1. B. lénta, L. (Cherry B. Sweet or Black Birch.) Bark of trunk dark brown, close (outer layers scarcely laminate), very sweet-aromatic; leaves ovate or oblong-ovate from a more or less heart-shaped base, acuminate, sharply and finely doubly serrate all round, when mature shining or bright green above and glabrous except on the veins beneath; fruiting catkins oblong-cylindrical (1–1¼´ long), the scales with short and divergent lobes.—Rich woodlands, Newf. to N. Del., and south in the mountains, west to Minn., and S. Ind. Tree 50–75° high, with reddish bronze-colored spray; wood rose-colored, fine-grained, valuable for cabinet-work.
2. B. lùtea, Michx. f. (Yellow or Gray Birch.) Bark of trunk yellowish- or silvery-gray, detaching in very thin filmy layers, within and the twigs much less aromatic; leaves (3–5´ long) slightly or not at all heart-shaped and often narrowish toward the base, duller-green above and usually more downy on the veins beneath; fruiting catkins oblong-ovoid (1´ or less in length, 6–9´´ thick), the thinner scales (5–6´´ long) twice as large as in n. 1, and with narrower barely spreading lobes.—Rich moist woodlands, Canada and N. Eng. to Del., west to Minn.; also along high peaks to Tenn. and N. C. Often 60–90° high at the north; wood whiter and less valuable.
[*][*] Trees, with chalky-white bark separable in thin sheets, ovate or triangular leaves of firmer texture, on long slender petioles; fruiting catkins cylindrical, usually hanging on rather slender peduncles; their scales glabrous, with short diverging lobes, freely deciduous; wing of the fruit much broader than its body.
3. B. populifòlia, Ait. (American White Birch. Gray Birch.) Trunk usually ascending (15–30° high); leaves triangular (deltoid), very taper-pointed (usually abruptly), truncate or nearly so at the broad base, smooth and shining both sides, except the resinous glands when young. (B. alba, var. populifolia, Spach.)—Poor sandy soils, N. Brunswick to Del., west to L. Ontario. Bark much less separable than the next; leaves on slender petioles, tremulous as those of the aspen.
4. B. papyrífera, Marshall. (Paper or Canoe Birch. White Birch.) Leaves ovate, taper-pointed, heart-shaped or abrupt (or rarely wedge-shaped) at base, smooth and green above, pale, glandular-dotted, and a little hairy on the veins beneath, sharply and unequally doubly serrate, 3–4 times the length of the petiole. (B. papyracea, Ait.)—Rich woodlands and stream-banks, N. Eng. to N. Penn., N. Ill., and Minn., and far north and westward. Tree 50–75° high, with bark freely splitting into paper-like layers.—Var. minor, Tuckerman, is a dwarf form of the alpine region of the White Mts.
[*][*][*] Tree, with greenish-brown bark, somewhat laminate, and reddish twigs, ovate leaves whitish beneath, and soft-downy peduncled fruiting catkins.
5. B. nìgra, L. (River or Red Birch.) Leaves rhombic-ovate, acutish at both ends, irregularly doubly serrate, whitish and (until old) downy underneath; petioles and peduncle of nearly the same length (3–7´´) and with the oblong catkin tomentose; the bracts with oblong linear nearly equal lobes; fruit broadly winged.—Banks of streams, Mass, to Fla., west to Minn., E. Kan., and Tex. Tree 50–75° high, with light-colored wood and somewhat Alder-like leaves.
[*][*][*][*] Shrubs, with brownish bark, rounded or wedge-shaped crenate and mostly small leaves of thickish or coriaceous texture, and oblong or cylindrical glabrous and mostly erect catkins, on short peduncles.
6. B. pùmila, L. (Low Birch.) Stems (2–8° high) erect or ascending, not glandular; young branches and lower face of young leaves mostly soft-downy; leaves obovate, roundish, or orbicular (6–16´´ long), pale beneath, veinlets on both faces finely reticulated; wing of the fruit mostly narrower than the body.—Bogs, W. Conn, and N. J. to Ind. and Minn., and northward throughout Canada. Leaves usually not at all resiniferous or glandular-dotted.
7. B. glandulòsa, Michx. (Dwarf Birch.) Stems erect or mostly spreading (1–4° high), or when alpine procumbent; branchlets glabrous, conspicuously dotted with resinous wart-like glands; leaves roundish wedge-obovate or sometimes orbicular (6–9´´ long), green and glabrous both sides, less reticulated; fruiting catkins mostly shorter and oblong or oval; wing of the fruit narrower than or sometimes equalling the body.—High mountains of N. Eng. and N. Y., to L. Superior, and far northward.