- 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.