Birch is a deciduous tree or shrub. The bark is white, gray, yellow white, reddish brown, brown or black brown, smooth, transverse, longitudinal, thin- or massive. Buds sessile, with several imbricate bud scales. Leaves simple, alternate, usually glandular punctate below, margin doubly serrate, rarely simple serrate, veins pinnate, petiolate; stipules separate, caducous. Flowers are unisexual and monoecious; 2-4 male inflorescences are clustered on the branches of the previous year, with bracteoles and 3 male flowers; perianth is membranous and the is conjoined; stamens are usually 2, with short filaments and bifurcated ends, and anthers have 2 separate anthers, with hairs or no hairs on the top; pollen grains are broadly elliptic in equatorial view, with edges in polar view, most of them have 3 holes, triangular in shape, rarely 2, 4, 5-8 holes, The two s of the outer wall are thickened, and no pore chamber is formed, and the outer has no obvious belt thickening at the hole; the female inflorescence is single 1 or 2-5 at the top of the short branch, cylindrical, oblong or nearly spherical, upright or drooping; the bract scales are arranged in tile shape, with 3 female flowers in each bract scale; the female flowers have no perianth, the ovary is flat, 2 chambers, 1 inverted ovule in each chamber, and 2 styles, separated. The fruit bracts are leathery, scalelike and exfoliated. They are made up of 3 bracts, with 3 lobes and 3 nutlets.