An oak is a hardwood tree or shrub in the genus Quercus of the beech family. They have spirally arranged leaves, often with lobed edges, and a nut called an acorn, borne within a cup. The genus is widely distributed in the Northern Hemisphere; it includes some 500 species, both deciduous and evergreen. Fossil oaks date back to the Middle Eocene. Molecular phylogeny shows that the genus is divided into Old World and New World clades, but many oak species hybridise freely, making the genus's history difficult to resolve.
Oak
Quercus robur habit
Illustration of Quercus lusitanica showing staminate (left) and pistillate flowers, which mature into acorns (right)
Catkins of Quercus alba containing the staminate or 'male' flowers
Hardwood is wood from dicot trees. These are usually found in broad-leaved temperate and tropical forests. In temperate and boreal latitudes they are mostly deciduous, but in tropics and subtropics mostly evergreen. Hardwood contrasts with softwood.
Beech is a popular hardwood
SEM images showing the presence of pores in hardwoods (oak, top) and absence in softwoods (pine, bottom)