Aromatic trees or shrubs, with dentate or serrate leaves, scaly buds and flowers of both kinds in aments expanding before or with the leaves, the pistillate erect or spreading. Staminate flowers about 3 together in the axil of each bract, consisting of a membranous, usually 4-toothed perianth, 2 stamens, and subtended by 2 bractlets; filaments short, deeply 2-cleft, each fork bearing an anther-sac. Pistillate flowers 2 or 3 (rarely 1) in the axil of each bract, the bracts 3-lobed, or sometimes entire, deciduous with the fruits; perianth none; ovary sessile, 2-celled; styles 2, stigmatic at the apex, mostly persistent. Nut small, compressed, membranous-winged on each side (a samara), shorter than the bracts. [The ancient name.]

About 35 species of the north temperate and arctic zones. Type species: Betula alba L. Trees (except mountain and boreal races of no. 3).

Fruiting aments peduncled.

Fruiting aments not tomentose.

Leaves long-acuminate.

Leaves bright green, irregularly toothed; bark not readily peeling, chalky white.

1.

B. populifolia.

Leaves dull green; bark readily peeling, white to bronze.

2.

B. coerulea.

Leaves merely acute; bark white to bronze, readily peeling.

3.

B. papyrifera.

Fruiting aments tomentose.

4.

B. nigra.

Fruiting aments sessile.

Fruiting bracts 2"-21 /2" long; leaves mostly cordate.

Fruiting bracts glabrous; bark dark brown, close.

5.

B. lenta.

Fruiting bracts ciliate; bark gray to yellow brown, close or peeling.

6.

B. alleghanensis.

Fruiting bracts 3 1/2"-5" long, ciliate; bark yellow-gray, freely peeling.

7.

B. lutea.

Shrubs (nos. 8 and 10 sometimes forming small trees;.

Twigs glandular-warty.

Samara-wings broader than the nut.

8.

B. fontinalis.

Samara-wings narrower than the nut.

9.

B. glandulosa.

Twigs not glandular-warty.

Samara-wings-broader than the nut.

10.

B. Sandbergi.

Samara-wings narrower than the nut.

Young foliage densely pubescent; fruiting bracts all 3-lobed.

11.

B. pitmila.

Leaves glabrous; at least the upper fruiting bracts entire.

12.

B. nana.