Fls unisexual, small; ♂ in spikes, per. with 4-6 imbricate lobes, stamens few to ∞; ♀ solitary, surrounded by an involucre of imbricate scales; ovary inferior, 3-6-celled, with as many styles; ovules 2 per cell, one us. aborting. Fr. a nut enclosed in the often hardened involucre. Trees or shrubs with simple, alt., stipulate lvs, of temperate and sub-tropical regions except Africa.

Deciduous or evergreen, monoecious trees or rarely shrubs. Buds with imbricate scales. Lvs alternate, simple, penninerved; stipules caducous. Fls usually axillary on young shoots; perianth (calyx) 4-6-(7)-lobed, imbricate; ♂ fls in slender spikes (catkins); ♀ fls solitary or in groups of 3, sometimes at base of ♂ spike. Corolla 0. Stamens generally as many as or twice as many as calyx lobes, occasionally to c. 40; filaments free, filiform. ♀ fls within an involucre of imbricate bracts, often numerous; perianth adnate to ovary. Ovary inferior, 3-6-celled, and each cell with 2 ovules. Styles as many as ovary cells. Fr. a 1-seeded nut; involucre cupulate, free or adnate to and partly or completely enclosing the fr., often hard, tuberculate or spiny. Seed 1, non-endospermic.