This is an important order, including nearly all our indigenous timber trees, besides a large number of exotic ornamental and useful species. Leaves deciduous in nearly all the hardy species, alternate, simple, entire or lobed or toothed, stipulate. Flowers monoecious. Perianth composed of 5 or more segments, or absent in the male flowers, which are solitary or clustered or in catkins with or without bracts. Female flowers with an adnate perianth, sessile in a coriaceous involucre formed of free or connate bracts; ovary inferior, 2- or 3- or several-celled, with one or two ovules in each cell. Fruit a glans or nut, seated on or enclosed within the cupular involucre, by abortion usually 1-celled and 1-seeded, the other cells becoming obliterated, rarely 2-seeded. Seeds large, destitute of albumen, and having large thick eotyledons. 12 genera, and about 280 species belong to this group. The species are most abundant in northern temperate regions, but occur in the south, and sparingly on the mountains of tropical countries.