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PLASTERS. 97 PLATEAU. weak muscles, to keep dressings in place, to cover ulcers, to limit efl'usions, etc. PLASTIDS (from Gk. irAao-TiSs, plastos, mold- ed, from irXdiro-cii', plassein, to form) , or Chbomato- PHORES. Living organs of the cell, usually im- bedded in the cytoplasm, of various form, having functions differing according to their ultimate mode of development. They differ from the nucleus in form and composition, and in multi- plying by direct division. They are of three kinds — chloroplasts, chromoplasts, and leuco- plasts. Chloroplasts (q.v. ) contain chlorophyll and carotin, with often other subordinate pig- ments which add their special tint to the general yellow-green of the ordinary chloroplast ; chro- moplasts (q.v.) have usually a red or yellow color, due to a predominance of carotin : leuco- plasts (q.v.) are colorless and have as their spe- cial functions the formation of starch grains, proteid granules, or oil drops, out of material absorbed from the adjacent protoplasm. The oil- formers have been distinguished as elaioplasts. Multiplication of plastids by direct divison con- sists in the severance of the protoplasmic matrix into two parts by its contraction, which con- tinues near the middle until the two portions are completely disjoined, Plastids are minute and undifferentiated in embryos, and take on special form and character under the influence of the external conditions. Thus, plastids which de- velop near the surface of the plant are likely to form chlorophyll and become chloroplasts : those which develop in subterranean organs or thick aerial parts are likely to become leucoplasts and act as starch or proteid formers ; while the plas- tids in flower leaves, the fruit, or even those in the root (e.g., the carrot) may develop as chro- moplasts. See Cell; jNIitosis". PLATA, pla'ta, Rio de la. The estuary of the combined Parana and Uruguay rivers (qq.v.) in South America. It forms a large marine inlet be- tween the republics of Uruguay and Argentina (Jlap: Argentina, J 14). It is 143 miles wide at its mouth and tapers gradually inland for a distance of 200 miles to the delta of the Parana. The Plata is the outlet for an enormous volume of water, amounting during floods to over two million cubic feet per second. Vast quantities of sediment are also brought down by the two rivers, and are gradually filling up the estuary, which formerly extended more than 100 miles farther inland. Shoals are everywhere forming rapidly, and, together with the strong currents and violent storms, make navigation dangerous. The best natural harbor on the estu- ary is Montevideo; artificial harbors have been constructed at La Plata and Buenos Ayres by means of jetties projecting beyond the shallows which line the Argentine shore. The Plata estu- ar' was discovered in 1.509 by Diaz de Solis. and received its present name ('silver river') from Sebastian Cabot. PLAT.a:'A, or PLAT.a;.a: (Lat., from Gk. IXXarata Phifaia, IIXaToiof, Plataiai) . An ancient city in the extreme southeastern part of Boeotia, on the borders of Attica, at the foot of Mount Cithoe- ron. Involved in war with its powerful neighbor Thebes, it sought help of the Spartans and was referred to Athens, as a nearer ally, apparently about B.C. 500. Received and defended by the Athenians, the Platseans showed their gratitude by coming with their whole force to the help of the Athenians at Marathon. For this they suf- fered severely in B.C. 480 when Xerxes destroyed the town. The great victory of the Greeks near the town in the following year restored them under a national agreement that guaranteed their independence. Their devotion to Athens brought on them the hatred of the Thebans, who made an unsuccessful attempt to take the town by sur- prise in 431, at the opening of the Peloponnesian War, and when it was finally forced to surrender to the Spartans (B.C. 427) after two years' siege, secured the death of the defenders. Re- stored after the Peace of Antalcidas (B.C. 387 ) , it was once more destroyed by the Thebans in 373. Finally under Alexander the Great it was again rebuilt, but sank into insig- nificance, though it was still of some importance in the time of Justinian, who rebuilt its walls. The present walls seem to belong to the later city and inclose a much greater circuit than can have belonged to the fifth-century town. For a full discussion of the topography of the neigh- borhood, with detailed maps, consult Grundy, The Battle of PJatcca (London, 1894). PLAT-BAND (Fr. platband, platebande, OF. plattehande, lintel, flat band). In architecture, a flat fascia or band, with less projection than breadth, between curvilinear moldings. PLATE, Sheffield. A kind of table ware made by coating a baser metal with silver. Previous to the introduction of electro-plating, the method generally pursued was that which has acquired the name of Sheflield plating, from the large extent to which it was carried on in that to^Ti. It consisted in soldering on to one or both sides of an ingot of the baser metal a thin plate of silver. The ingot is always of an oblong shape, and is most carefully prepared on the sur- faces which are to receive the silver, so that nothing shall prevent the complete union of the two. The soldering is a process requiring much care and nicety; the plates of silver are thinly coated with a concentrated solution of borax, and are then applied to the prepared surfaces of the ingot, to which they are firmly bound with iron wire, and then placed in the plating-furnace, and subjected to a strong heat. This furnace is so arranged that the interior can be constantly watched, and when the proper temperature is at- tained the workman knows the exact instant to withdraw it. The act of soldering is almost in- stantaneous, and fusion would immediately fol- low if the ingot was not quickly withdrawn. When cooled, the wire is taken off, and the ingot is taken to the rolling-mill, where it is passed baclcward and fonvard, of course with the silver above and below, until it is rolled out into a sheet of the exact thickness required. How- ever thin it may be made, it is found that the relative thickness between the ingot and its lay- ers of silver is always the same. Practically all plating of metals is now performed by the process of electroplating. See Electro-Chemistry. Ix- DUSTBiAL; Tableware, Silver-Plated. PLATEAtr, pla-td' (Fr., diminutive of plat, plate). An elevated area of fairly level land with approximately horizontal strata. The chief difference between a plain and a plateau is that the plateau is higher; but this very difference in altitude gives the streams a chance to cut deeper valleys than in a plain. Plateaus are al- most uniformly associated with mountains, being