oak, any tree or shrub of the genus Quercus of the family Fagaceae (beech family). This complex genus includes as many as 600, found chiefly in north temperate zones and also in Polynesia. The more southerly species, ranging into the tropics, are usually evergreen. Oaks are cultivated for ornament and are prized as the major source of hardwood lumber. The wood is durable, tough, and attractively grained; it is especially valued in shipbuilding and construction and for flooring, furniture, railroad ties, barrels, tool handles, and veneer (particularly highly burled oak). The oaks are commonly divided into two groups, the black (or red) and the white. The former (e.g., the scarlet, pin, Spanish, willow, laurel, and shingle oaks) are characterized by leaves with sharp-tipped lobes and by acorns that mature in two years. The white oaks (e.g., the white, post, bur, cork, and holly oaks) are characterized by smooth-lobed leaves and acorns that mature in one year. Q. alba, the white oak, is the most important timber tree of the oak genus. Lumber-yielding species of chestnut (genus Castanea) are included in the white oak group when the term is used as a timber classification. The live oaks, evergreen species common in the S and SW United States, are sometimes considered a separate group. The bark of some oaks has been employed in medicine, in tanning, and for dyes; that of the cork oak supplies the cork of commerce. The galls caused by certain insects are utilized commercially. The Mediterranean kermes oak (Q. coccifera) is host to the kermes insect, source of the world's oldest dyestuff. Acorns, the fruit of oak trees, have long been employed as a source of hog feed, tannin (chiefly from valonia, the acorn cup of the Turkish oak, Q. aegilops), oil, and especially food. Acorns were one of the most important foods of the North American forest Native Americans; they were pulverized, leached to extract the bitter taste, and then cooked in various ways. Acorns have also been used as food in other regions where they are native. A symbol of strength, the oak has been revered for both historical and mythological associations. It was the favorite of Jove and Thor and especially sacred to the druids. St. Louis administered justice under an oak, and the Charter Oak is legendary in America. Several unrelated plants are also called oak, e.g., the Jerusalem oak (a lobe-leaved annual of the goosefoot family) and the poison oak of the sumac family (see poison ivy). Oaks are classified in the division Magnoliophyta, class Magnoliopsida, order Fagales, family Fagaceae.
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