by bats. Cacti and yucca plants have needles or swords for protection, and some angiosperms trap and consume insects. A few species of flowering plants grow during a short season in Antarctica; others grow near the tops of all but the highest mountains.

HOW FLOWERS FORM

Although a vast body of descriptive data is available on plant development, many problems remain unsolved. In both plants and animals the cells divide and multiply, under normal conditions, at the right time and place. A precise schedule for development is followed, and the role of genetic material in determining this process is acknowledged; however, the nature of the mechanism of development is not fully understood.

The formation of flowers in plants provides an appropriate model for the study of development. The plant stem grows by the division of a group of cells near the tip of the stem, with a few other dividing cells scattered in the stem down to a few centimeters below the tip. Regions of active cell division and growth in plants are called meristems. When a tree grows, a layer of meristem cells, called the cambium, forms between the bark and the wood. As these cells divide and grow, wood is produced on the inside and bark on the outside.

At some time during the life of the plant the meristems at the stem tips or in the lateral buds stop producing more stems and more leaves and produce flowers instead. Frequently, this occurs in response to changes in the environment. It is as if an environmental signal detected by the plant is translated into a signal within the plant, and this causes buds to grow into flowers instead of into stems and leaves.

In the early part of the 20th century it was thought that flowering resulted from a balance in internal nutrients. This continues to be a valuable concept with a few species. Tomato plants, for example, flower more and bear more fruit when nitrogen fertilizers are withheld after they are mature. Fruit trees respond in much the same way, but many plants flower more instead of less when nitrogen is increased. Other factors have proved to be far more important.

   Temperature and Light

Winter annuals and biennials form flowers in response to the low temperatures of winter. This phenomenon was noted in the early 19th century, but it was not until the early 20th century that it was first documented and recognized by scientists. Many species of plants are induced to flower by several days to weeks of temperatures close to or just above the freezing point of water. The flowering of summer vegetables is promoted by a brief exposure to lower temperatures; a few perennials also respond this way.

Light also variously affects flowering plants. A few species seem to flower in response to increased light levels; others respond to dimmer light. However, most species respond not to the brightness of light but to its duration, or the duration of the dark period (night), or to a combination of both.

In 1920, W. W. Garner and H. A. Allard reported that Maryland mammoth tobacco plants remained vegetative in the fields during the summer but flowered profusely in the winter greenhouse. They tested several different environmental factors, one of which was the length of day. When their test plants were placed in cabinets in midsummer at about 4:00 PM and removed at 8:00 AM the next morning, the plants flowered profusely, just as they had in the winter greenhouses.

Garner and Allard found that several species responded to short days (or, as later studies would suggest, long nights), and they called these short-day plants. Examples are cocklebur, chrysanthemum, poinsettia, and morning glory. Other species had the opposite response: they would bloom when the days got longer. These were called long-day plants. Beets, dill, Darnel ryegrass, spinach, henbane, radish, a tobacco species, and various cereals are good examples. The flowering time of a few species such as tomato, cucumber, globe amaranth, sunflower, a tobacco species, and garden pea, seems to be unaffected by day length, although the number or size of flowers or fruit set may be influenced strongly. Garner and Allard called this phenomenon photoperiodism.

   Photoperiodism

In the 1930s, in the USSR, Mikhail Chaila-khyan noted that the leaf was the part of the plant that responded to the length of day or night. If the leaf of a short-day plant is covered with a black bag, for example, the plant will flower, even though the stems and the buds (which will become flowers) remain under long-day conditions. Long-day plants will not flower when their leaves are covered long enough to give the leaf only short-day conditions.

Thus the leaf detects the day length and sends a signal to the bud, where flowers actually form. It is conceivable that this signal is an electrical, or nervous, impulse, but it seems much more likely that it is a chemical substance, or hormone. For one thing, the signal moves quite slowly--only a few centimeters per hour--which seems far too slow for an electrical stimulus. Another evidence for a hormone is that plants that have been induced to flower by the proper environmental treatment can be grafted onto plants that have been maintained in a vegetative condition. This causes the vegetative plants to flower, even if they do not experience the environmental conditions normally required to induce flowering.

Individual species have specific requirements for a minimum light or dark period. Cocklebur, a short-day plant, requires a minimum of about 8.5 hours of darkness to induce flowering (the critical night). The plant flowers maximally with 12 to 16 hours of darkness. Henbane, a long-day plant, requires more than about 12 hours of light to produce flowers.

In 1938, Karl Hamner and James Bonner gave cocklebur plants a 16-hour night and interrupted it briefly with light after about 8 hours. The plants responded as though they were experiencing long days instead of short days, remaining vegetative. This discovery of the night-interruption phenomenon opened up several avenues for future research. The intensity of light required to produce this effect varies from species to species. The light of the full Moon can be slightly effective in certain species if they are exposed to it for the entire night.

Orange-red wavelengths are the most effective in producing this phenomenon. It was discovered in the early 1950s that the effects of a night interruption with orange-red light can be almost completely reversed if that exposure is followed by one to far-red wavelengths. Thus, if a cocklebur plant is given a 16-hour dark period that is interrupted after 8 hours with orange-red light, it remains vegetative; a subsequent exposure to far-red leads again to flowering--unless this is in turn followed by orange-red light. The last exposure determines the response.

The discovery of the night-interruption phenomenon was extremely significant because it was found that many plant responses follow part of the same pattern. If lettuce seeds absorb water and are then exposed to orange-red light, they germinate; if they are later exposed to far-red light, they do not. Orange-red light also causes dark-grown stems to stop elongating, leaves to expand, hooks on seedlings to unfold, apple skins to turn red, and other phenomena to occur. In each case, subsequent exposure to far-red light reverses the effect.

It was postulated that a plant pigment exists that is converted from one form to another by orange-red light and back to the original form by far-red light. This pigment was called phytochrome, and in 1959 this important protein was first extracted from plant tissues. Phytochrome seems to be the means by which the plant "knows" whether it is in the light or the dark. Most light sources, including sunlight, act primarily as orange-red sources. During the 1970s and 80s much was learned about the action of phytochrome in flowering. It now seems clear that two kinds of phytochrome play separate roles in the flowering process. Time measurement, however, consists of much more than phytochrome.

   Biological Clock

Virtually all living organisms (except perhaps the prokaryotes- -bacteria and blue-green algae) have a BIOLOGICAL CLOCK. In plants this can be studied not only in the phenomenon of photoperiodism but in such phenomena as the diurnal movements of leaves. A bean plant, for example, has its leaves in a nearly horizontal position at noon and in an almost vertical position after midnight. These leaf movements continue to occur even when plants are placed under constant conditions of light and temperature, receiving no obvious external cues about the daily environmental cycle.

Actually, under such conditions, rhythms of both plants and animals tend to run either fast or slow (usually slow), often as much as an hour or more each day. After just a few days the leaf position of the bean plant is far out of phase with the daily cycle going on outside the laboratory; leaves may be in the midnight position at noon. This is almost conclusive evidence that organisms have an internal clock capable of measuring time, although not very accurately. In the natural environment, accuracy is not necessary because the clock is reset by the daily rising and the setting of the Sun.

Flower formation as exhibited in photoperiodism is an excellent example of both phytochrome action and the biological clock in plants.

THE CLASSIFICATION OF FLOWERING PLANTS

Some of the earliest systems of plant classification were highly artificial. Plants were not grouped according to genetic relationship but with respect to certain arbitrary features, such as number of stamens. One artificial ecological scheme might place all the plants of the prairie in one group and desert plants in another. A useful and long-used artificial approach is to classify plants as herbs, shrubs, or trees. Taxonomists seek to develop a natural system in which species are related to one another on the basis of common ancestry in their evolutionary development.

Within the angiosperms, there are clearly two groups of plants: the Monocotyledonae and the Dicotyledonae, usually called monocots and dicots. (The COTYLEDON is the embryonic seed leaf.) Examples of the monocots are lilies, rushes, sedges, grasses, irises, orchids, and palm trees. Dicots include honey- suckles, sunflowers, buttercups, roses, all deciduous broad- leaf trees except the ginkgo, mustards, mallows, primroses, phloxes, snapdragons, mints, goosefoots, and geraniums.

Monocots are characterized especially by having only one cotyledon (often not easy to distinguish) as part of the embryo in the seed; the dicots have two cotyledons (for example, the two halves of a bean seed). The veins in a monocot leaf are typically parallel; those in a dicot leaf typically form a network. Monocot flower parts usually occur in threes; dicot flower parts occur in twos, fours, or usually fives. Monocot stems have their vascular bundles (groups of transporting cells) enclosed in a sheath of cells and scattered in a matrix of pith cells; dicot stems have vascular bundles arranged in a ring, and often there is a cambium (layer of dividing cells) that allows the stem to become thicker in diameter from year to year. Monocot stems, even palm trees, do not grow in thickness from year to year. Dicots often have a branching taproot, while monocots typically have a fibrous root system. A few exceptions occur in each of these generalizations, but for the most part it is easy to distinguish between monocots and dicots by keeping all the characteristics in mind.

One problem in angiosperm taxonomy concerns the manner in which the genera are arranged in relation to one another. It is a convention of both botanists and zoologists that groups of genera should be classified into families, groups of families into orders, groups of orders into classes, groups of classes into divisions (phyla in zoology), and groups of divisions into kingdoms. Sometimes subclasses, suborders, and other subdivisions are also used.

Taxonomists also disagree as to whether the angiosperms should be considered a division, a class, or a subclass; so even at this level problems remain to be resolved. A few taxonomists consider the dicots and the monocots to represent classes (if the angiosperms form a division), but most taxonomists call the dicots and the monocots subclasses. One system considers them a series between subclasses and orders. All taxonomists agree, however, that the monocots and dicots form logical groups within the angiosperms. There is also general agreement about the arrangement of monocot orders.

The question of how to arrange dicot orders is not entirely resolved. Two systems exist that date almost from the time of Carolus LINNAEUS (1753). One of these assumes that petal structure is especially important. Those plants having united petals would form one major group within the dicots; plants with separate petals would form another group. Within these groups some orders are thought to be more primitive than others in the evolutionary sequence of plants. The second system assumes that ovary position is the most important criterion. Those dicots having superior ovaries would form one evolutionary sequence, while those orders with inferior ovaries would form another line. At present most taxonomists feel that the second system comes the closest to expressing a natural classification scheme. In this scheme united petals with irregular corollas occur in both major lines of evolution and thus would be examples of parallel evolution, having originated independently in each line. In the other scheme inferior ovaries would be an example of parallel evolution.

There is some disagreement about the number of families of flowering plants, but one authority lists a total of 306 families: 249 dicot and 57 monocot families. Nearly half of the species of angiosperms occur in 19 families, which are listed approximately in the order of their size (largest first): aster, orchid, pea, grass, gardenia (coffee, quinine), spurge, sedge, lily, rose, mint, verbena, snapdragon, mustard, myrtle, fig (hemp, mulberry), heath, carrot, potato, and palm.

THE IMPORTANCE OF FLOWERS

Flowers and flowering plants form an exceedingly important part of nature. They also provide color to the surroundings--not only the flowers themselves but also the brilliant colored leaves that cover the hillsides during the autumn in temperate regions. Gymnosperms, which were predominant until the angiosperms appeared 140 million years ago, have no colorful flowers; a few exhibit autumn colors.

Flowers are used in plantings and in many ceremonies; their seeds, fruits, roots, stems, and leaves also provide food, as has been noted in several examples. In turn, most of the animals that humans eat depend upon flowering plants for their food supply.

Frank B. Salisbury