Britannica Concise 2005

placer mining Oldest method of recovering gold from alluvial depos¬ its. It takes advantage of gold’s high density, which causes it to sink more rapidly from moving water than the lighter siliceous materials with which it is found. Panning, used by 19th-century miners, employed a pan in which a few handfuls of the gold-bearing soil or gravel and a large amount of water were placed; by swirling the pan’s contents, the miner washed the siliceous material over the side, leaving the gold and heavy materials behind. Dredging is the most important placer-mining method today. Used worldwide is the bucket-ladder dredge, with its continuous chain of buck¬ ets rotating around a rigid adjustable frame called the ladder. In sluicing, a slightly sloping wooden trough called a box sluice, or a ditch cut in hard gravel or rock called a ground sluice, is used as a channel along which gold-bearing gravel is carried by a stream of water. Riffles placed trans¬ versely along the bottom of the sluice cause the water to eddy into small basins, retarding the current so that gold may settle.

Placid, Lake See Lake Placid

plagioclase \ , pla-je-o- l klas\ Any member of the series of abundant feldspar minerals that usually occur as light- to medium-grey-coloured,

transparent to translucent grains or crystals. Plagioclase ranges in com¬ position from albite to anorthite. It is used in the manufacture of glass and ceramics; iridescent varieties are valued as gemstones. The primary importance of plagioclase, however, derives from its role as a rock¬ forming mineral.

plague \'plag\ Infectious fever caused by the bacterium Yersinia pestis , carried by the rat flea. It usually spreads to humans only when the flea runs out of rodent hosts. It takes three forms. Bubonic, the mildest, has characteristic swollen lymph nodes (buboes) and is spread only by the flea. It accounts for three-fourths of plague cases. Pneumonic plague has exten¬ sive lung involvement and is spread in droplets from the lungs; it is often fatal in three or four days without treatment. In septicemic plague, bac¬ teria overwhelm the bloodstream and often cause death within 24 hours, before other symptoms have a chance to develop. In the 14th century, plague ravaged Europe and Asia and was called the Back Death. Plague does not respond to penicillin, but other antibiotics are effective. Sanitary measures against fleas and rodents, quarantine, and extreme caution in handling infectious materials help to suppress epidemics. A vaccine can prevent plague.

plaice Commercially valuable European fatfish ( Pleuronectes platessa). At most 36 in. (90 cm) long, the plaice normally has both eyes on the right side of the head and four to seven bony bumps near its eyes. It is brown with red or orange spots. The American plaice ( Hippoglossoides platessoides) is found in both Europe (where it is called the rough dab) and the U.S. It is reddish or brownish and grows to about 24 in. (60 cm) long.

plain Any relatively level area of the Earth’s surface that exhibits gentle slopes and small local relief (differences in elevation). Occupying slightly more than one-third of the terrestrial surface, plains are found on all continents except Antarctica. Some are tree-covered, and others are grassy. Still others support scrub brush and bunch grass, and a few are nearly waterless deserts. With certain exceptions, plains have become the sites of major centres of population, industry, commerce, and transportation.

Plain, the French la Plaine In the French Revolution, the centrist deputies in the National Convention. They formed the majority of the assembly’s members and were essential to the passage of any measures. Their name derived from their place on the floor of the assembly; above them sat the members of the Mountain, or Montagnard. Led by Emmanuel- Joseph Sieyes, the Plain initially voted with the moderate Girondins but later joined the Montagnards in voting for the execution of Louis XVI. In 1794 they helped overthrow Maximilien de Robespierre and other extreme Jacobins (see Jacobin Club).

Plain of Esdraelon See Plain of Esdraelon Plain of Reeds See Plain of Reeds Plain of Sharon See Plain of Sharon

Plains Indian Any member of various Native American tribes that for¬ merly inhabited the Great Pains of the U.S. and southern Canada. Plains Indians are popularly regarded as the typical American Indians. They were essentially big-game hunters, the buffalo being a primary source of food and equally important as a source of materials for clothing, shelter, and tools. Until supplanted by the white settlers from the 16th century onward, the Plains Indians occupied the area between the Mississippi River and the Rocky Mountains, which includes portions of both the United States and Canada. It is a vast grassland stretching from northern Alberta and Saskatchewan in Canada to the Rio Grande border of Texas. The Arapaho, Assiniboin, Backfoot, Cheyenne, Comanche, Plains Cree, Crow, Hidatsa, Kiowa, Mandan, Osage, Pawnee, and Sioux are Plains Indians.

Plains of Abraham See Plains of Abraham

planarian \pta-'nar-e-3n\ Any of about 3,000 species of widely distrib¬ uted, mostly free-living fatworms of the family Planariidae and related families (class Turbellaria), usually found in freshwater but also in marine and terrestrial environments. The soft, ciliated body is leaf-shaped when elongated. The spade-shaped head has two eyes and sometimes tentacles. The tail is pointed. The mouth is on the lower side, often more than halfway

Plaice (Pleuronectes platessa)

JACQUES SIX

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1508 I Planck ► plant virus

toward the tail. Most species are 0.1-0.6 in. (3-15 mm) long; some grow to about 1 ft (30 cm). Planarians swim with an undulating motion or creep like slugs. Most feed at night on protozoans, snails, and other worms.

Planck \'plaqk\, Max (Karl Ernst Ludwig) (b. April 23, 1858, Kiel, Schleswig—d. Oct. 4, 1947, Gottingen, W.Ger.) German physicist. He studied at the Universities of Munich and Kiel, then became professor of theoretical physics at the University of Berlin (1889-1928). His work on the second law of thermodynamics and blackbody radiation led him to formulate the revolutionary quantum theory of radiation, for which he received a Nobel Prize in 1918. He also discovered the quantum of action, now known as Planck’s constant, h. He championed Albert Einstein’s spe¬ cial theory of relativity, but he opposed the indeterministic, statistical worldview introduced by Niels Bohr, Max Born, and Werner Heisenberg after the advent of quantum mechanics. As the influential president of the Kaiser Wilhelm Society (later the Max Planck Society) until his resigna¬ tion in 1937, he appealed to Adolf Hitler to reverse his devastating racial policies. His son was later implicated in the July Plot against Hitler and was executed.

plane tree Any of ten species of large trees that make up the genus Plcitanus, sole genus of the family Platanaceae, native to North America, eastern Europe, and Asia. Plane trees are planted widely in cities for their resistance to diseases and to air pollution and because they grow rapidly and furnish quick shade. They are characterized by scaling bark; large, deciduous, usually lobed leaves; and globular heads of flower and seed. Ball-shaped smooth or bristly seed clusters, which dangle singly and often persist after leaf fall, are key identifiers. Winter bark is patchy and pic¬ turesque; as the outer bark flakes off, inner bark shows shades of white, gray, green, and yellow.

planer Metal-cutting machine tool in which the workpiece is firmly attached to a horizontal table that moves back and forth under a single¬ point cutting tool. The tool-holding device is mounted on a crossrail so that the tool can be moved across the table in small sideward movements. Since the cutting tool can be moved at almost any angle, a wide variety of grooves and surfaces can be generated. Mechanical planers, or surfacers, are also used to smooth wood to an even thickness. Planers perform the same opera¬ tions as shapers but can machine workpieces up to 50 ft (15 m) long.

planet Any large body orbiting the Sun or another star (see planets of other stars); comets, asteroids, meteoroids (see meteor), and natural satel¬ lites are excluded. The word comes from the Greek for “wanderer,” because their positions change relative to those of the stars. The nine major planets known to revolve around the Sun, in order of increasing distance, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Nep¬ tune, and Pluto. The first four are called terrestrial planets and the next four Jovian planets. Pluto, distinct from either group, resembles a giant comet or one of the icy satellites of the Jovian planets. The terrestrial p_ planets are less than 8,000 mi (13,000 km) in diameter and rocky, with

comparatively thin or negligible atmospheres. The Sun’s heat is thought wm to have prevented the abundant gases in the original solar nebula from

condensing in them. The Jovian planets formed farther out, where the gases were cool enough to condense, so the planets grew very massive tm and accumulated huge atmospheres of light gases, mainly hydrogen and

y helium. Called gas giants, the Jovian planets appear to be similar in struc¬

ture; none has an accessible surface. Pluto is by far the smallest body tra¬ ditionally called a major planet, although many astronomers now consider it the largest known member of the Kuiper belt. Additional bodies rivaling Pluto in size may exist in this region of the solar system; one about half its size was discovered in 2002. The term minor planet is sometimes used (less now than formerly) to refer to any of the asteroids that lie between the terrestrial and Jovian planets. In astrology great importance is placed on the planets’ positions in the 12 constellations of the zodiac. See also planetesimal; solar system.

Planet X Supposed distant planet of the solar system, hypothesized on the basis of calculations of effects on the orbits of Uranus and Neptune. The term was first used by Percival Lowell c. 1905, and though his pre¬ diction eventually resulted in the discovery of Pluto (1930), Pluto’s mass was insufficient to explain the apparent perturbations of Uranus and Nep¬ tune. Proposals and searches for other planets beyond Neptune or Pluto continued sporadically during the remainder of the 20th century. How¬ ever, Voyager spacecraft data gathered about the masses of the outer plan¬ ets indicate that no additional large bodies are needed to explain the planets’ observed orbits. See also Kuiper belt.

planetarium Institution devoted to popular education in astronomy and related fields, especially space science, whose main teaching tool is a hemispheric screen onto which images of celestial objects as seen from Earth are projected from an instrument also known as a planetarium, or planetarium projector. Major planetariums have extensive exhibit space, museum collections, sizable staffs, projection domes 80 ft (25 m) or more across, and seating capacities of over 600.

planetary nebula Any of a class of bright NEBULAe that may some¬ what resemble planets when viewed through a small telescope but are, in fact, expanding shells of luminous gas around dying stars. A planetary nebula is the outer envelope shed by a red giant star not massive enough to become a supernova. Instead, the star’s intensely hot core becomes exposed (see white dwarf star) and ionizes the surrounding shell of gas, which is expanding at tens of miles per second.

planetesimal N.pla-no-'te-so-moH One of a class of hypothetical bod¬ ies that joined to form the planets after condensing from dust and gas early in the history of the solar system. According to the nebular hypothesis, a rotating cloud of interstellar dust and gas underwent gravitational con¬ traction, eventually forming a solar nebula comprising a disk of matter with a central hub. As the disk contracted further, clumps of dust left behind in its midplane coalesced successively into bodies the size of pebbles and boulders and then into planetesimals up to several hundred miles across. These then combined under the force of gravity to form pro¬ toplanets, precursors of the current planets.

planets of other stars or extrasolar planets Planets that orbit stars other than the Sun. The existence of extrasolar planets, many light- years from Earth, was confirmed in the early 1990s with the detection of three bodies circling a pulsar. The first planet revolving around a more sunlike star, 51 Pegasi, was reported in 1995. Within a decade over 100 stars with one or more planets were known. Current detection methods, based on the planets’ gravitational effects on the stars they orbit, have revealed only planets much more massive than Earth; some are several times the size of Jupiter. A number of them have highly elliptical orbits, and many are closer to their stars than Mercury is to the Sun. These find¬ ings have raised questions regarding astronomers’ ideas of how Earth’s solar system formed and how typical it is.

plankton Marine and freshwater organisms that, because they are unable to move or are too small or too weak to swim against water cur¬ rents, exist in a drifting, floating state. Plankton is the productive base of both marine and freshwater ecosystems, providing food for larger animals and indirectly for humans, whose fisheries depend on plankton. As a human resource, plankton has only begun to be developed and used. The plantlike community of plankton is called phytoplankton, and the animal¬ like community is called zooplankton, but many planktonic organisms are better described as protists. Most phytoplankton serves as food for zoo¬ plankton, but some of it is carried below the light zone. Zooplankton is used directly as food by fish (including herring) or mammals (including whales), but several links on the food chain usually have been passed before plankton is available for human consumption.

plant Any organism in the kingdom Plantae, consisting of multicellu¬ lar, eukaryotic life forms (see eukaryote) with six fundamental character¬ istics: photosynthesis as the almost exclusive mode of nutrition, essentially unlimited growth at meristems, cells that contain cellulose in their walls and are therefore somewhat rigid, the absence of organs of movement, the absence of sensory and nervous systems, and life histories that show alternation of generations. No definition of the kingdom completely excludes all nonplant organisms or even includes all plants. Many plants, for example, are not green and thus do not produce their own food by photosynthesis, being instead parasitic on other living plants (see parasit¬ ism). Others obtain their food from dead organic matter. Many animals possess plantlike characteristics, such as a lack of mobility (e.g., sponges) or the presence of a plantlike growth form (e.g., some corals and bryo- zoans), but in general such animals lack other plant characteristics. Some past classification systems (see taxonomy) placed difficult groups such as protozoans, bacteria, algae, slime molds, and fungi (see fungus) in the plant kingdom, but structural and functional differences between these organ¬ isms and plants have convinced most scientists to classify them elsewhere.

plant virus Any of various viRUSes that can cause plant disease (e.g., the tobacco mosaic virus). Plant viruses are economically important because many of them infect crop and ornamental plants. Numerous plant viruses are rodlike and can be extracted readily from plant tissue and crystallized.

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Plantagenet ► plastics I 1509

Most lack the fatty membrane found in many animal viruses, and all con¬ tain RNA. Plant viruses are transmitted in various ways, most importantly through insect bites, mainly by aphids and plant hoppers. Symptoms of virus infection include colour changes, dwarfing, and tissue distortion. The appearance of streaks of colour in certain tulips is caused by a virus. See also reovirus.

Plantagenet Nplan-'ta-jo-natV House of or House of Anjou

Van-jii\ Royal house of England (1154-1485) that provided 14 kings, including six from the cadet houses of Lancaster and York. The line descended from Geoffrey, count of Anjou (died 1151), and the empress Matilda, daughter of the English king Henry I. Some historians apply the name House of Anjou, or Angevin dynasty, to only Henry II, Richard I, and John, and label their successors, including Edward I, Edward II, and Edward III, as Plantagenets. The name may have originated as a nickname (Plante- geneste) for Count Geoffrey, who planted broom shrubs (Latin Genista ) to improve his hunting covers. The Wars of the Roses saw the defeat of the last Plantagenet king, Richard III, in 1485. The legitimate line ended with Edward of Warwick (died 1499).

plantain Tall plant ( Musa paradisiaca) of the banana family that is closely related to the common banana (M. sapientum). Believed to have originated in Southeast Asia, the plantain grows 10-33 ft (3-10 m) tall and has a conical false “trunk” formed by the leaf sheaths of its spirally arranged, long, thin leaves. The green-coloured fruit is larger than that of the banana and contains more starch. Because the starch is maximal before the fruit ripens, the fruit is not eaten raw but is boiled or fried, often with coconut juice or sugar as flavouring. It may also be dried for later use in cooking or ground for use as meal, which can be further refined to a flour. The plantain is a staple food and beer-making crop for East African peoples and is also eaten in the Caribbean and Latin America.

plantain \'plan-t 3 n\ Any of about 265 species of familiar garden, lawn, and roadside weeds in the genus Plantago of the family Plantaginaceae. Distinctively, the leaves lack a proper blade. What appears to be a blade is an expanded petiole (leafstalk), with several parallel main veins, emerg¬ ing at the base of the stalk. Small flowers are borne in spikes or heads atop long leafless stalks. The greater plantain (P. major) provides seed spikes for bird food. Ribwort, or English, plantain (P. lanceolata ) and hoary plantain (P media) are troublesome weeds. Some species have been useful in medicine (e.g., as an ingredient in laxatives).

plantation walking horse See Tennessee walking horse

Plante, (Joseph) Jacques (Omer) (b. Jan. 17, 1929, Shawinigan Falls, Que., Can.—d. Feb. 26, 1986, Geneva, Switz.) Canadian ice-hockey player. A goalie, he recorded a shutout in his first NHL game. He was an integral member of the powerful Montreal Canadiens team that won a record five successive Stanley Cups (1956-60). He was the first goalie to wear a protective face mask (1959). Honoured throughout his 18-year NHL career, he was one of the most successful of all NHL goaltenders.

Planudes \pla-'nu-dez\, Maximus orig. Manuel Planudes (b.

1260, Nicomedia, Byzantine Empire—d. c. 1310, Constantinople) Greek Orthodox scholar, anthologist, and polemicist. He established a school in Constantinople that gained a strong reputation for its humanities curricu¬ lum. His Greek translations of Latin philosophy and literature and of Ara¬ bic mathematics publicized these areas of learning throughout the Greek Byzantine cultural world. Though chiefly known for his theological writ¬ ings, he made a distinctive contribution to the history of Greek literature with his revision of the Greek Anthology, a collection of Greek prose and poetry c. 700 bc-ad 1000.

plasma Vplaz-m9\ Liquid part of blood (including dissolved chemicals but not the cells and platelets). This straw-coloured fluid serves as the blood’s transport medium, helps maintain blood pressure, distributes body heat, and maintains the pH balance in the bloodstream and body. More than 90% consists of water, about 7% proteins, and the rest other sub¬ stances, including waste products of metabolism. Important plasma proteins include albumin, coagulation factors, and globulins, including gamma globu¬ lin and a hormone that stimulates erythrocyte formation. Serum is the liq¬ uid part of the blood that remains after clotting.

plasma Electrically conducting medium in which there are roughly equal numbers of positively and negatively charged particles, produced when the atoms in a gas become ionized (see ionization). Plasma is some¬ times called the fourth state of matter (the first three being solid, liquid, and gas). A plasma is unique in the way it interacts with itself, with elec¬

tric and magnetic fields, and with its environment. It can be thought of as a collection of ions, electrons, neutral atoms and molecules, and photons in which some atoms are being ionized at the same time as electrons are recombining with other ions to form neutral particles, while photons are continuously being produced and absorbed. It is estimated that more than 99% of the matter in the universe exists in the plasma state.

plasmid Vplaz-madN Genetic element not contained within a chromo¬ some. It occurs in many bacterial strains. Plasmids are circular DNA mol¬ ecules that replicate independently of the bacterial chromosome. They are not essential for the bacterium but may give it a selective advantage. Some plasmids determine the production of proteins that can kill other bacte¬ ria; others make bacteria resistant to antibiotics. Plasmids are extremely valuable tools in the fields of molecular biology and genetics, specifically in the area of genetic engineering.

plasmodium \plaz-'mo-de-9m\ Any of the parasitic protozoans that make up the genus Plasmodium, the cause of malaria. Infecting red blood cells in mammals, birds, and reptiles, plasmodia occur worldwide, espe¬ cially in tropical and temperate zones. They are transmitted to humans by the bite of the Anopheles mosquito. From the bloodstream, young plas¬ modia enter liver cells, where they divide and form an adult stage that is then released back into the bloodstream and infects red blood cells. Rapid division of the parasites results in the destruction of the red blood cells, which release toxins that cause the periodic chill and fever cycles typical of malaria.

plaster of paris Quick-setting gypsum plaster consisting of a fine white powder, calcium sulfate hemihydrate, which hardens when moistened and allowed to dry. It is made by heating gypsum to 250-360°F (120-180°C). Used since ancient times, plaster of paris is so called because of its prepa¬ ration from the abundant gypsum found in Paris. It is used to make molds and casts for ceramics and sculptures, to precast and hold ornamental plasterwork on ceilings and cornices, and for orthopedic bandages (casts). In medieval and Renaissance times, gesso (plaster of paris mixed with glue) was applied to wood panels, plaster, stone, or canvas to provide the ground for tempera and oil painting.

plastic surgery Surgery to correct disfigurement, restore function, or improve appearance. It may involve reshaping or moving tissues to fill a depression, cover a wound, or improve appearance. Cosmetic surgery solely to improve appearance is not the main focus of plastic surgery. It is utilized after disfigurement by bums or tumour removal or for recon¬ structive work, and it may involve hiding incisions in skin folds or using buried sutures to hold wounds closed. Reconstructive plastic surgery cor¬ rects severe functional impairments, fixes physical abnormalities, and compensates for tissue lost to trauma or surgery. Microsurgery and com¬ puterized diagnostic imaging techniques have revolutionized the field.

plasticity Ability of certain solids to flow or to change shape perma¬ nently when subjected to STRESSes between those that produce temporary deformation, or elastic behaviour, and those that cause failure of the mate¬ rial, or rupture (see fracture). Plasticity allows a solid under the action of outside forces to become permanently deformed without rupturing; elas¬ ticity enables a solid to return to its original shape after the load is removed. Plastic deformation occurs in many metal-forming processes (rolling, pressing, forging, wire drawing) and in geologic processes (rock folding and rock flow within the Earth under extremely high pressures and at elevated temperatures).

plastics Polymers that can be molded or shaped, usually by heat and pressure. Most are lightweight, transparent, tough organic compounds that do not conduct electricity well. They fall into two classes: Thermoplas¬ tics (e.g., polyethylene, polystyrene) can be melted and formed again and again; thermosetting plastics, or thermosets (e.g., polyurethane, epoxy), once formed, are destroyed rather than melted by heating. Few plastics contain only the polymer resin; many also contain plasticizers (to change the melting point and make them softer), colorants, reinforcements, and fillers (to improve mechanical properties such as stiffness), and stabiliz¬ ers and antioxidants (to protect against aging, light, or biological agents). Traditional plastics are not biodegradable (see biodegradability); recycling of plastics, especially thermoplastics, has become an important industry, and the development of low-cost biodegradable plastics and plastic sub¬ stitutes is a significant pursuit of industrial research. Major commercial uses of plastics include cars, buildings, packaging, textiles, paints, adhe¬ sives, pipes, electrical and electronic components, prostheses, toys, brushes, and furniture. Common plastics include polyethylene terephtha-

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1510 I Plata ► platelet

rigidly connected to their respective plates and move as the plates move, a few inches a year.

© MERRIAM-WEBSTER INC.

late, or PET (beverage bottles), PVC (pipes and hoses), foamed polysty¬ rene, or Styrofoam (insulated food containers), and Lucite (shatterproof windows). See also Leo Baekeland.

Plata, Rio de la Estuary of the Parana and Uruguay rivers, between Uruguay and Argentina. It is about 180 mi (290 km) long, with a maxi¬ mum width of 136 mi (219 km) at its mouth, but it narrows gradually to the northwest as it approaches Buenos Aires. Discovered by the Spanish in 1516, it was explored by Ferdinand Magellan in 1520 and by Sebastian Cabot (1526-29). The first permanent settlement in the area was at Asun¬ cion in 1537.

Plata, Viceroyalty of the Rio de la See Viceroyalty of the Rio de

la Plata

Plataea \pb-'te-9\ Ancient city, Boeotia, east-central Greece, south of Thebes. Plataea was settled by Boeotians who expelled the earlier Bronze Age inhabitants. The Plataeans fought along with the Athenians against the Persians at the Battle of Marathon (490 bc). Plataea was the scene of the Greek victory over the invading Persians in the Battle of Plataea (479 bc). It was destroyed by the Spartans in 427 bc but was rebuilt under the Macedonian kings Philip II and Alexander the Great as a symbol of Greek courage in resisting Persia.

Plataea, Battle of (479 bc) Battle between Greek and Persian forces near Plataea (modern Plataiaf) in Boeotia on the slopes of Mount Cith- aeron. A largely Spartan force, including helots, defeated the Persian army of Xerxes I, led by Mardonius; the victory marked this battle as the final Persian attempt to invade mainland Greece.

plate tectonics Theory that the Earth’s lithosphere (the crust and upper portion of the mantle) is divided into about 12 large plates and several small ones that float on and travel independently over the asthenosphere. The theory revolutionized the geological sciences in the 1960s by com¬ bining the earlier idea of continental drift and the new concept of seafloor spreading into a coherent whole. Each plate consists of rigid rock created by upwelling magma at oceanic ridges, where plates diverge. Where two

plates converge, a subduction zone forms, in which one plate is forced under another and into the Earth’s mantle. The majority of the earthquakes and volcanoes on the Earth’s surface occur along the margins of tectonic plates. The interior of a plate moves as a rigid body, with only minor flex¬ ing, few earthquakes, and relatively little volcanic activity.

plateau Extensive area of flat upland, usually bounded by an escarp¬ ment on all sides but sometimes enclosed by mountains. Plateaus are extensive, and together with enclosed basins they cover about 45% of the Earth’s land surface. The essential criteria for a plateau are low relative relief and some altitude. Low relief distinguishes plateaus from mountains, although their origin may be similar. Plateaus, being high, often create their own local climate; the topography of plateaus and their surround¬ ings often produce arid and semiarid conditions.

Plateau Indian Any member of various North American Indian peoples that inhabited the high plateau between the Rocky Mountains on the east and the Cascade Range on the west. The Plateau culture was not stable. By ad 1200-1300 the “classic” phase had emerged, characterized by permanent winter villages with semi-subterranean earth lodges along the main rivers and by summer camps with mat-covered conical lodges in the meadows. During the centuries that followed, the Plateau area was influenced by cultural elements from the highly specialized Northwest Coast culture. Part of this diffusion was possibly brought about by a Chi¬ nook group, the Wishram, who migrated from the coast into the Cascade Mountains. During the 18th century there were influences from the east. The Shoshone had acquired horses by that time and furnished their clos¬ est neighbours on the Plains and the Plateau with horses. Other elements of Plains culture, such as beaded dresses, warbonnets, and tepees, came with the horse. Among this group were the Coeur d’Alene, Flathead, Kla¬ math, Kutenai, Modoc, Nez Perce, Spokan, Thompson, and Salish.

platelet or thrombocyte Small, colourless, irregular blood cell cru¬ cial in coagulation. Produced in bone marrow and stored in the spleen, platelets accumulate to block a cut in a blood vessel and provide a sur¬ face for fibrin strands to adhere to, contract to pull the strands together.

© 2006 Encyclopaedia Britannica, Inc.

Plateresque ► platyhelminth I 1511

and take part in the conversion sequence of coagulation factors. They also store and transport several chemicals.

Plateresque \,pla-t9-'resk\ Main architectural style in Spain and its New World colonies in the late 15th and 16th centuries. The name (which comes from a comparison to the intricate work of silversmiths) came to be generally applied to late Gothic and early Renaissance Spanish archi¬ tecture, which was characterized by minutely detailed relief ornament derived from Moorish, Gothic, and Italian Renaissance sources and applied without regard for structure. Favourite motifs for this ornament included twisted columns, heraldic escutcheons, and sinuous scrolls; clus¬ ters of ornament often contrasted with broad expanses of flat wall surface. Over time, the style evolved so that ornamentation became purer and more unified with the overall structure. Outstanding examples of the style include Diego de Siloe’s Granada Cathedral (1528—43), Diego de Riano’s work on the Seville Cathedral (c. 1530), and Rodrigo Gil de Hontanon’s facade of the University of Alcala de Henares (1541-53).

platform tennis Variation of paddle tennis, played on a platform enclosed by a wire fence. It was devised in 1928 in Scarsdale, N.Y., U.S. The short-handled oval paddles are made of perforated plywood; the balls are made of sponge rubber. The rules are the same as for tennis, except that balls may be played off back or side walls after first striking inside the court.

Plath, Sylvia (b. Oct. 27, 1932, Boston, Mass., U.S.—d. Feb. 11. 1963, London, Eng.) U.S. poet. The daughter of an entomologist, Plath was driven to excel as a writer from an early age and published her first poem at age eight. At Smith College she made an early suicide attempt and sub¬ mitted to electroshock treatment. While attending Cambridge University on a Fulbright grant, she married the poet Ted Hughes. After their sepa¬ ration, she committed suicide at age 30. Though she was not widely rec¬ ognized in her lifetime, her reputation grew rapidly afterward; by the 1970s she was considered a major contemporary poet. Her works, often confessional and preoccupied with alienation, death, and self-destruction, include the volumes The Colossus (1960), Ariel (1965), and The Collected Poems (1981, Pulitzer Prize) and a semiautobiographical novel. The Bell Jar (1963).

plating Coating a metal or other material, such as plastic or china, with a hard, nonporous metallic surface to improve durability and beauty. Early plated goods (“old Sheffield plate”) are made by the process invented by Thomas Boulsover, and consist of a sandwich of copper between two lay¬ ers of silver. Today surfaces such as gold, silver, stainless steel, palladium, copper, and nickel are applied by dipping an object into a solution con¬ taining the desired surface material, which is deposited by chemical or electrochemical action (see electroplating). Much plating is done for deco¬ rative purposes, but still more is done to increase the durability and cor¬ rosion resistance of softer materials. Most automotive parts, appliances, housewares and flatware, hardware, plumbing and electronic equipment, wire goods, aircraft and aerospace products, and machine tools are plated for durability. See also galvanizing, ternepiate, tinplating.

platinum Metallic chemical element, one of the transition elements, chemical symbol Pt, atomic number 78. A very heavy, silvery white pre¬ cious metal, it is soft and ductile, with a high melting point (3,216 °F [1,769 °C]) and good resistance to corrosion and chemical attack. Small amounts of iridium are commonly added for a harder, stronger alloy that retains platinum’s advantages. Platinum is found usually as alloys of 80-90% purity in placer deposits or more rarely combined with arsenic or sulfur. It is indispensable in high-temperature laboratory work for elec¬ trodes, dishes, and electrical contacts that resist chemical attack even when very hot. Platinum is used in dental alloys and surgical pins; alloys typically containing 90-95% platinum are used in expensive jewelry. The international primary standard for the kilogram is made of 90% platinum, 10% iridium. Platinum has valence 2 or 4 in its compounds, which include many coordination complexes. It and some compounds are useful cata¬ lysts, particularly for hydrogenation and in catalytic converters for reduc¬ ing automobile emissions.

Plato \'pla-to\ orig. Aristocles (b. 428/427, Athens, or Aegina, Greece—d. 348/347 bc, Athens) Greek philosopher, who with his teacher Socrates and his student Aristotle laid the philosophical foundations of Western culture. His family was highly distinguished; his father claimed descent from the last king of Athens, and his mother was related to Cri- tias and Charmides, extremist leaders of the oligarchic terror of 404. Plato (whose acquired name refers to his broad forehead, and thus his range of

knowledge) must have known Socrates from boyhood. After Socrates was put to death in 401, Plato fled Athens for Megara, then spent the next 12 years in travel. Upon his return, he founded the Academy, an institute of scientific and philosophical research, where Aristotle was one of his stu¬ dents. Building on but also departing from Socrates’ thought, he devel¬ oped a profound and wide-ranging philosophical system, subsequently known as Platonism. His thought has logical, epistemological, and meta¬ physical aspects, but much of its underlying motivation is ethical. It is presented in his many dialogues, in most of which Socrates plays a lead¬ ing role. See also Neoplatonism.

Platonic solid also known as regular polyhedron Geometric solid all of whose faces are identical regular polygons and all of whose angles are equal. There are only five such polyhedrons. The cube is constructed from the square, the dodecahedron from the regular pentagon, and the tetrahedron, octahedron, and icosahedron (with 20 faces) from the equi¬ lateral triangle. They are known as the Platonic solids because of Plato’s attempt to relate each to one of the five elements that he believed formed the world.

Platonism Vpla-ta-.ni-zsmV Any philosophy that embodies some major idea of Plato’s, especially in taking abstract forms as metaphysically more basic than material things. Though there was in antiquity a tradition about Plato’s “unwritten doctrines,” Platonism then and later was based primar¬ ily on a reading of the dialogues. It is characterized by an intense con¬ cern for the quality of human life—always ethical, often religious, and sometimes political, based on a belief in unchanging and eternal realities (the Platonic Forms), independent of the changing things of the physical world perceived by the senses. This belief in absolute values rooted in an eternal world distinguishes Platonism from the philosophies of Plato’s immediate predecessors and successors and from later philosophies inspired by them. See also Neoplatonism.

platoon Principal subdivision of a military company, battery, or troop. Usually commanded by a lieutenant, it consists of 25-50 soldiers orga¬ nized into two or more squads led by noncommissioned officers. The term was first used in the 17th century to refer to a small body of musketeers who fired together in a volley alternately with another platoon. It has been used in U.S. military manuals since 1779, and throughout the 19th cen¬ tury it meant half a company. It was reintroduced into the British army in 1913. See also military unit.

Platt, Orville Hitchcock (b. July 19, 1827, Washington, Conn., U.S.——d. April 21, 1905, Washington) U.S. politician. He served in the Connecticut legislature (1861-62, 1864, 1869) and later in the U.S. Sen¬ ate (1879-1905), where he sponsored legislation concerning patents and copyrights. He was chairman of the committee on territories (1887-93), which recommended admission of six new western states. He is remem¬ bered for sponsoring the Platt Amendment (1901), which became the basis for the withdrawal of U.S. troops from Cuba following the Spanish- American War of 1898.

Platt Amendment (1901) Rider appended to a U.S. Army appropria¬ tions bill stipulating conditions for withdrawing of U.S. troops remaining in Cuba after the Spanish-American War. The amendment, which was added to the Cuban constitution of 1901, affected Cuba’s rights to nego¬ tiate treaties and permitted the U.S. to maintain its naval base at Guantanamo Bay and to intervene in Cuban affairs “for the preservation of Cuban independence.” In 1934 Pres. Franklin Roosevelt supported abro¬ gation of the amendment’s provisions except for U.S. rights to the naval base. See also Good Neighbor Policy.

Platte River River, central Nebraska, U.S. Formed by the confluence of the North Platte and South Platte rivers, it is 310 mi (500 km) long. It flows southeast into a big bend at Kearney, Neb., then empties into the Missouri River at Plattsmouth, south of Omaha. It is too shallow for navi¬ gation and is almost dry for much of the year. Its waters are used for irri¬ gation and municipal water systems.

Plattsburgh City (pop., 2000: 18,816), northeastern New York, U.S. Situated on the western shore of Lake Champlain, south of the Canadian border, Plattsburgh was founded by Zephaniah Platt in 1784. It was the scene of an important U.S. victory during the War of 1812 on Lake Champlain that saved New York from British invasion via the Hudson River valley. Incorporated as a city in 1902, it became the centre of the Lake Champlain resort area.

platyhelminth See flatworm

© 2006 Encyclopaedia Britannica, Inc.

1512 I platypus ► Plekhanov

platypus or duckbill Monotreme amphibious mammal ( Ornithorhyn - chus anatinus ) of lakes and streams in eastern Australia and Tasmania. About 23 in. (60 cm) long, the squat-bodied platypus has a ducklike snout, short legs, webbed feet, and a beaverlike tail. Each day it eats nearly its own weight in crustaceans, fishes, frogs, mollusks, tadpoles, and earth¬ worms; lacking teeth, it crushes its food with ridges in the bill. The female lays one to three eggs in a nest in a long twisting passage above the waterline. The young are weaned about four months after hatching. The male’s heel bears a spur connected to a poison-secreting gland. Large fishes and perhaps snakes prey on platypuses. Formerly trapped for their dense, soft fur, they are now protected by law.

Plautus \'plo-t3s\ (b. c. 254, Sarsina, Umbria?—d. 184 bc) Roman comic playwright. Little is known for certain about his life, but tradition holds that he was associated with the theatre from an early age. Like other Roman playwrights, he borrowed plots and dramatic techniques from Greek authors, especially New Comedy playwrights such as Menander. His plays, written in verse, were often farces marked by cases of mistaken identity and opportunities for slapstick, and he popularized character types such as the braggart soldier and the sly servant. Twenty-one of his comedies—among the earliest surviving works in Latin—are extant, including The Pot of Gold, The Captives, The Two Menaechmuses, The Braggart Warrior, and Pseudolus. His work influenced European comedy from the Renaissance onward, notably William Shakespeare’s Comedy of Errors (1592-93) and Moliere’s The Miser (1669).

play See theatre

play In zoology, actions that have all the elements of purposeful behav¬ iour but are performed for no apparent reason. Play has been documented only among mammals and birds. It is most common among immature animals, but adult animals also play. Horses, cattle, and other ungulates run and kick up their heels even when not fleeing from predators or defending themselves. Dogs adopt an aggressive posture to entice others to join in mock combat. Otters are well known for their mud sliding. Male birds may spontaneously perform their territorial songs when there is no intruding rival.

playa VplI-9\ or pan or flat or dry lake Flat-bottomed depression that is periodically covered by water. Playas occur in interior desert basins and adjacent to coasts in arid and semiarid regions. The water that peri¬ odically covers the playa slowly filters into the groundwater system or evaporates into the atmosphere, causing the deposition of salt, sand, and mud along the bottom and around the edges of the depression.

Player, Gary (Jim) (b. Nov. 1, 1935, Johannesburg, S.Af.) South African golfer. Player entered the U.S. PGA circuit in 1957. He won the British Open three times (1959, 1968, 1974), the Masters three times (1961, 1974, 1978), the PGA twice (1962, 1972), and the U.S. Open once (1965). Player was the third man (after Gene Sarazen and Ben Hogan) to win the four tournaments composing the modern grand slam of golf (the Masters, the U.S. Open, the British Open, and the PGA Championship).

player piano Piano that mechanically plays music encoded as perfo¬ rations on a paper roll. An early ver¬ sion, patented in 1897 by the American engineer E.S. Votey, was a cabinet placed in front of an ordinary piano, with wooden “fingers” pro¬ jecting over the keyboard. A paper roll with perforations corresponding to the notes passed over a tracker bar to activate the release of air by pneu¬ matic devices that set the fingers in motion; the user could control tempo and loudness by levers and pedals.

Soon this mechanism was built into the piano itself. The later “reproduc¬ ing piano” could reproduce the nuances of tempo and dynamics in great performances, the roll having been produced by the performance itself. After the 1920s the phono¬ graph led to the instrument’s quick decline. Modern versions, such as the Yamaha Disklavier, are operated by digital memory on a computer disk.

playing cards Small rectangular cards used for playing games and sometimes for divination and conjuring. Modem cards are divided into four suits: spades, hearts, diamonds, and clubs. A complete pack, or deck, of cards includes 13 cards in each suit (10 numbered cards and 3 court cards—king, queen, and jack, or knave); 2 extra cards, called jokers (many times portraying a medieval jester), are often included as well. The ori¬ gin of playing cards is obscure—China and India being the two most likely sources—as is the meaning of their symbols. The earliest reference to cards in Europe occurs in Italy in 1299. The 52-card French deck is now standard throughout the world, but decks with fewer cards evolved in Germany and Spain. Other suit emblems were also used (e.g., bells in Germany, cups in Spain and Italy). See also tarot.

plea bargaining Negotiation of an agreement between the prosecu¬ tion and the defense whereby the defendant pleads guilty to a lesser offense or (in the case of multiple offenses) to one or more of the offenses charged, in exchange for more lenient sentencing, recommendations, a specific sentence, or a dismissal of other charges. Supporters claim plea bargaining speeds court proceedings and guarantees a conviction; oppo¬ nents believe it prevents justice from being served.

plebeian \ple-'be-3n\ (Latin, plebs) Member of the general citizenry, as opposed to the patrician class, in the ancient Roman republic. Plebeians were originally excluded from the Senate and from all public offices except military tribune, and they were forbidden to marry patricians. Seeking to acquire equal rights, they carried on a campaign called Conflict of the Orders, developing a separate political organization and seceding in pro¬ test from the state at least five times. The campaign ceased when a ple¬ beian dictator (appointed 287 bc) made measures passed in the plebeian assembly binding on the whole community.

plebiscite Vple-bs-.shA Vote by the people of an entire country or dis¬ trict to decide an issue. Voters are asked to accept or reject a given pro¬ posal rather than choose between alternative proposals. By means of plebiscites, intermediaries such as political parties can be bypassed. Because plebiscites offer a way to claim a popular mandate without per¬ mitting an opposition party, totalitarian regimes have used them to legiti¬ mize their power. See also referendum and initiative.

Plehve \'pla-vyi\ A Vyacheslav (Konstantinovich) (b. April 20, 1846, Kaluga province, Russia—d. July 28, 1904, St. Petersburg) Russian government administrator. In 1881 he was appointed director of the secret police in the ministry of the interior. He became head of the imperial chancellery (1894), state secretary for Finland (1899), and minister of the interior (1902). Concerned with upholding autocratic principles, he sup¬ pressed revolutionary and liberal movements, harshly pursued Russifica¬ tion policies against minority nationality groups, and backed police- controlled labour unions. He was assassinated by a member of the Socialist Revolutionary Party.

Pleiades \'ple-9-,dez\ Open cluster of stars in the constellation Taurus, about 400 light-years from Earth. It contains a large amount of bright nebulous material and several hundred stars, of which six or seven can be seen by the unaided eye and have figured prominently in the myths and literature of many cultures. In the Northern Hemisphere, the rising of the Pleiades near dawn in spring has from ancient times marked the opening, and their morning setting in autumn the end, of seafaring and farming seasons.

Pleistocene Vplls-t9-,sen\ Epoch Earlier and longer of the two epochs that constitute the Quaternary Period. The Pleistocene began c. 1.8 mil¬ lion years ago and ended c. 10,000 years ago. It was preceded by the Pliocene Epoch of the Tertiary Period and followed by the Holocene Epoch. At the height of the Pleistocene glacial ages, more than 30% of the land area of the Earth was covered by glacial ice; during the interglacial stages, probably only about 10% was covered. The animals of the Pleistocene began to resemble those of today, and new groups of land mammals, including humans, appeared. At the end of the epoch, mass extinctions occurred: in North America more than 30 genera of large mammals became extinct within a span of roughly 2,000 years. Of the many causes that have been proposed for these extinctions, the two most likely are changing environment with changing climate and disruption of the eco¬ logical pattern by early humans.

Plekhanov \pli-'ka-nof\, Georgy (Valentinovich) (b. Dec. 11, 1856, Gudalovka, Russia—-d. May 30, 1918, Terioki, Fin.) Russian Marx¬ ist theorist. From 1874 he was active in the Populist movement and

Steinway-Welte player piano, 1910; in the British Piano and Musical Museum, Brentford, Middlesex, Eng.

COURTESY OF THE BRITISH PIANO AND MUSICAL MUSEUM, BRENTFORD, MIDDLESEX, ENG.

© 2006 Encyclopaedia Britannica, Inc.

Plessy v. Ferguson ► plover I 1513

became a leader of the Land and Freedom group (1877-80). To avoid arrest, he went into a long exile in Geneva (1880-1917). In 1883 he founded the first Russian Marxian revolutionary organization, Liberation of Labour, which became the Russian Social-Democratic Workers' Party (1898). In Socialism and Political Struggle (1883) and Our Differences (1885), he described a two-phase revolutionary scheme that influenced Russian Marxist thought. Followers in the 1890s included Vladimir Ilich Lenin. After the party split (1903), Plekhanov joined the Mensheviks but spent years trying to reunite the party. He supported the Allies in World War I and opposed the Bolshevik seizure of power. After returning briefly to Russia in 1917, he died in exile in Finland.

Plessy v. Ferguson (1896) U.S. Supreme Court decision that estab¬ lished the legality of racial segregation so long as facilities were “sepa¬ rate but equal.” The case involved a challenge to Louisiana laws requiring separate railcars for African Americans and whites. Though the laws were upheld by a majority of 8 to 1, a famous dissent by John Marshall Har¬ lan advanced the idea that the U.S. Constitution is “color-blind.” The Plessy decision was overturned in 1954 by Brown v. Board of Education.

Pleven \pla-‘ven\, Rene (b. April 13 or 15, 1901, Rennes, France—d. Jan. 13, 1993, Paris) French politician and premier (1950-51, 1951-52). A lawyer and industrial executive, he served in Charles de Gaulle’s war¬ time government (1940-45). Elected to the National Assembly in 1945, he cofounded with Francois Mitterrand the Democratic and Socialist Union of the Resistance, serving as its president (1946-53). He held sev¬ eral postwar cabinet posts, including minister of defense (1949-50, 1952— 54). He sponsored the Pleven Plan for a unified European army, but the European Defense Community, based on his plan, collapsed in 1954. He served as minister of justice (1969-73).

Plexiglas See Lucite

Pleyel \ple-'yel,\ English VplI-oL, Ignace Joseph or Ignaz Josef Pleyel (b. June 18, 1757, Ruppersthal, Austria—d. Nov. 14, 1831, Paris, France) Austrian-French music publisher, piano maker, and composer. He studied with Joseph Haydn (c. 1772-77), then held posts in Vienna and Strasbourg and traveled in Italy before settling in Paris. His publishing house, Maison Pleyel, published Haydn’s complete string quartets (1801), and in 1802 it published the first miniature scores for study. It ceased publishing in 1834, but the piano-manufacturing company he founded in 1807 lasted until the 1960s. Pleyel’s music, including 45 symphonies and more than 70 string quartets, was once very popular.

Plimsoll Vplim-solV Samuel (b. Feb. 10, 1824, Bristol, Gloucester¬ shire, Eng.—d. June 3,1898, Folkestone, Kent) British reformer. A London coal merchant, he served in Parliament (1868-80). With his book Our Sea¬ men (1873), he helped overcome resistance to the Merchant Shipping Act, which instituted such reforms as the loading limit for cargo ships. A load line (the Plimsoll mark or line) was marked on the hull of every cargo ship, indicating the maximum depth to which the ship could be safely loaded.

Pliny Vplin-e\ the Elder Latin Gaius Plinius Secundus (b. ad 23,

Novum Comum, Transpadane Gaul—d. Aug. 24, 79, Stabiae, near Mt. Vesuvius) Roman scholar. Descended from a prosperous family, Pliny pursued a military career, held official positions (including procurator of Spain), and later spent years in semiretirement, studying and writing. His fame rests on his Natural History (ad 77), an encyclopaedic work of uneven accuracy that was the European authority on scientific matters up to the Middle Ages. Six other works ascribed to him were probably lost in antiquity. He died while observing the great eruption of Vesuvius.

Pliny the Younger Latin Gaius Plinius Caecilius Secundus (b.

ad 61/62, Comum—d. c. 113, Bithynia, Asia Minor) Roman author and administrator. The nephew of Puny the Elder, he practiced law and held official posts, including consul and head of the military and senatorial treasuries. He is known for the nine books of private letters he published in ad 100-109. The carefully composed letter, at that time a fashion among the wealthy, was transformed by Pliny into an art. His are charming and meticulous occasional pieces on diverse literary, social, and domestic themes that intimately illustrate public and private life in the heyday of the Roman Empire.

Pliocene \'pli-3-,sen\ Epoch Last and shortest epoch of the Tertiary Period, from c. 5.3 to c. 1.8 million years ago. It follows the Miocene Epoch and precedes the Pleistocene Epoch of the Quaternary Period. Pliocene environments were generally cooler and drier than those of preceding Tertiary epochs. In general, Pliocene mammals grew larger than those of

earlier epochs. The more advanced primates continued to evolve, and it is possible that the australopithecines (see Australopithecus), the first crea¬ tures that can be termed human, developed late in the Pliocene.

Plisetskaya Vpli-'set-sko-yoV Maya (Mikhaylovna) (b. Nov. 20, 1925, Moscow, Russia, U.S.S.R.) Russian prima ballerina. She studied at the Bolshoi Ballet’s school in Moscow and joined the company as a solo¬ ist in 1943. Noted for her technical virtuosity and ability to integrate act¬ ing with dancing, she toured worldwide with the Bolshoi Ballet and appeared as a guest artist with several companies, including the Paris Opera. She also danced in several films, including Swan Lake (1957). In the 1980s she was a guest ballet director in Rome and then in Madrid.

Maya Plisetskaya in Swan Lake, 1961.

PARIS MATCH-PICTORIAL PARADE

PLO See Palestine Liberation Organization

ploidy \'ploi-de\ Number of sets of chromosomes in the nucleus of a cell. In normal human body cells, chromosomes exist in pairs, a condition called diploidy. During meiosis the cell produces sex cells (gametes), each containing half the normal number of chromosomes, a condition called haploidy. When an egg and a sperm unite in fertilization, the diploid con¬ dition is restored. Polyploid cells have three or more times the number of chromosomes found in haploid cells; polyploid organisms usually can¬ not reproduce. Aneuploid cells have an abnormal number of chromosomes that is not a multiple of the haploid number. Aneuploidy is most often caused by an error leading to an unequal distribution of chromosomes during division.

Plotinus \plo-'tI-n9s\ (b. ad 205, Lyco, or Lycopolis, Egypt?—d. 270, Campania) Egyptian-Roman philosopher. At age 27 he traveled to Alex¬ andria, where he studied philosophy for 11 years. In about 242 he joined the expedition of the Roman emperor Gordian III against Persia in order to learn about the philosophies of the Persians and Indians. He went to Antioch and then to Rome, where he settled at age 40, becoming the cen¬ tre of an influential circle of intellectuals. His attempt to form a Platonic republic in Campania c. 265 was halted by the emperor Galuenus. He was the founder of the school of philosophy known as Neoplatonism; his col¬ lected works, the Enneads (from Greek, enneas: “set of nine”), arranged by his disciple Porphyry (232?-c. 305), are the first and greatest collec¬ tion of Neoplatonic writings. For Plotinus, philosophy was not only a matter of abstract speculation but also a way of life and a religion. His works strongly influenced early Christian theology, and his philosophy was widely studied and emulated for many centuries.

Plovdiv \'pl6v-,dif\ City (pop., 2001: 340,638), south-central Bulgaria. It is situated on the Maritsa River, north of the Rhodope Mountains. In 341 bc it fell to Philip II and was renamed Philippopolis. From ad 46 it was called Trimontium and was the capital of the Roman province of Thrace. It changed hands repeatedly during the Middle Ages until 1364, when it was captured by the Turks. After the Russo-Turkish Wars (1877-78), it became the capital of Turkish Eastern Rumelia, which united with Bulgaria in 1885. It officially assumed its present name after World War I. It is a major railroad junction and a food-processing centre with diversified industries.

plover Vplo-vor, 'plo-vorX Any of about 36 species (family Charadriidae, order Charadriiformes) of plump-breasted shorebirds found almost world¬ wide. Plovers are 6-12 in. (15-30 cm) long and have long wings, long-

© 2006 Encyclopaedia Britannica, Inc.

1514 I plow ► Pluto

ish legs, a short neck, and a straight, short bill. Many species are plain brown, gray, or sandy above and whitish below. Others, including the golden and black-bellied plovers, are finely patterned above and black below in breeding season. Many spe¬ cies run along the shoreline, snap¬ ping up small aquatic invertebrates.

They have a melodious whistled call.

Both parents incubate the two to five eggs and care for the young. See also KILLDEER.

plow or plough Most important agricultural implement since the beginning of history, used to turn and break up soil, to bury crop residues, and to help control weeds. The forerunner of the plow is the prehistoric digging stick. The earliest plows were undoubtedly digging sticks with handles for pulling or pushing. By Roman times, plows were pulled by oxen or horses, and today they are drawn by tractors.

plum Any of various trees in the genus Prunus of the rose family, and their edible fruits. In the U.S. and Europe, plums are the most extensively distributed of the stone (drupe) fruits, most varied in native and cultivated kinds, and most adapted to a wide range of soils and climatic conditions. The fruits show a wide range of size, flavour, colour, and texture. They are widely eaten fresh, cooked, or baked in pastries. In full bloom, plum trees are covered with densely packed, showy flower clusters. The smooth-skinned fruit has a fleshy, juicy exterior and a hard interior stone or pit. Plum varieties that can be or have been dried without resulting in fermentation are called prunes.

plumbago See graphite

plumbing System of pipes and fixtures installed in a building for the distribution of potable water and the removal of waterborne wastes. Plumbing is usually distinguished from water and sewage systems, which serve a group of buildings or a city. Improvement in plumbing systems was very slow, with virtually no progress made from the time of the Roman system of aqueducts and lead pipes until the 19th century. Even¬ tually the development of separate, underground water and sewage sys¬ tems eliminated open sewage ditches. Present-day water pipes are usually made of steel, copper, brass, plastic, or other nontoxic material. A build¬ ing’s waste-disposal system has two parts: the drainage system and the venting system. The drainage portion comprises pipes leading from vari¬ ous fixture drains to the central main, which is connected to the sewage system. The venting system consists of pipes leading from an air inlet (usually on the roof) to various points within the drainage system; by pro¬ viding the circulation of air within the system, it protects the trap seals of fixtures from siphonage and back pressure. See also sewage system, water pollution, water-supply system. See illustration opposite.

Plunket, Saint Oliver (b. 1629, Loughcrew, County Meath, Ire.—d. July 1, 1681, London, Eng.; canonized 1975; feast day July 11) Irish prel¬ ate, the last man to suffer martyrdom for the Catholic faith in England. He was ordained in Rome, where he taught theology and represented the Irish bishops at the Holy See. In 1669 he was appointed archbishop of Armagh and primate of Ireland, and he worked to restore the disorga¬ nized church in Ireland. Renewed persecution forced him into hiding in 1673. In the anti-Catholic hysteria caused by the Titus Oates plot (1678), he was betrayed and imprisoned in Dublin in 1679. After a farcical trial in London, he was convicted of treason and was hanged, drawn, and quartered. His head, originally sent to Rome, is preserved at Drogheda, and his body is at Downside Abbey, near Bath.

pluralism In political science, the view that in liberal democracies power is (or should be) dispersed among a variety of economic and ideo¬ logical pressure groups and is not (or should not be) held by a single elite or group of elites. Pluralism assumes that diversity is beneficial to soci¬ ety and that the disparate functional or cultural groups of which society is composed—including religious groups, trade unions, professional orga¬ nizations, and ethnic minorities—should be autonomous. Pluralism was stressed most vigorously during the early 20th century by a group of English writers that included Frederic W. Maitland and Harold J. Laski; it was defended in the later 20th century by the American scholars Robert Dahl and David B. Truman.

The basic components of a plumbing fixture include the water-supply pipes, a valve or faucet for controlling the flow of water, and a drainpipe to carry wastewater away. An air chamber may be added to the supply line to cushion the effects of water hammer. A trap in the drain line leaves a plug of water in the pipe to prevent unwanted sewer gases from entering the room via the drain.

© MERRIAM-WEBSTER INC.

pluralism In metaphysics, the doctrine opposed to monism. Whereas monists such as Parmenides, Benedict de Spinoza, and G.W.F. Hegel main¬ tain that reality consists of only one ultimate substance, pluralists assert that reality consists of manifold entities of many different types and that the diversity of things is more striking and important than their unity. In A Pluralistic Universe (1909), William James held that it is characteristic of empirically minded thinkers to note the changeability of things, the multiplicity of their being and their relations with one another, and the unfinished character of the world.

plurality system Electoral process in which the candidate who polls more votes than any other candidate is elected. It is distinguished from the majority system, in which, to win, a candidate must receive more votes than all other candidates combined. It is the most common method of selecting candidates for public office. Its chief advantage is that it avoids the need for runoffs to produce a winner. Its chief disadvantage is that it may result in a winner who has received a minority of the votes cast. It operates best in a two-party system, where the small vote for any third party will rarely result in an outcome seriously at odds with the voters’ will.

Plutarch Vplii-.tarkV Greek Plutarchos Latin Plutarchus (b. ad 46,

Chaeronea, Boeotia—d. after 119) Greek biographer and author. The son of a biographer and philosopher, Plutarch studied in Athens, taught in Rome, traveled widely, and made many important friends before returning to his native town in Boeotia. His literary output was immense, but his popularity rests primarily on his Parallel Lives, a series of pairs of biographies of famous Greeks and Romans. Displaying impressive learning and research, the Lives exhibit noble deeds and characters and provide model patterns of behaviour. The Moralia, or Ethica, contains his surviving writings on ethi¬ cal, religious, physical, political, and literary topics. His works profoundly influenced the evolution of the essay, biography, and historical writing in 16th-19th-century Europe, especially through translations such as Sir Tho¬ mas North’s Lives of the Noble Grecians and Romanes (1579), William Shakespeare’s source for his Roman history plays.

Pluto Ninth planet from the Sun. It was discovered in 1930 by Clyde W. Tombaugh (1906-97) and named after the Greek god of the underworld. It is usually the outermost of the known planets, averaging about 3.7 billion

Golden plover (Pluvial is apricaria).

KENNETH W. FINK-ROOT RESOURCES

© 2006 Encyclopaedia Britannica, Inc.

plutonium ► Pobedonostsev I 1515

mi (5.9 billion km) from the Sun (within the Kuiper belt), but its eccentric orbit brings it closer to the Sun than Neptune for 22 years during its 248- year orbit. Its axis is tipped 120°, so it rotates nearly on its side and “back¬ ward” (see retrograde motion) once every 6.39 days, locked synchronously with the orbit of its largest moon, Charon, discovered in 1978. Two addi¬ tional, small moons were discovered in 2005. Pluto has a diameter of about 1,455 mi (2,340 km), roughly two-thirds that of the Moon, and has less than 1% of Earth’s mass and only about 6% of its surface gravity. Its estimated average surface temperature is near -390 °F (-235 °C). Its thin atmosphere contains nitrogen, methane, and perhaps other heavier gases. Pluto is thought to be made of frozen gases with a significant fraction of rocky material. Its size, composition, and orbital location in the Kuiper belt have sparked debate over its classification as a major planet.

plutonium Radioactive (see radioactivity) metallic chemical element, chemical symbol Pu, atomic number 94. A member of the actinide series of transition elements, it is the most important transuranium element because of its use in certain types of nuclear reactors (see nuclear power) and in nuclear weapons. It is found in nature only in traces produced by natural neutron irradiation in uranium ores. It is produced artificially by neutron irradiation of uranium-238. Plutonium is a silvery metal that tarnishes in air; it is warm because of energy released in alpha decay. Its isotopes, all radioactive, are highly toxic radiological poisons (see radiation injury) because they give off alpha particles and are specifically absorbed by bone marrow.

Plymouth City (pop., 1999 est.: 255,800) in Devon, southwestern England. Located on Plymouth Sound southwest of London, the city was named Sudstone in Domesday Book (1086); its harbour is called Sutton Harbour. It was the port from which the English fleet sailed against the Spanish Armada in 1588. In 1690 its dockyard was built on the western bank of the River Tamar. During World War II Plymouth suffered bomb damage from air raids. The rebuilt city has some of the finest commer¬ cial, shopping, and civic centres in Britain and new bridges over the Riv¬ ers Plym and Tamar.

Plymouth City (pop., 2000: 51,701), southeastern Massachusetts, U.S. Located on Plymouth Bay, it was the site of the first permanent European settlement in New England, the Colony of New Plymouth, founded by the Pilgrims in 1620 and governed under the Mayflower Compact until 1691, when it became part of Massachusetts Bay Colony. Its seaside loca¬ tion and historic associations make Plymouth an outstanding summer resort. A tourist-based economy is supplemented by light industry, fish¬ ing, and cranberry growing. Historical attractions include Plimoth Plan¬ tation (a recreation of the original Pilgrim village) and many restored early colonial houses.

Plymouth Company (1606-09) Commercial trading company char¬ tered by the English crown to colonize the eastern coast of North America in present-day New England. Also called the Virginia Colony of Plymouth, its shareholders were merchants of Plymouth, Bristol, and Exeter. Its twin company was the more successful London Company. The Plymouth Com¬ pany established a colony on the coast of Maine in 1607 but soon aban¬ doned it. Inactive after 1609, it was reorganized under a new charter in 1620 as the Council for New England. See also chartered company.

plywood Manufactured panel made up of three or more thin plies (lay¬ ers) of wood. Each ply is laid down with its grain running perpendicular to the one before it. Like other glued-wood products, plywood provides a strong, inexpensive alternative to solid wood. It is widely used both in cabinetmaking (for chests, dressers, wardrobes, and tables) and in house construction (for walls, ceilings, subfloors, doors, and in forms for cast¬ ing concrete).

PMS See premenstrual syndrome

pneumatic device \nu-'ma-tik\ Any of various tools and instruments that generate and use compressed air. Examples include rock drills, pave¬ ment breakers, riveters, forging presses, paint sprayers, blast cleaners, and atomizers. Compressed-air power is flexible, economical, and safe. In general, pneumatic systems have relatively few moving parts, contribut¬ ing to high reliability and low maintenance costs.

pneumatic structure Membrane structure that is stabilized by the pres¬ sure of compressed air. Air-supported structures are supported by inter¬ nal air pressure. A network of cables stiffens the fabric, and the assembly is supported by a rigid ring at the edge. The air pressure within this bubble is increased slightly above normal atmospheric pressure and maintained by compressors or fans. Air locks are required at entrances to prevent loss

of internal air pressure. Air-supported membranes were first devised by Walter Bird in the late 1940s and were soon put to use as covers for swimming pools, temporary warehouses, and exhibition buildings. Air- inflated structures are supported by pressurized air within inflated build¬ ing elements that are shaped to carry loads in a traditional manner. Pneumatic structures are perhaps the most cost-effective type of building for very long spans.

pneumococcus V.nu-mo-'ka-kosX Spheroidal bacterium ( Streptococcus pneumoniae ) that causes human diseases including pneumonia, sinusitis, ear infection, and meningitis. Usually occurring in the upper respiratory tract, this gram-positive (see gram stain) coccus is often found in a chain configuration and surrounded by a polysaccharide capsule. Pneumococci are separated into types depending on the specific capsular polysaccha¬ ride formed. Their disease-causing ability resides in the capsule, which delays or prevents their destruction by cells in the bloodstream that nor¬ mally engulf foreign material.

pneumoconiosis X.nu-mo-.ko-ne-'o-sssV Any lung disease caused by inhalation of organic or inorganic dusts or chemical irritants, usually over time. Some dusts (e.g., silica minerals, asbestos) produce grave reactions in small quantities. Dust collects in the pulmonary alveoli, causing inflam¬ mation that scars lung tissue, reducing its elasticity. Chest tightness and shortness of breath may progress to chronic bronchitis and emphysema. Cigarette smoking worsens many types. The most common cause of severe disease is silica (see silicosis). Asbestos (see asbestosis), beryllium, and aluminum dusts can cause worse disease, often on brief exposure. Organic causes include mold spores (see allergy) and textile fibres. Chemi¬ cal irritants such as ammonia, acid, and sulfur dioxide are soon absorbed by the lung lining; their irritant effect can cause pulmonary edema, and they can lead to chronic bronchitis.

pneumonia \nu-'mon-y9\ Inflammation and solidification of lung tissue caused by infection, foreign particle inhalation, or irradiation but usually by bacteria. Mycoplasma pneumoniae is the most common cause in healthy individuals. The bronchi and alveoli may be inflamed. Coughing becomes severe and may bring up flecks of blood. It can be serious but is rarely fatal. Streptococcus pneumoniae is more common and generally more severe but usually affects only those with low resistance, especially in hospitals. A highly lethal form caused by Klebsiella pneumoniae is almost always con¬ fined to hospitalized patients with low immunity. Other bacterial pneumo¬ nias include Pneumocystis carinii pneumonia (rare except in AIDS) and Legionnaire disease. Most respond to antibiotic treatment. ViRUses set the stage for bacterial pneumonia by weakening the individual’s immune sys¬ tem more often than they cause pneumonia directly. Fungal pneumonia usually occurs in hospitalized persons with low resistance, but contami¬ nated dusts can cause it in healthy individuals. It can develop rapidly and may be fatal. X-ray treatment (see radiation therapy) of structures in the chest may cause temporary lung inflammation.

pneumothorax Condition in which air accumulates in the pleural sac, causing it to expand and thus compress the underlying lung, which may then collapse. (The pleural sac is a cavity formed by the two pleural membranes that line the thoracic cavity and cover the lungs.) Traumatic pneumothorax is the accumulation of air caused by penetrating wounds (knife stabbing, gunshot) or other injuries to the chest wall, after which air is sucked through the opening and into the pleural sac. Spontaneous pneumothorax is the passage of air into the pleural sac from an abnormal connection created between the pleura and the bronchial system as a result of tuberculosis or some other lung disease. The symptoms of spontane¬ ous pneumothorax are a sharp pain in one side of the chest and shortness of breath.

Po Chu-i See Bo Juyi Po Hai See Bo Hai

Po River ancient Padus River, northern Italy. The country’s longest river, it is about 405 mi (652 km) long. It rises in the Cottian Alps on the western frontier and flows northeast to Turin, then east across Piedmont and Lombardy into the Adriatic Sea. Its delta is one of the most complex of any European river, with at least 14 mouths. It is navigable from its mouth to Pavia. Industrial cities in its valley include Milan, Padua, and Verona. It has suffered devastating autumn floods, including those in 589, 1438, 1951, and 1966.

Pobedonostsev Vpo-byi-do-'nost-syiA, Konstantin (Petrovich)

(b. May 21, 1827, Moscow, Russia—d. March 23, 1907, St. Petersburg)

© 2006 Encyclopaedia Britannica, Inc.

1516 I Pocahontas ► pogrom

Russian government administrator. He wrote and lectured on civil law at Moscow University (1859-65), tutored the sons of Tsar Alexander II, and later became a close adviser to Alexander III. influencing him to adopt reactionary policies. As director general of the Holy Synod of the Rus¬ sian Orthodox church (1880-1905), he assumed great power over domes¬ tic policy in education, religion, and censorship. Nicknamed “the Grand Inquisitor,” he became the symbol of Russian monarchical absolutism.

Pocahontas \,po-k3-'han-t3s\ (b. c. Va. [U.S.]—d. March 1617, Grave¬ send, Kent, Eng.) Powhatan Indian woman. Daughter of the powerful chief Powhatan, Pocahontas helped maintain peace between English colonists and Native Americans by befriending the settlers at Jamestown, Va. By the account of colonial leader John Smith, Pocahon¬ tas intervened to save his life after he had been taken prisoner by her father’s men. She subsequently con¬ verted to Christianity and wedded the colonist John Rolfe, which fur¬ thered efforts toward peace. She traveled to England, where she was received at court, but soon afterward she died, probably of lung disease.

pocket billiards See pool pocket gopher See gopher

1596, near present-day Jamestown,

Pocahontas, detail of a portrait by an unknown artist, 1616.

LIBRARY OF CONGRESS, WASHINGTON, D.C.

pocket mouse Any of about 30 species of nocturnal North American rodents constituting the genus Perognathus (family Heteromyidae), hav¬ ing fur-lined, external cheek pouches that open alongside the mouth. Pocket mice are yellowish brown to dark gray and are 2.5-5 in. (6-13 cm) long, excluding a tail of about the same length. They are usually soli¬ tary and inhabit dry and desert regions. They carry food (mainly seeds) in their pouches and store it in their burrows. Spiny pocket mice (most in the genera Liomys and Heteromys\ family Heteromyidae), found from Mexico through Central America, are gray, brown, or black nocturnal burrowers that inhabit wet, forested regions as well as dry country.

podesta \,po-d3-'sta\ (Italian: “power”) In medieval Italian communes, the highest judicial and military magistrate. The office was instituted by Frederick I Barbarossa in an attempt to govern rebellious Lombard cities. From the late 12th century the communes usually elected their own pod¬ esta, often a nobleman with legal training. Serving for six months or a year, the podesta summoned councils, led the communal army, and admin¬ istered civil and criminal jurisdiction.

Podgorica Vpod-.gor-et-sa \ formerly (1946-92) Titograd City (pop., 1999 est.: 130,875), capital of Montenegro, southern Serbia and Montene¬ gro. As a feudal state capital in the early Middle Ages, it was known as Ribnica; it was called Podgorica from 1326. It fell to the Turks in 1474 but was restored to Montenegro in 1878. It was occupied in 1916 by Austria, in 1941 by Italy, and in 1943 by Germany. In 1946 it became Titograd in honour of Marshal Tito, but its former name was restored in 1992 after the collapse of communism. It is the site of a university (founded 1974).

Podgorny \p6d-'g6r-ne\, Nikolay (Viktorovich) (b. Feb. 18,1903, Karlovka, Ukraine, Russian Empire—d. Jan. 12, 1983, Moscow, Russia, U.S.S.R.) Soviet politician. He worked in engineering jobs in the sugar industry, then was appointed deputy commissar of the Soviet food¬ processing industry in 1940. He rose through the Communist Party hier¬ archy to become chair of the Ukraine party committee (1957-63), a full member of the Politburo (1960), and head of the party’s Central Com¬ mittee (1963-65). In a power struggle with Leonid Brezhnev, Podgorny was demoted to the ceremonial post of chairman of the Presidium of the Supreme Soviet (1965-77).

podiatry \p3-'dI-3-tre\ or chiropody \,k3-'rap-3d-e\ Medical specialty dealing with the foot. Podiatrists diagnose and treat foot diseases, disabili¬ ties, and deformities by means of physical medicine and rehabilitation, special shoes and other mechanical devices, drugs, and minor surgery.

podium In architecture, a pedestal on a large scale. It may be any of various elements that form the base of a structure, such as the platform

forming the floor and substructure of a Classical temple, a low wall sup¬ porting columns, or the structurally or decoratively emphasized lowest portion of a wall. The term is also applied to other types of raised plat¬ form, such as an orchestra conductor’s dais.

Poe, Edgar Allan (b. Jan. 19, 1809, Boston, Mass., U.S.—d. Oct. 7, 1849, Baltimore, Md.) U.S. poet, critic, and short-story writer. Poe was raised by foster parents in Richmond, Va., following his mother’s death in 1811. He briefly attended the University of Virginia and then returned to Boston, where in 1827 he published a pamphlet of youthful, Byronic poems. By 1835 he was in Richmond as editor of the Southern Literary Messenger, the first of several periodicals he was to edit or write for. There he married a 13-year-old cousin, who died in 1847. At various times he lived in Baltimore, New York, and Philadelphia. Alcohol, the bane of his irregular and eccentric life, caused his death at age 40. His works are famous for his cultivation of mystery and the macabre. Among his tales are “The Fall of the House of Usher,” “The Masque of the Red Death,” “The Black Cat,” “The Tell-Tale Heart,” and “The Pit and the Pendulum.” “The Murders in the Rue Morgue” and “The Purloined Letter” initiated the modern detective story. His poems (less highly regarded now than for¬ merly) are musical and sensuous, as in “The Bells,” a showcase of sound effects; they include touching lyrics inspired by women (e.g., “Annabel Lee”) and the uncanny (e.g., “The Raven”).

poet laureate Title first granted in England for poetic excellence. Begun in 1616, the office was formally established in 1668 and has been continuous since then. Its holder, a salaried member of the British royal household, was formerly expected to compose poems for court or national occasions, but since the appointment of William Wordsworth in 1843 the office has been a reward for eminence in poetry and has carried no spe¬ cific duties. In 1985 the U.S. government created the title of poet laure¬ ate, to be held by the consultant in poetry to the Library of Congress.

poetry Writing that formulates a concentrated imaginative awareness of experience in language chosen and arranged to create a specific emotional response through its meaning, sound, and rhythm. It may be distinguished from prose by its compression, frequent use of conventions of metre and rhyme, use of the line as a formal unit, heightened vocabulary, and free¬ dom of syntax. Its emotional content is expressed through a variety of techniques, from direct description to symbolism, including the use of metaphor and simile. See also prose poem; prosody.

Poetry U.S. poetry magazine founded in Chicago in 1912 by Harriet Monroe, who became its longtime editor. It became the principal organ for modern poetry of the English-speaking world and survived through World War II. Because its inception coincided with the Chicago literary renaissance, it is often associated with the raw, local-colour poetry of Carl Sandburg, Edgar Lee AAasters, Vachel Lindsay, and Sherwood Anderson, but it also championed new formalistic movements, including Imagism. Ezra Pound was its European correspondent; among the authors it published were T.S. Eliot, Wallace Stevens, Marianne Moore, D.H. Lawrence, and William Carlos Williams.

Poggio Bracciolini Vpod-jo-.brat-cho-'le-neX, Gian Francesco (b.

Feb. 11, 1380, Terranuova, Tuscany—d. Oct. 30, 1459, Florence) Italian humanist and calligrapher. While working as a copyist of manuscripts, Poggio invented the humanist script, which later became the prototype for Roman fonts in printing. He traveled to various monasteries in Europe, uncovering lost, forgotten, or neglected classical Latin manuscripts, including works by Cicero and Lucretius. He also translated works by Lucian, Xenophon, and others into Latin. His own writings include moral dialogues and Facetiae (1438-52), a collection of humorous tales con¬ taining satires of his contemporaries.

pogrom \p3-'gram\ Russian "devastation" or "riot" Mob attack, condoned by authorities, against persons and property of a religious, racial, or national minority. The term is usually applied to attacks on Jews in Russia in the late 19th and early 20th centuries. After the assassination of Tsar Alexander II (1881), false rumours associating Jews with the mur¬ der aroused Russian mobs in more than 200 cities and towns to attack Jews and destroy their property. Mob attacks diminished in the 1890s, but they again became common in 1903-06. Although the government did not organize pogroms, its anti-Semitic policy (1881-1917) and reluctance to stop the attacks led many anti-Semites to believe that their violence was legitimate. Pogroms also occurred in Poland and in Germany during Adolf Hitler’s regime.

© 2006 Encyclopaedia Britannica, Inc.

pogy ► poison ivy I 1517

pogy See menhaden

Poincare \pwa n -ka-'ra\, (Jules-) Henri (b. April 29, 1854, Nancy, France—d. July 17, 1912, Paris)

French mathematician, theoretical astronomer, and philosopher of sci¬ ence. Born into a distinguished fam¬ ily of civil servants (see Raymond Poincare), he excelled at mental cal¬ culation and possessed an unusually retentive memory. He wrote a doc¬ toral dissertation on differential equa¬ tions (1879), then joined the University of Paris (1881), where he remained the rest of his life. Work¬ ing in celestial mechanics and math¬ ematical analysis, he independently obtained many of Albert Einstein’s results relating to the special theory ’~* enri Poincar ®' 1 ?09.

ot relativity and published them in a -

paper on the dynamics of the elec¬ tron (1906). He later wrote books for the general public on the meaning and importance of science and mathematics.

Poincare \pwa"-ka-'ra\, Raymond (b. Aug. 20, 1860, Bar-le-Duc, France—d. Oct. 15, 1934, Paris) French politician. A lawyer, he served in the Chamber of Deputies (1887-1903) and the Senate (1903-12) and as minister of education (1893, 1895) and finance minister (1894, 1906). As prime minister and foreign minister (1912-13), he strengthened France’s ties with Russia and Britain. As president of the Third Republic (1913-20), he vigorously supported national unity. As prime minister again in 1922-24 and 1926-29, he was credited with solving France’s financial crisis by sta¬ bilizing the franc, enabling a period of new prosperity.

poinsettia \p6in-'se-te-9\ Popular flowering plant (Euphorbia pulcher- rima), best-known member of the diverse spurge family. Native to Mexico and Central America, it grows in moist, wet, wooded ravines and on rocky hillsides. What appear to be flower petals are actually coloured leaflike bracts that surround a central cluster of tiny yellow flow¬ ers. Cultivated varieties are available with white, pink, mottled, and striped bracts, but the solid red vari¬ eties are in greatest demand during the Christmas season. Milky latex in the stems and leaves can be irritating to sensitized persons or animals, but the claim that poinsettias are deadly poisonous is greatly exaggerated.

Point Four Program U.S. policy of technical assistance and economic aid to less-developed countries. Pres.

Harry S. Truman proposed the plan as the fourth point of his 1949 inau¬ gural address. It was approved by Congress and administered by a spe¬ cial State Department agency until it was merged with other foreign-aid programs in 1953. Technical assistance, mainly in agriculture, public health, and education, was provided through contracts with U.S. business and educational organizations. Organizations such as the Inter-American Development Bank were created to help develop the program.

Pointe-Noire V.pwa'VnwarV City (pop., 1992 est.: 576,206) and port, southwestern Republic of the Congo. It was the capital (1950-58) of the Middle Congo region of French Equatorial Africa. With independence in 1958, it was replaced by Brazzaville as the national capital, but it remained important for trade. Its port facilities were completed in 1939 and expanded during World War II. The country’s second largest city, it is a principal port and commercial centre, especially for the oil industry.

Poinsettia (Euphorbia pulcherrima).

GRANT HEILMAN-EB INC.

pointer Dog breed, of hound, spaniel, and setter ancestry, first recorded c. 1650 in Great Britain and named for the dog’s rigid posture in the direction of quarry. Pointers were originally used to point out hares and were later trained as bird dogs. The pointer stands 23-28 in. (58-71 cm) tall and weighs 50-75 lb (23-34 kg); it has a long muzzle, hanging ears,

tapered tail, and a short, smooth coat, usually white with dark markings. The German short-haired pointer, another sporting breed, tracks, points, and retrieves; it is about the size of a pointer and has a short coat of solid liver colour or liver and gray-white.

German short-haired pointer

SALLY ANNE THOMPSON

pointillism \'pwa n -te-,yi-z3m, 'poin-Ul-.i-zomV In painting, the practice of applying small strokes or dots of contrasting colour to a surface so that from a distance they blend together. The term (and its synonym, division- ism) was first used to describe the paintings of Georges Seurat. See also Camille Pissarro; Paul Signac.

Poiret \pwa-'re\, Paul (b. April 20, 1879, Paris, Fr.—d. April 30, 1944, Paris) French fashion designer. After working in the Parisian fashion house of Charles Frederick Worth, he opened his own shop in 1902. In 1908 he revived the Empire style, popular in France during the reign of Napoleon I. Seeking to restore naturalness to female garb, he was prin¬ cipally responsible for the decline of the corset. He is best known for the hobble skirt, to which he later added draped and belted knee-length tunics. Fringed and tasseled capes, multicoloured feathers, and fox stoles imparted a theatrical look to his designs. His flowing Greek costumes were extremely popular in the prewar era, but his popularity faded in the 1920s and he died in poverty.

poison Any substance (natural or synthetic) that, at a certain dosage, damages living tissues and injures or kills. Poisons spontaneously pro¬ duced by living organisms are often called toxins, venoms if produced by animals. Poisons may be ingested, inhaled, injected, or absorbed through the skin. They do not always have an all-or-none effect; degrees of poi¬ soning may occur, and at a given dose some substances are far more toxic than others (e.g., a pinch of potassium cyanide can kill, whereas a single dose of ordinary table salt must be massive to kill). Poisoning may be acute (a single dose does significant damage) or chronic (repeated or con¬ tinuous doses produce an eventual effect, as with chemical carcinogens). The effects produced by poisons may be local (hives, blisters, inflamma¬ tion) or systemic (hemorrhage, convulsions, vomiting, diarrhea, clouding of the senses, paralysis, respiratory or cardiac arrest). Agricultural pesti¬ cides are often poisonous to humans. Some industrial chemicals can be very toxic or carcinogenic. Most therapeutic drugs and health-care prod¬ ucts can be poisons if taken inappropriately or in excess. Most forms of radiation can be toxic (see radiation injury). See also antidote; arsenic poi¬ soning; fish poisoning; food poisoning; lead poisoning; medicinal poisoning;

MERCURY POISONING; MUSHROOM POISONING.

poison hemlock Any of several poisonous herbaceous plants of the parsley family, especially Conium maculatum, believed to be the plant that killed Socrates. It is now common in the U.S. as well as in Europe. A tall biennial, this plant has green stems spotted with red or purple, large com¬ pound leaves, and white flowers. Though the poison is concentrated in the seeds, the entire plant is dangerous to livestock when fresh. Despite their common name, poison hemlocks are not conifers (see hemlock). Water hemlocks (Cicuta species) are similar and also dangerous.

poison ivy Either of two North American species of white-fruited woody vines or shrubs of the sumac, or cashew, family. The species found

© 2006 Encyclopaedia Britannica, Inc.

1518 I poison oak ► Poland

in eastern North America ( Toxico¬ dendron radicans) is abundant; a western species, known as poison oak, is less common. Both species are sometimes classified as genus Rhus. A key identifier is leaves with three mitten-shaped leaflets. Contact with urushiol, an oil produced by the plant, can cause severe inflammation and blistering of human skin. Urush¬ iol may be carried from the plant on clothing, shoes, tools, or soil; by ani¬ mals; or by smoke from burning plants. Because urushiol is nonvola¬ tile, a reaction may result from wear¬ ing clothing a year or more after its contact with the plant.

poison oak Species of poison ivy ( Toxicodendron diversilobum) native to western North America and classified in the sumac (or cashew) family. Like many other lobe-leafed plants commonly called oak, poison oak is not an OAK tree (genus Quercus).

poison sumac also called poison elder Attractive, narrow shrub or small tree ( Rhus vernix or Toxicodendron vernix ) of the sumac, or cashew, family. It is native to swampy acidic soils of eastern North America. Unlike the upright reddish, fuzzy fruit clusters of other sumacs, whitish waxy berries droop loosely from its stalks. The clear sap, which black¬ ens on exposure to air, is extremely irritating to the skin for many people.

Poitier \'pwa-te-,a\, Sidney (b. Feb. 20, 1927?, Miami, Fla., U.S.) U.S. actor. He was raised in The Bahamas, then studied and acted with the American Negro Theatre in New York City. He made his film debut in No Way Out (1950) and gave notable performances in Blackboard Jungle (1955) and The Defiant Ones (1958). He won acclaim on Broadway for his role in A Raisin in the Sun (1959). He became the first African Ameri¬ can actor to win an Academy Award, for his role in Lilies of the Field (1963). He used his critical and commercial success to push for integrated film crews. He went on to star in notable films such as In the Heat of the Night (1967), To Sir with Love (1967), and Guess Who’s Coming to Din¬ ner (1967). He also directed several films, including Buck and the Preacher (1972), Let’s Do It Again (1975), and Stir Crazy (1980). In 1997 he was appointed The Bahamian ambassador to Japan.

Poitiers, Battle of (732) See Battle of Tours/Poitiers

Poitiers \pwa-'tya\, Battle of (September 19, 1356) Catastrophic defeat of the French king John II in the Hundred Years' War between France and England. English troops under Edward the Black Prince were under pursuit from the probably superior French forces. South of Poitiers the English secured themselves in thickets and marshes, where the French knights became bogged down and made easy targets for the English archers. John II was taken prisoner and for his freedom had to consent to the disadvantageous Treaties of Bretigny and Calais.

Poitou \pwa-'tu\ Historical region, western central France. It was bounded by Brittany, Anjou, Touraine, Marche, and the Atlantic Ocean. It was inhabited by the ancient Gallic tribe of Pictones and became part of Roman Aquitania. A meeting place of northern and southern cultures, its golden age (11th—12th century) was characterized by great Romanesque art and architecture. The counts of Poitiers were succeeded by the Angevin kings of England, but by 1375 the French had won the region back. It was a province of France until the French Revolution, when it was divided into three departments. It is predominantly a rural area; regional specialties include seafood and white wine.

poker Any of several card games in which a player bets that the value of his or her hand is greater than that of the hands held by others. Each subsequent player must either equal or raise the bet or drop out. The pot is eventually won by either the player showing the best hand when it comes to a showdown or the only player left when everyone else has dropped out, or “folded.” In this case the winner need not show his hand and could conceivably have won the pot with a lower hand than any other at the table. It is for this reason that poker is described as a game of bluff. Three principal forms of the game have developed: straight poker, in which all cards of the standard five-card hand are dealt facedown; stud poker, in which some but not all of a player’s cards are dealt faceup; and

Poison ivy (Toxicodendron radicans)

WALTER CHANDOHA

community-card poker, in which some cards are exposed and used by all the players to form their best hand. In draw poker, the main variant of straight poker, cards may be discarded and additional cards drawn. The traditional ranking of hands is (1) straight flush (five cards of the same suit in sequence, the highest sequence—ace, king, queen, jack, ten—being called a royal flush), (2) four of a kind, (3) full house (three of a kind, plus a pair), (4) flush (five of a single suit), (5) straight (five in sequence), (6) three of a kind, (7) two pair, (8) one pair.

pokeweed also called pokeberry or poke Strong-smelling shrub¬ like plant ( Phytolacca americana) with a poisonous root resembling the shape of horseradish, native to wet or sandy areas of eastern North America. It has white flowers, reddish black berries, and dark green leaves that often are red-veined or borne on red leafstalks. The berries contain a red dye used to colour wine, candies, cloth, and paper. Like the roots, the red or purplish mature stalks are poisonous. Very young green shoots (up to about 6 in. [15 cm]), however, are edible.

Pokrovsky Cathedral See Saint Basil the Blessed

Pol Pot orig. Saloth Sar (b. May 19, 1925/28, Kompong Thom prov¬ ince, Cambodia—d. April 15, 1998, near Anlong Veng) Prime minister of Cambodia. He joined the anti-French resistance under Ho Chi Minh in the 1940s and became a member of the Cambodian Communist Party in 1946. After studying radio technology in Paris, he returned to Cambodia in 1953 and taught French until he fell under police suspicion in 1963. For the next 12 years he built up the Communist Party. U.S. anticommunist activity in Cambodia in the early 1970s, including helping to depose Norodom Siha¬ nouk and a bombing campaign in the countryside, drove many to join him. In 1975 his Khmer Rouge forces captured Phnom Penh, which he ordered entirely evacuated. Even the sick were wheeled out in hospital beds, and in the first weeks it is estimated that tens of thousands died. The ruthlessness with which he pursued his intention to return to “year zero” and create an ethnically pure, agrarian, communist state resulted in the deaths of one to two million people. Overthrown by the Vietnamese in 1979, he led an anti- Vietnamese guerrilla war until he was repudiated by the Khmer Rouge and sentenced to life imprisonment (1997); he died the next year.

Poland officially Republic of Poland Country, central Europe. Area: 120,728 sq mi (312,685 sq km). Population (2005 est.): 38,164,000. Capi¬ tal: Warsaw. Most of the people are Polish; there are minorities of Ukrai-

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© 2006 Encyclopaedia Britannica, Inc.

Poland ► pole vault I 1519

and the Carpathian Mountains. The Vistula and Oder, the principal river systems, both drain into the Baltic Sea. Industries include mining, manu¬ facturing, and public utilities. Poland is a republic with two legislative houses; its chief of state is the president, and its head of government is the prime minister. Established as a kingdom in 922 under Mieszko I, Poland was united with Lithuania in 1386 under the Jagiellon dynasty (1386-1572) to become the dominant power in east-central Europe, enjoying a prosperous golden age. In 1466 it wrested western and east¬ ern Prussia from the Teutonic Order, and its lands eventually stretched to the Black Sea. Wars with Sweden (see First Northern War; Second Northern War) and Russia beginning in the late 17th century led to the loss of considerable territory. In 1697 the electors of Saxony became kings of Poland, virtually ending Polish independence. In the late 18th century Poland was divided between Prussia, Russia, and Austria (see partitions of Poland) and ceased to exist. After 1815 the former Polish lands came under Russian domination, and from 1863 Poland was a Russian prov¬ ince, subjected to intensive Russification. After World War I an indepen¬ dent Poland was established by the Allies. The invasion of Poland in 1939 by the U.S.S.R. and Germany precipitated World War II, during which the Nazis sought to purge Poland’s culture and its large Jewish popula¬ tion in the Holocaust. Reoccupied by Soviet forces in 1945, Poland was controlled by a Soviet-dominated government from 1947. In the 1980s the Solidarity labour movement led by Lech Walesa achieved major politi¬ cal reforms, and free elections were held in 1989. An economic austerity program instituted in 1990 sped the transition to a market economy. Poland became a member of NATO in 1999 and the European Union in

Poland, partitions of (1772, 1793, 1795) Territorial divisions of Poland by Russia, Prussia, and Austria that progressively reduced its ter¬ ritory until it ceased to exist as a state. In the First Partition (1772), a Poland weakened by civil war and Russian intervention agreed to a treaty signed by Russia, Prussia, and Austria that deprived it of half its popu¬ lation and almost one-third of its land area. In the Second Partition (1793), Poland was forced to cede additional lands to Prussia and Russia. To quell a nationalist uprising led by Tadeusz Kosciuszko, Russia and Prussia invaded Poland and divided the remnants of the state among themselves and Austria in the Third Partition (1795). Only with the establishment of the Polish Republic in 1918 were the results of the partitions reversed.

Polanski \p3-'lan-ske\, Roman (b. Aug. 18, 1933, Paris, France) Polish-French film director. He grew up in Poland and survived a trau¬ matic wartime childhood under the Nazis. His first feature film, Knife in the Water (1962), brought him international fame. He left Poland that year for Britain, where he made Repulsion (1965), and later the U.S., where his Rosemary’s Baby (1968) was highly successful. In 1969 his new wife, the actress Sharon Tate, was murdered by followers of Charles Manson. He directed a graphic adaptation of Macbeth (1971) and the acclaimed film noir Chinatown (1974). In 1977 Polanski was arrested and eventually pleaded guilty to a charge of statutory rape. He subsequently jumped bail and fled to France, where he remained active in both the theatre and motion pictures. His subsequent films include Tess (1979), Frantic (1988), Bitter Moon (1992), Death and the Maiden (1994), and The Pianist (2002), which won the Gold Palm for best film at the Cannes International Film Festival and earned a best director Academy Award for Polanski.

polar bear White semiaquatic bear ( Ursus maritimus) found through¬ out Arctic regions, generally on drift¬ ing oceanic ice floes. A swift, wide- ranging traveler and a good swimmer, it stalks and captures its prey. It primarily eats seal but also fish, seaweed, grass, birds, and cari¬ bou. The male weighs 900-1,600 lbs (410-720 kg) and is about 5.3 ft (1.6 m) tall at the shoulder and 7-8 ft (2.2-2.5 m) long. It has a short tail.

The hairy soles of its broad feet pro¬ tect it from the cold and help it move across the ice. Though shy, it is dan¬ gerous when confronted.

polar wandering Migration of the magnetic poles of the Earth through geologic time. Scientific evidence indicates that the magnetic poles have slowly and erratically wandered across the surface of the Earth. Pole locations calculated from measurements on rocks younger than about

20 million years do not depart from the present pole locations by very much, but successively greater “virtual pole” distances are revealed for rocks older than 30 million years, indicating that substantial deviations occurred. Calculations of polar wandering formed one of the first impor¬ tant pieces of evidence for continental drift.

Polaris \p3-'lar-3s\ or North Star Earth’s present northern polestar (the star visible from the Northern Hemisphere toward which Earth’s axis points), at the end of the “handle” of the Little Dipper in the constella¬ tion Ursa Minor. Polaris is actually a triple star, composed of a binary star and a Cepheid variable. Precession of Earth’s axis made the star Thuban, in the constellation Draco, the North Star in ancient Egyptian times; it will cause the North Pole to point toward Vega, in the constellation Lyra, 12,000 years from now.

polarization Property of certain types of electromagnetic radiation in which the direction and magnitude of the vibrating electric field are related in a specified way. The electric vector representing the magnitude and direction of the electric field in a wave of light is perpendicular to the direction in which the wave is moving. Unpolarized light consists of waves moving in the same direction with their electric vectors pointing in random orientations about the axis of propagation. Plane-polarized light consists only of waves that vibrate in one direction. In circular polariza¬ tion the electric vector rotates about the propagation direction. Light may be polarized by reflection or by passing it through polarizing filters, such as certain crystals, that transmit vibrations in one plane but not in others. Polarized light has useful applications in crystallography, liquid-crystal displays, optical filters, and the identification of optically active chemi¬ cal compounds.

Polaroid Corp. Major U.S. manufacturer of photographic equipment and supplies. The company was established as Land-Wheelwright Labo¬ ratories in 1932 by Edwin Land and George Wheelwright to produce Land’s first invention, an inexpensive plastic-sheet light polarizer (see polarization). By 1936 Land was using polarized material in sunglasses and other optical devices, and in 1937 the company was incorporated under the Polaroid name. After World War II Land invented the first instant camera, marketed in 1947 as the Polaroid Land camera, which delivered a finished sepia-toned print 60 seconds after exposure. The com¬ pany introduced colour film for its Polaroid cameras in the 1960s and instant motion pictures in 1977. Other Polaroid products include film, digital cameras, and identification systems.

Pole, Reginald (b. March 3, 1500, Stourton Castle, Staffordshire, Eng.—d. Nov. 17, 1558, London) English Catholic prelate. A cousin of Henry VII, Pole was sent by Henry VIII to study in Italy (1521-27) and given minor offices in the church. Critical of Henry’s antipapal policies, he wrote In Defense of Ecclesiastical Unity (1536) to defend the pope’s spiritual authority. As cardinal, he was sent by Pope Paul III on missions to persuade Catholic monarchs to depose Henry. These efforts angered Henry, who executed Pole’s brother, Lord Montague (1538), and his mother, Margaret, countess of Salisbury (1541). Pole was named papal governor of the Patrimony of St. Peter and later was presiding legate at the Council of Trent. When the Catholic Mary Tudor became queen as Mary I in 1553, he was appointed legate for England; there he instituted church reforms and was a strong influence on the queen. He was appointed archbishop of Canterbury (1556), but a conflict between the papacy and England’s ally Spain caused the pope to cancel Pole’s authority and declare him a heretic. Demoralized, he died 12 hours after the death of Queen Mary.

pole construction Method of building that dates back to the Stone Age. Excavations in Europe show rings of stones that may have braced huts made of wooden poles or weighted down the walls of tents made of animal skins supported by central poles. Two types of Native American pole structures were the wigwam and longhouse. Pole-and-thatch dwell¬ ings are common in the Caribbean, Mesoamerica, and the Pacific Islands; bamboo-pole dwellings constructed on piles are found in many wet areas of Asia. A southern African method utilizes a ring of poles that is inserted into the ground, brought together in a crest, and expertly thatched. Today, pole construction employs a vertical structure of pressure-treated wood poles firmly embedded in the ground as a pier foundation. See also tent STRUCTURE.

pole vault Track-and-field event consisting of a vault for height over a crossbar with the aid of a long pole. It became a competitive sport in the mid-19th century and was included in the first modem Olympic Games.

Polar bear (Ursus maritimus)

FRANCOIS GOHIER/ARDEA LONDON

© 2006 Encyclopaedia Britannica, Inc.

1520 I polecat ► political action committee

In competition, each vaulter is given three chances to clear a specific height. The bar is raised progressively until a winner emerges.

polecat Any of several carnivores of the weasel family (Mustelidae), found in Eurasia and Africa. The polecat hunts at night, principally on the ground, feeding on small mammals, birds, reptiles, frogs, fishes, and eggs. Species differ in size and colour. The European, or common, polecat (Mustela putorius), also called foul marten for its odour, weighs 1-3 lb (0.5-1.4 kg) and is 14-21 in. (35-53 cm) long, excluding the 5-8-in. (13- 20-cm) bushy tail. Its long, coarse fur is brown above, black below. In the U.S., skunks are often called polecats. See also ferret.

European polecat (Mustela putorius)

RUSS KINNE-PHOTO RESEARCHERS

police Body of agents organized to maintain civil order and public safety, enforce the law, and investigate crime. Characteristics common to most police forces include a quasi-military organization, a uniformed patrol and traffic-control force, plainclothes divisions for criminal investigations, and a set of enforcement priorities that reflects the community’s way of life. Administration may be centralized at the national level downward, or decentralized, with local police forces largely autonomous. Recruits usu¬ ally receive specialized training and take an exam. The modem metro¬ politan police force began with Sir Robert Peel in Britain c. 1829. Secret police are often separate, clandestine organizations established by national governments to maintain political and social orthodoxy, which typically operate with little or no restraint.

police power Power of a government to exercise reasonable control over people and property within its jurisdiction in the interest of general security, health, safety, morals, and welfare. It is generally regarded as one of the powers reserved to the states under the U.S. Constitution. In considering cases involving the exercise of police power, the courts have applied a doctrine called “balance of interests” to determine when the public’s right to health and well-being outweighs private or individual concerns. Of equal concern is that due process of law be observed.

poliomyelitis N.po-le-o-.ml-a-'lI-tosV or polio or infantile paraly¬ sis Acute infectious viral disease that can cause flaccid paralysis of muscles. Severe epidemics killed or paralyzed many people, mostly chil¬ dren and young adults, until the 1960s, when Jonas Salk’s injectable killed vaccine and Albert B. Sabin’s oral attenuated live vaccine controlled polio in the developed world. Flulike symptoms with diarrhea may progress to back and limb pain, muscle tenderness, and stiff neck. Destruction of spi¬ nal cord motor cells causes paralysis, ranging from transient weakness to complete, permanent paralysis, in fewer than 20% of patients. Patients may lose the ability to use their limbs, to breathe, or to swallow and speak. They may need physical medicine and rehabilitation, mechanical breathing assistance, or tracheal suction to remove secretions. A “postpolio syn¬ drome” occurs decades later in some cases, with weakness of muscles that had recovered.

polis Vpa-l9s\ In ancient Greece, an independent city and its surround¬ ing region under a unified government. A polis might originate from the natural divisions of mountains and sea and from local tribal and cult divi¬ sions. Usually the town was walled and contained a citadel on raised ground (acropolis) and a marketplace (agora). Government was centred in the town; usually there was an assembly of citizens, a council, and magistrates. Ideally, all citizens participated in the government and in the cults, as well as in defense and economy. Women, minors, metics, and

slaves were not citizens. Hellenism spread many of the institutions into the Middle East. See also Athens; city-state; Sparta; Thebes.

Polisario N.po-li-'sa-re-oX officially Popular Front for the Libera¬ tion of Saguia el Hamra and Rio de Oro Sahrawi political and military group. Initially an insurgent against Spanish control of Western Sahara, it turned to agitation against Morocco and Mauritania when the Spanish withdrew in 1976 and those two countries partitioned the coun¬ try. Mauritania made peace with the group in 1979, whereupon Morocco annexed the whole territory. Polisario continued its resistance, mostly from bases in Algeria. In 1991 it agreed to a ceasefire and a referendum, which has been repeatedly postponed by Morocco, to determine the state’s fate. See also Saharan Arab Democratic Republic.

Polish Corridor Strip of land that gave Poland access to the Baltic Sea. Transferred to the newly constituted state of Poland as part of the Treaty of Versailles (1919), the corridor, 20 to 70 mi (30 to 110 km) wide, separated eastern Prussia from the main part of Germany. The Germans resented the transfer, though the region had been historically Polish before the partitions of Poland and was inhabited by a Polish majority. When Poland refused to accede to Adolf Hitler’s demands for extraterritorial highways across the corridor and cession of the free port city of Danzig (Gdansk), Germany seized the pretext to invade Poland (1939), beginning World War II.

Polish language West Slavic language of Poland, spoken by more than 41 million people, including 2-3 million in North America and perhaps 1.5 million in the former Soviet Union. The earliest continuous text in Polish dates from the 14th century. The standard language, formulated in the 16th century, combines features of western and southeastern dialects. Polish is written in the Latin alphabet and utilizes both digraphs (combi¬ nations of letters) and diacritics to distinguish its fairly elaborate reper¬ tory of consonants. Stress is fixed on the next-to-last syllable.

Polish Succession, War of the (1733-38) European conflict waged ostensibly to determine the successor to Augustus II. Austria and Russia supported his son Augustus III, while most Poles, France, and Spain sup¬ ported Stanislav/ I. a former Polish king (1704-09) and father-in-law of France’s Louis XV. Stanislaw was elected king in 1733, but a Russian threat forced him to flee, and Augustus was elected in his place. France, with Sardinia and Spain, declared war on Austria (1733), seeking to reclaim territory in Italy held by Austria. An inconclusive campaign ended in the preliminary Peace of Vienna (1735), which redistributed the dis¬ puted Italian territory and recognized Augustus as king. A final treaty was signed in 1738.

Politburo \'pa-l9t-,byur-o\ Supreme policy-making body of the Com¬ munist Party of the Soviet Union, the model for the politburos in other countries. The first Politburo, created in 1917 to provide leadership dur¬ ing the Bolshevik uprising, was dissolved when the coup was accom¬ plished. The party congress of 1919 instructed the Central Committee to elect a new Politburo, which soon overshadowed the Central Committee in power. In 1952 it was replaced by a larger Presidium of the Central Committee; after Joseph Stalin’s death, stress was placed on “collective leadership” to correct for his abuses. The name Politburo was revived in 1966. Its members included the general secretary of the Communist Party, the minister of defense, the head of the KGB, and the heads of the most important republics or urban party branches. It was dissolved with the breakup of the Soviet Union in 1991.

Politian \po-Ti-shon\ orig. Angelo Poliziano or Angelo Ambrogini (b. July 14, 1454, Montepulciano, Tuscany—d. Sept. 28/29, 1494, Florence) Italian poet and humanist. He demonstrated his poetic abilities early and became a friend and protege of Lorenzo de’ Medici. One of the foremost classical scholars of the Renaissance, he produced between 1473 and 1478 Latin and Greek verses that are among the best examples of humanist poetry. He was also, with Lorenzo, a leader in the revaluation of literature in Italian. His vernacular works include Stanzas Begun for the Tournament of the Magnificent Giuliano de ’ Medici (1475— 78), a masterpiece in ottava rima, and the drama Orfeo (1480).

political action committee (PAC) In U.S. politics, an organization whose purpose is to raise and distribute campaign funds to candidates seeking political office. PACs rose to prominence after the Federal Elec¬ tion Campaign Act (1971) limited the amount of money any corporation, union, or private individual could give to a candidate. PACs were able to circumvent these limits by soliciting smaller contributions from a much

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political convention ► pollack I 1521

larger number of individuals. During the late 20th and early 21st centu¬ ries the vast amounts of money raised by PACs greatly increased the cost of running for office and led to efforts to reform this method of financing campaigns.

political convention or party conference In politics, a meeting of members of a political party at the local, state, or national level to select party leaders and candidates for office and to determine party policy. Dur¬ ing presidential election years in the U.S., the main parties hold conven¬ tions that serve to showcase their presidential and vice presidential candidates and to boost the morale of party members for the campaigns that follow. Conventions were instituted in the U.S. in the 1830s to replace the often exclusive and secretive caucus system; it was hoped that the conventions’ openness would make them less vulnerable to control by party bosses. Most candidates for political office at all levels in the U.S. are now nominated through primary elections, and the conventions merely ratify the candidates already selected by the voters. Political parties in other countries (e.g., Great Britain) often hold annual party conferences.

political economy Academic discipline that explores the relationship between individuals and society and between markets and the state, using methods drawn from economics, political science, and sociology. The term is derived from the Greek terms polis (city or state) and oikonomos (one who manages a household). Political economy is thus concerned with how countries are managed, taking into account both political and eco¬ nomic factors. The field today encompasses several areas of inquiry, including the politics of economic relations, domestic political and eco¬ nomic issues, the comparative study of political and economic systems, and the study of international political economy.

political machine In U.S. politics, a political organization that con¬ trols enough votes to maintain political and administrative control of its community. The rapid growth of cities in the 19th century created huge problems for city governments, which were often poorly organized and unable to provide services. Enterprising politicians were able to win sup¬ port by offering favours, including patronage jobs and housing, in exchange for votes. Though machines often helped to restructure city governments to the benefit of their constituents, they just as often resulted in poorer service (when jobs were doled out as political rewards), cor¬ ruption (when contracts or concessions were awarded in return for kick- backs), and aggravation of racial or ethnic hostilities (when the machine did not reflect the city’s diversity). Reforms, suburban flight, and a more mobile population with fewer ties to city neighbourhoods have weakened machine politics. Famous machines include those of William AAarcy Tweed (New York), James Michael Curley (Boston), Thomas Pendergast (Kansas City, Mo.), and Richard J. Daley (Chicago). See also civil service.

political party Group of persons organized to acquire and exercise political power. Formal political parties originated in their modern form in Europe and the U.S. in the 19th century. Whereas mass-based parties appeal for support to the whole electorate, cadre parties aim at attracting only an active elite; most parties have features of both types. All parties develop a political program that defines their ideology and sets out the agenda they would pursue should they win elective office or gain power through extraparliamentary means. Most countries have single-party, two- party, or multiparty systems (see party system). In the U.S., party candi¬ dates are usually selected through primary elections at the state level.

political philosophy Branch of philosophy that analyzes the state and related concepts such as political obligation, law, social justice, and con¬ stitution. The first major work of political philosophy in the Western tra¬ dition was Plato’s Republic. Aristotle’s Politics is a detailed empirical study of political institutions. The Roman tradition is best exemplified by Cicero and Polybius. St. Augustine’s City of God began the tradition of Christian political thinking, which was developed by Thomas Aquinas. Niccolo Machiavelli studied the nature and limits of political power. Tho¬ mas Hobbes’s Leviathan (1651) raised the problem of political obligation in its modem form. Hobbes was followed by Benedict de Spinoza, John Locke, and Jean-Jacques Rousseau in the exposition of a social-contract theory. This was rejected by David Hume and also by G.W.F. Hegel, whose Philosophy of Right (1821) was fundamental for 19th-century political thought. Hegel’s defense of private property stimulated Karl Marx’s cri¬ tique of it. John Stuart Mill developed Jeremy Bentham’s utilitarian theory of law and political institutions, so as to reconcile them with individual liberty. In the 20th century John Dewey sought to counteract the dehu¬ manizing aspects of modem capitalist society through a freer form of

education. Until the end of the Cold War, the field of political philoso¬ phy was characterized by a division between Marxists and more tradi¬ tional liberal thinkers, as well as by disagreements between left- and right-leaning liberals, such as John Rawls and Robert Nozick (1938— 2002), respectively. From the 1970s, feminist political philosophy drew attention to the apparent gendered nature of many concepts and problems in Western political philosophy, especially autonomy, rights, liberty, and the public-private distinction.

political science Academic discipline concerned with the empirical study of government and politics. Political scientists have investigated the nature of states, the functions performed by governments, voter behaviour, political parties, political culture, political economy, and public opinion, among other topics. Though it has roots in the political philosophies of Plato and Aristotle, political science in the modern sense did not begin until the 19th century, when many of the social sciences were established. Its empirical and generally scientific orientation is traceable to the work of Henri de Saint-Simon and Auguste Comte. The first institution dedicated to its study, the Free School of Political Science, was founded in Paris in 1871.

polka Lively couple dance of Bohemian folk origin. The polka (Czech for “Polish woman”) is characterized by three quick steps and a hop and is danced to music in 2 A, or duple, time. It originated in the early 19th century and became popular in ballrooms across Europe and in North and South America. It has remained popular into the 21st century as both a folk dance and a ballroom dance.

poll tax Tax of a uniform amount levied on each individual. The most famous British poll tax was the one levied in 1380, a main cause of the 1381 Peasants' Revolt. In the U.S., poll taxes were used as a voting pre¬ requisite in the southern states; when payment was made a prerequisite to voting, impoverished blacks (and often poor whites) were effectively denied the vote. In 1966 the U.S. Supreme Court ruled that states could not levy a poll tax as a prerequisite for voting in state and local elections.

pollack or pollock Vpa-tak\ Either of two commercially important North Atlantic species of food fish in the COD family (Gadidae). Pollachius (or Gadus) virens, called saithe or coalfish in Europe, is deep green with a pale belly. It has a small chin barbel (fleshy protuberance) and three dorsal and two anal fins. A carnivorous, lively, usually schooling fish, it grows to about 3.5 ft (1.1 m) long and weighs up to 35 lbs (16 kg). The other species, Theragra chalcogramma, or walleye pollack, closely resembles P. virens.

1795, Mecklenburg county, N.C.,

Polk, James K(nox) (b. Nov. 2,

U.S.—d. June 15, 1849, Nashville,

Tenn.) 11th president of the U.S.

(1845-49). He was a friend and sup¬ porter of Andrew Jackson, who helped Polk win election to the U.S.

House of Representatives in 1825.

He left the House in 1839 to become governor of Tennessee. At the dead¬ locked 1844 Democratic Party con¬ vention Polk was nominated as the compromise candidate; he is consid¬ ered the first dark-horse presidential candidate. A proponent of western expansion, he openly laid claim to the whole territory that extended as far north as latitude 54° 40’ with the slogan “Fifty-four Forty or Fight”

(see Oregon Question). Elected at the age of 49, the youngest president to that time, he successfully con¬ cluded the Oregon border dispute with Britain (1846) and secured pas- sage of the Walker Tariff Act (1846),

which lowered import duties and helped foreign trade. He led the pros¬ ecution of the Mexican War, which resulted in large territorial gains but reopened debate over the extension of slavery. His administration also established the Department of the Interior, the U.S. Naval Academy, and the Smithsonian Institution; oversaw revision of the treasury system; and proclaimed the validity of the Monroe Doctrine. Though an efficient and competent president and deft in his handling of Congress, he was exhausted by his efforts and did not seek reelection; he died three months after leaving office.

James K. Polk, daguerreotype by Mathew Brady, 1849.

LIBRARY OF CONGRESS, WASHINGTON, D.C.

© 2006 Encyclopaedia Britannica, Inc.

1522 I Pollaiuolo ► polonaise

Pollaiuolo \ I pol-ll-'w6-lo\ # Antonio and Piero del orig. Anto¬ nio and Piero di Jacopo d'Antonio Benci (respectively b. Jan. 17, 1432/33, Florence, Republic of Florence—d. Feb. 4, 1498, Rome; b. 1443, Florence—d. 1496, Rome) Italian sculptors, painters, engravers, and goldsmiths. Antonio probably trained in goldsmithing and metalwork¬ ing with Lorenzo Ghiberti, while Piero may have studied painting with Andrea del Castagno. The brothers collaborated consistently after 1460, producing their works under a combined signature, and their individual contributions are hard to determine. Antonio is recognized as a superb draftsman and was among the first to practice anatomical dissection in the study of the human form; Piero’s individual work is less artistically sig¬ nificant. In Florence they created an altarpiece in San Miniato al Monte and The Martyrdom of St. Sebastian (1475) in the Pucci Chapel of the Church of the Santissima Annunziata. In Rome their works included the tombs of Popes Sixtus IV and Innocent VIII. Antonio’s famous Battle of Nudes is one of the largest and most important Italian engravings of the 15 th century.

pollen Mass of microscopic spores in a seed plant that appears usually as a fine dust. Each pollen grain is tiny, varies in shape and structure, is formed in the stamens in seed plants, and is transported by various means (see pollination) to the pistil, where fertilization occurs. The outer layer of a pollen grain is very resistant to disintegration; treatment with intense heat, strong acids, or strong bases has little effect on it. Because the grains often are very distinctive, some plant species may be identified by their pollen grains alone. Common components of both recent and ancient geo¬ logic sediments, pollen grains have provided much information on the origin and geologic history of plant life on land. Pollen is produced in such quantities that it is a significant part of the airborne components of earth’s atmosphere. The protein-containing substance in many pollen grains (e.g., ragweed and many grasses) causes the allergic reaction com¬ monly known as hay fever.

pollination Transfer of pollen grains in seed plants from the stamens, where they form, to the pistil. Pollination is required for fertilization and the production of seeds. On the surface of the pistil the pollen grains ger¬ minate (see germination) and form pollen tubes that grow downward toward the ovules. During fertilization, a sperm cell in a pollen tube fuses with the egg cell of an ovule, giving rise to the plant embryo. The ovule then grows into a seed. Since the pollen-bearing parts of the stamens are rarely in direct contact with the pistil, plants commonly rely on external agents for pollen transport. Insects (especially bees) and wind are the most important pollinators; other agents include birds and a few mammals (notably certain bats). Water transport of pollen is rare. An egg may be fertilized by self-pollination (when the sperm comes from pollen produced by the same flower or by another flower on the same plant) or by cross¬ pollination (when the sperm comes from the pollen of a different plant).

Pollini, Maurizio (b. Jan. 5, 1942, Milan, Italy) Italian pianist. He made his debut at age nine and won the Warsaw Chopin Competition in 1960. He first played in the U.S. in 1968. His recordings and performances range from works by Johann Sebastian Bach to Ludwig van Beethoven to Karlheinz Stockhausen. His combination of intellectual seriousness and extraordinary technical brilliance have given him a unique standing in the concert world.

polliwog See tadpole pollock See poliack

Pollock, Sir Frederick, 3rd Baronet (b. Dec. 10, 1845, London, Eng.—d. Jan. 18, 1937, London) British legal scholar. He taught at the University of Oxford (1883-1903) and was made a king’s counsel in 1920. He was noted for his History of English Law Before the Time of Edward I (1895; written with Frederic W. Maitland) and several standard textbooks. He maintained a 60-year correspondence with Oliver Wendell Holmes, Jr.; the Holmes-Pollock Letters were published in 1941.

Pollock \'pa-bk\, (Paul) Jackson (b. Jan. 28, 1912, Cody, Wyo., U.S.—d. Aug. 11, 1956, East Hampton, N.Y.) U.S. painter. He grew up in California and Arizona. In the early 1930s he studied in New York City under Thomas Hart Benton, and later he was employed on the WPA Fed¬ eral Art Project. In 1945 he married the artist Lee Krasner. Two years later, after several years of semiabstract work stimulated by psychotherapy, Pollock began to lay his canvas on the floor and pour or drip paint onto it in stages. This process permitted him to record the force and scope of his gestures in trajectories of enamel or aluminum paint that “veiled” the

figurative elements found in his ear¬ lier work. The results were huge areas covered with complex and dynamic linear patterns that fuse image and form and engulf the vision of the spectator in their scale and intricacy. Pollock believed that art derived from the unconscious and judged his work and that of others on its inherent authenticity of personal expression. He became known as a leading practitioner of Abstract Expressionism, particularly the form known as action painting. Champi¬ oned by critic Clement Greenberg and others, he became a celebrity. When he died in a car crash at 44, he was one of the few American painters to be recognized during his lifetime and afterward as the peer of 20th-century European masters of modem art.

Pollock v. Farmers' Loan and Trust Company (1895) U.S. Supreme Court ruling that declared the federal income tax unconstitutional, thus voiding portions of an 1894 act that imposed a direct tax on the incomes of U.S. citizens and corporations. The decision was mooted in 1913 when ratification of the 16th Amendment to the U.S. Constitution gave Congress the power “to lay and collect taxes on incomes.”

pollution See air pollution, water pollution

Pollux, Castor and See Dioscuri

polo Game played by teams of players on horseback. Players use mal¬ lets with long flexible handles to drive a wooden ball through goalposts. It was first played in Persia in the 6th century bc; from there it spread to Arabia, Tibet (polo is Balti for “ball”). South Asia, and the Far East. The first British polo clubs were formed in India in the mid-19th century; the game came to the U.S. a few decades later. Polo has long been primarily played by the wealthy, because of the expense of acquiring and main¬ taining a stable of polo “ponies” (actually full-sized adult horses, bred for docility, speed, endurance, and intelligence). The standard team is made up of four players whose positions are numbered 1-4. A game consists of six 7.5-minute periods called chukkers or chukkas. The field is 300 yards (274.3 m) long by 160 yards (146.3 m) wide; an indoor version of the game is played on a smaller field.

Polo, Marco (b. c. 1254, Venice or Curzola, Venetian Dalmatia—d. Jan. 8, 1324, Venice) Venetian merchant and traveler who journeyed from Europe to Asia (1271-95). Born into a Venetian merchant family, he joined his father and uncle on a jour¬ ney to China, traveling along the Silk Road and reaching the court of Kublai Khan c. 1274. The Polos remained in China for about 17 years, and the Mongol emperor sent Marco on sev¬ eral fact-finding missions to distant lands. Marco may also have gov¬ erned the city of Yangzhou (1282—

87). The Polos returned to Venice in 1295, after sailing from eastern China to Persia and then journeying overland through Turkey. Captured by the Genoese soon after his return,

Marco was imprisoned along with a writer, Rustichello, who helped him to write the tale of his travels. The book, II milione, was an instant suc¬ cess, though most medieval readers considered it an extravagant romance rather than a true story.

polonaise \,pa-l3-'naz\ Dignified ceremonial dance in 3 A time, frequently employing dotted rhythms, that often opened court balls in the 17th—19th century. It likely began as a

Jackson Pollock painting in his studio on Long Island, New York, 1950.

© HANS NAMUTH

Marco Polo, title page of the first printed edition of The Travels of Marco Polo , 1477.

COURTESY OF THE COLUMBIA UNIVERSITY LIBRARIES, NEW YORK

© 2006 Encyclopaedia Britannica, Inc.

Poltava ► polymer I 1523

warrior’s triumphal dance and had been adopted by the Polish court as a formal march as early as 1573. The dancers promenaded with gliding steps accented by bending the knee slightly on every third step. It often appeared in ballets, and it was used as a musical form by composers such as George Frideric Handel, Ludwig van Beethoven, and especially Frederic Chopin, whose piano polonaises were martial and heroic.

Poltava Vpol-'ta-voV Battle of (June 1709) Decisive victory of Rus¬ sia over Sweden in the Second Northern War. The battle was fought near Poltava, Ukr., between 80,000 Russian troops under Peter I the Great and Aleksandr Menshikov and 17,000 Swedish troops under Charles XII. Despite the lack of reinforcements for his depleted forces, Charles besieged Pol¬ tava in May 1709. The Russians set up a countersiege line and forced the Swedes to attack. Charles planned a daring charge through the Russian line, but he had been injured, and his commanders failed to execute the attack. The Russian counterattack killed or captured the entire Swedish army except for Charles and 1,500 followers. Sweden’s defeat ended its status as a major power and marked the beginning of Russian supremacy in eastern Europe.

Polybius \p3-'lib-e-3s\ (b. c. 200, Megalopolis, Arcadia, Greece—d. c. 118 bc) Greek statesman and historian. Son of an Achaean statesman, Polybius was one of 1,000 eminent Achaeans deported to Rome in 168 bc for supposed disloyalty to the Romans. There he became mentor to Scipio Africanus the Younger, with whom he witnessed the destruction of Carthage soon after his political detention had ended. When hostilities broke out between Achaea and Rome, he negotiated for his countrymen and sought to reestablish order. His reputation rests on his history of the rise of Rome; of its 40 volumes, only 5 survive in their entirety.

polychaete \'pa-le-,ket\ Any of about 5,400 species of marine worms of the annelid class Polychaeta, having a segmented body with many setae (bristles) on each segment. Species, often brightly coloured, range from less than 1 in. (2.5 cm) to about 10 ft (3 m) long. Most body segments bear two bristly parapodia (lobelike outgrowths). The head has short sen¬ sory projections and tentacles. Adults may be free-swimming or seden¬ tary; larvae are free-swimming. Found worldwide, polychaetes are important for turning over sediment on the ocean bottom. One species, the bloodworm, is a popular saltwater fish bait. See also tube worm.

Polyclitus or Polycleitus or Polykleitos V.pa-li-'kll-tosV (fl. 5th cen¬ tury bc, Greece) Greek sculptor. His Spear Bearer (c. 440 bc) was known as “the Canon” because it illustrated his book of that name, which set forth his theory of the ideal mathematical proportions of the human body and proposed that the sculptor strive for a dramatic counterbalance between the relaxed and tense body parts and the directions in which they move. His balanced and rhythmical bronze statues of young athletes, such as Man Tying on a Fillet (c. 420 bc), demonstrated his principles and freed Greek sculpture from its tradition of rigid frontal poses. With Phidias, Polyclitus was the most important Greek sculptor of his age.

Polycrates Xpo-'lik-ro-.tezV (fl. 6th century bc) Tyrant of the Aegean island of Samos c. 535-522 bc. He seized sole control during a festival of Hera, eliminating his two brothers, who shared his power. He quickly became notorious for piracy, as he sought to dominate nearby islands and Ionia. Initially a supporter of Egypt, he joined the Persians against Egypt in 525. Attempts by his opponents to remove him were unsuccessful until the Persian governor of Sardis lured him to the mainland and had him crucified. Polycrates brought wealth and prominence to Samos and was a patron of writers, including Anacreon.

polycystic ovary syndrome or Stein-Leventhal syndrome

Endocrine disorder in women, characterized by high androgen levels and infrequent or absent ovulation (see reproductive system). It causes a high proportion of female infertility cases. Symptoms vary but often include hirsutism, acne, and obesity. Menstruation may be irregular, absent, or excessive. The ovaries are usually enlarged and contain cysts. The disease may remain undiagnosed until a woman tries to conceive. The underly¬ ing cause is not fully understood. Treatment attempts to reduce androgen production. Infertility may be treated with clomiphene citrate or gona¬ dotropins to induce ovulation or with laparoscopy.

polyester Organic compound, any of a class of polymers formed by ester linkages between monomers. They are usually prepared from equivalent amounts of glycols and dibasic carboxylic acids, which undergo conden¬ sation polymerization to produce the polyester and water. Polyesters are strong, colourfast, and resistant to corrosion and chemical attack but tend

to build up a static electric charge. In addition to the familiar fibres and films (e.g., Dacron, Mylar), polyesters are used to make reinforced plas¬ tics, automotive parts, boat hulls, foams, laminates, tapes, piping, bottles, disposable filters, encapsulations, and coatings.

polyethylene (PE) Any of the polymers of ethylene, the largest class of plastics. Its simple basic structure, of ethylene monomers, can be linear (high-density and ultrahigh-molecular-weight polyethylene; HDPE and UHMWPE, respectively) or branched to a greater or lesser degree (low- density and linear low-density polyethylene; LDPE and LLDPE, respec¬ tively). The branched polyethylenes have similar structural characteristics (e.g., low crystalline content), properties (high flexibility), and uses (pack¬ aging film, plastic bags, mulch, insulation, squeeze bottles, toys, and housewares). HDPE has a dense, highly crystalline structure of high strength and moderate stiffness; uses include beverage bottles, liquid detergent jugs, crates, barrels, and luggage. UHMWPE is made with molecular weights 6-12 times that of HDPE; it can be spun and stretched into stiff, highly crystalline fibres with a tensile strength many times that of steel; uses include bulletproof vests.

polygamy \p9-'li-g3-me\ Marriage to more than one spouse at a time. Although the term may also refer to polyandry (marriage to more than one man) it is often used as a synonym for polygyny (marriage to more than one woman), which appears to have once been common in most of the world and is still found widely in some cultures. Polygyny seems to offer the husband increased prestige, economic stability, and sexual com¬ panionship in cultures where pregnancy and lactation dictate abstinence, while offering the wives a shared labour burden and an institutionalized role where a surplus of unmarried women might otherwise exist. The polygynous family is often fraught with bickering and sexual jealousy; to preserve harmony, one wife may be accorded seniority, and each wife and her children may have separate living quarters. Polyandry is relatively rare; in Tibet and Nepal, where brothers may marry a single woman, the practice serves to limit the number of descendants and keep limited land within the household.

Polygnotus or Polygnotos N.pa-lig-'no-tosX (b. c. 500, Thasos, Thrace— d. c. 440 bc, Athens, Greece) Greek painter. None of his works are extant, but accounts exist of monumental wall paintings in a severely Classical style at the hall of the Cnidians at Delphi: Sack of Troy and Ulysses Visiting Hades. His compositions were noted for the expression of emotion on faces and for the distribution of figures throughout the composition rather than on a single base line, as was the convention of the day.

polygon In geometry, any closed curve consisting of a set of line seg¬ ments (sides) connected such that no two segments cross. The simplest polygons are triangles (three sides), quadrilaterals (four sides), and pen¬ tagons (five sides). If none of the sides, when extended, intersects the polygon, it is a convex polygon; otherwise it is concave. A polygon with all sides equal is equilateral. One with all interior angles equal is equi¬ angular. Any polygon that is both equilateral and equiangular is a regular polygon (e.g., equilateral triangle, square).

polygraph See lie detector

polyhedron \,pa-le-'he-dr3n\ In Euclidean geometry, a three- dimensional object composed of a finite number of polygonal surfaces (faces). Technically, a polyhedron is the boundary between the interior and exterior of a solid. In general, polyhedrons are named according to number of faces. A tetrahedron has four faces, a pentahedron five, and so on; a cube is a six-sided regular polyhedron (hexahedron) whose faces are squares. The faces meet at line segments called edges, which meet at points called vertices. See also Platonic solid; Euler's formula.

Polykleitos See Polyclitus

polymer Vpa-lo-morX Any of a class of natural or synthetic substances composed of macromolecules that are multiples of monomers. The mono¬ mers need not all be the same or have the same structure. Polymers may consist of long chains of unbranched or branched monomers or may be cross-linked networks of monomers in two or three dimensions. Their backbones may be flexible or rigid. Some natural inorganic materials (e.g., the minerals diamond, graphite, and feldspar) and certain man-made inor¬ ganic materials (e.g., glass) have polymer-like structures. Many impor¬ tant natural materials are organic polymers, including cellulose (from sugar monomers; see polysaccharide), lignin, rubber, proteins (from amino acids), and nucleic acids (from nucleotides). Synthetic organic polymers

© 2006 Encyclopaedia Britannica, Inc.

1524 I polymerase chain reaction ► polyvinylchloride

include many plastics, including polyethylene, the nylons, polyurethanes, polyesters, vinyls (e.g., PVC), and synthetic rubbers. The silicone polymers, with an inorganic backbone of silicon and oxygen atoms and organic side groups, are among the most important mixed organic-inorganic com¬ pounds.

polymerase \pa-'b-m3- I ras\ chain reaction (PCR) Laboratory technique used to make numerous copies of specific DNA segments quickly and accurately. These are needed for various experiments and procedures in molecular biology, forensic analysis (DNA fingerprinting), evolutionary biology (to amplify DNA fragments found in ancient speci¬ mens), and medicine (to diagnose genetic disease or detect low viral counts). Invented by Kary Mulus, PCR requires a DNA template (as little as one molecule) to copy, nucleotides to build the copies, and the enzyme DNA polymerase to catalyze the formation of bonds between the nucle¬ otide monomers. Each three-step cycle (separating the two strands of the DNA double helix, marking the ends of the segment to be copied, and catalyzing the formation of bonds), which takes only minutes to complete, doubles the number of DNA strands present in the reaction medium. Rep¬ etition of this cycle many times results in an exponential increase in the amount of DNA.

polymerization 'ypo-.li-mo-ro-'za-shonN Any process in which mono¬ mers combine chemically to produce a polymer. The monomer molecules— which in the polymer usually number from at least 100 to many thousands—may or may not all be the same. In nature, enzymes carry out polymerization under ordinary conditions to form proteins, nucleic acids, and carbohydrate polymers; in industry, the reaction is usually done with a catalyst, often under high pressure or heat. In addition polymerization, monomers are added successively to the reactive ends of a growing poly¬ mer molecule, similar to adding links to a chain; during the reactions, no by-products are formed. In condensation polymerization, growth of the polymer advances stepwise—monomers having reactive functional groups combine into larger molecules with their own functional groups; each reaction splits off a small molecule, often water, as a by-product.

polymorphism Discontinuous genetic variation that results in the occurrence of several different forms or types of individuals among the members of a single species. The most obvious example of polymorphism is the separation of most higher organisms into male and female sexes. Another example is the different blood types in humans. A polymorphism that persists over many generations is usually maintained because no one form has an overall advantage or disadvantage over the others in terms of natural selection. Some polymorphisms have no visible manifestations. The castes that occur in social insects are a special form of polymorphism that results from differences in nutrition rather than from genetic varia¬ tion.

Polynesia ^pa-lo-'ne-zhsN Island group, scattered across a huge trian¬ gular area of the east-central Pacific Ocean. A subdivision of Oceania, Polynesia includes New Zealand, Hawaii, Samoa, the Line Islands, French Polynesia, the Cook Islands, the Phoenix Islands, Tuvalu, Tonga, and Easter Island. Fiji is sometimes included because of its Polynesian population. The islands are mostly small coral atolls; some are of volcanic origin. Most of the inhabitants are Polynesians, some of whom might be related to the Malay. Their languages belong to a subfamily of the Austronesian languages. Contact with European culture began in the late 1700s with the arrival of Spanish explorers and radically altered life in Polynesia. Colonizers, imposing Western belief systems and cultural ways, effec¬ tively wiped out local traditions and customs. Samoa and Tonga retain more of the traditional culture than the other islands.

polynomial In algebra, an expression consisting of numbers and vari¬ ables grouped according to certain patterns. Specifically, polynomials are sums of monomials of the form ay! 1 , where a (the coefficient) can be any real number and n (the degree) must be a whole number. A polynomial’s degree is that of its monomial of highest degree. Like whole numbers, polynomials may be prime or factorable into products of primes. They may contain any number of variables, provided that the power of each variable is a nonnegative integer. They are the basis of algebraic equation solving. Setting a polynomial equal to zero results in a polynomial equa¬ tion; equating it to a variable results in a polynomial function, a particu¬ larly useful tool in modeling physical situations. Polynomial equations and functions can be analyzed completely by methods of algebra and cal¬ culus. See also orthoganal polynomial.

polynomial, orthogonal See orthoganal polynomial

polyoma V.pa-le-'o-moX virus Minute infectious agent normally present in extremely small amounts in wild mice without causing obvi¬ ous ill effects. It may induce cancerous tumours if grown in tissue cul¬ ture and injected in large quantities into newborn mice or young hamsters, guinea pigs, and rabbits. It belongs to the Papovaviridae family of viRUSes.

polyp \'pa-bp\ Growth projecting from the wall of a cavity lined with a mucous membrane. Shape varies widely; it may have a stalk or many lobes. Polyps most often occur in the nose, urinary bladder, and digestive tract, especially in the rectum and colon. Symptoms, if any, depend on location and size; they may result from pressure or from blockage of a passage. Polyps occasionally bleed. Because a small percentage are pre¬ cursors to cancers or actually contain cancers, it is advisable to have them removed and examined microscopically and to undergo routine colonos¬ copy after age 50.

polyp In zoology, one of two principal cnidarian body forms and, some¬ times, an individual in a bryozoan colony. The cnidarian polyp body is a hollow cylindrical structure. The lower end attaches to another body or surface. The upper, or free, end is directed upward and has a mouth sur¬ rounded by extensible tentacles that bear stinging structures called nem- atocysts. The tentacles capture prey, which is then drawn into the mouth. The polyp may be solitary (see sea anemone) or colonial (see coral). The body wall consists of three dermal layers. The other cnidarian body form is the MEDUSA.

Polyphemus V.pa-lo-'fe-mssV In Greek mythology, a Cyclops. He was the son of Poseidon and the nymph Thoosa. When Odysseus and his com¬ panions were cast ashore on the coast of Sicily, Polyphemus imprisoned them in his cave with the intention of eating them. Odysseus got the giant drunk and then blinded him by plunging a burning stake into his single eye. When Polyphemus opened his cave in the morning, Odysseus and the six men the giant had not yet devoured made their escape by clinging to the bellies of sheep let out to pasture.

polyphony See counterpoint

polyploidy See ploidy

polysaccharide V.pa-le-'sa-ko-.ridX Any of a large class of long-chain sugars composed of monosaccharides. Because the chains may be unbranched or branched and the monosaccharides may be of one, two, or occasionally more kinds, polysaccharides can be categorized in various ways. Cellulose, starch, glycogen, and dextran are all polysaccharides of glucose, with different configurations. Pectins are composed of a galac¬ tose derivative, chitin of a glucose derivative. Connective tissues, joint fluid, and cartilage contain two-component polysaccharides, including heparin. See also oligosaccharide.

polysiloxane See silicone

polytheism Belief in many gods. Though Judaism, Christianity, and Islam are monotheistic (see monotheism), most other religions throughout history have been polytheistic. The numerous gods may be dominated by a supreme god or by a small group of powerful gods. The gods originated as abstractions of the forces of nature such as the sky or the sea and of human and social functions such as love, war, marriage, or the arts. In many religions the sky god is powerful and all-knowing (e.g., Dievs), and the earth goddess is maternal and associated with fertility. Gods of death and the underworld (e.g., Osiris and Hel) are also important. In addition to many gods, polytheistic religions generally also include malevolent or benevolent spiritual forces or powers. See also god and goddess.

polyurethane Any of a class of very versatile polymers that are made into flexible and rigid foams, fibres, elastomers (elastic polymers), sur¬ face coatings, and adhesives. They are produced by reacting a diisocyan¬ ate (a compound with two functional groups of the type —NCO) with a diol (a compound with two hydroxyl, or —OH, groups). Foamed poly¬ urethanes, used for insulation and mattress and upholstery filler, are made with organic compounds containing carboxyl groups, causing a reaction that liberates carbon dioxide bubbles throughout the product. Spandex fibres are highly elastic and have replaced natural and synthetic rubber fibres for many textile purposes. Polyurethane elastomers are made into auto parts, rollers, flexible molds, medical equipment, and shoe soles. Polyurethane surface coatings are applied as sealants to wood, concrete, and machine parts and as linings for tanks and pipes; moisture-curing polyurethane resin is used as a general-purpose waterproof glue.

polyvinyl chloride See PVC