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-
© 2006 Encyclopaedia Britannica, Inc.
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).