Any tree or shrub of the genus Quercus. The oaks have
   alternate leaves, often variously lobed, and staminate flowers in
   catkins. The fruit is a smooth nut, called an acorn, which is more or
   less inclosed in a scaly involucre called the cup or cupule. There are
   now recognized about three hundred species, of which nearly fifty occur
   in the United States, the rest in Europe, Asia, and the other parts of
   North America, a very few barely reaching the northern parts of South
   America and Africa. Many of the oaks form forest trees of grand
   proportions and live many centuries. The wood is usually hard and
   tough, and provided with conspicuous medullary rays, forming the silver
   grain.