Leaves alternate, simple; stipules caducous. Male flowers in pendulous catkins, 3 together in a dichasium in the axil of each bract; bracteoles 2 to 4 to each group of flowers; perianth present. Female flowers in erect cylindrical or ovoid catkins, 2 or 3 together in a dichasium in the axil of each bract; bracteoles 2 to 4 to each group of flowers; perianth absent; ovary 2-celled; ovule one in each cell, anatropous, pendulous; styles 2, free. Fruit a flattened nut, single or in a group of 2 or 3 on the surface of a scale formed from the accrescent fused bract and bracteoles, in a dense cylindrical or cone-like strobilus; seed 1, endosperm absent.

Contains 2 genera and about 95 species in north temperate regions and tropical mountains; central Andes.

Betula L.

Deciduous, monoecious trees and shrubs. Trunk slender and extending well into the crown. Bark smooth and resinous, marked by long longitudinal lenticels, often separating into thin papery plates, often becoming thick, deeply furrowed and scaly at the base of old trunks. Twigs roughened by crowded leaf-scars of many years; young shoots clothed with varying amounts of lenticels, or lenticels absent; simple eglandular hairs or peltate glands are often diagnostic. Buds with numerous scales, which are fully grown and bright green at midsummer. Leaves usually alternate, sometimes in pairs or threes on short shoots, sometimes incised-lobed; stipules scarious and usually caducous. Male and female flowers in 3-flowered dichasia, arranged spirally in separate catkins, with the lateral flowers of the dichasium subtended by bracteoles adnate to the base of the scale. Male catkins long and pendulous, solitary or clustered, appearing in summer or autumn in the axils of the last leaves of a young shoot and near the ends of short lateral twigs, erect and naked during the winter; scales in the spring broadly ovate, rounded, short-stalked, yellow or orange below the middle and dark chestnut-brown above; flowers with a membranous 4-lobed calyx, often 2-lobed by suppression, with the anterior lobe obovate, rounded at the apex, much longer than the minute posterior lobe; the 2 bifid stamens inserted at the base of the calyx.