Deciduous trees or shrubs with simple alternate stipulate leaves and dioecious flowers usually preceding the leaves and destitute of a distinct perianth. Both male and female flowers are disposed in deciduous catkins and solitary at the base of the bracts. Stamens 1 to indefinite. Fruit a 1-celled many-seeded capsule dehiscing in two revolute valves. Seeds minute, furnished with a tuft of silky hairs at one end. There are two genera and about 200 species, chiefly in the north temperate and arctic zones, and unrepresented in Australasia.