1. Populus tremuloides Michx. Aspen. Quaking Asp.
Leaves ovate to broad-ovate or rarely reniform (var. _reniformis_ Tidestrom) abruptly short-pointed or acuminate at apex rounded or rarely cuneate at the wide base, closely crenately serrate with glandular teeth, thin, green and lustrous above, dull green or rarely pale below, up to 4½′ long and broad with a prominent midrib, slender primary veins and conspicuous reticulate veinlets; petioles slender, compressed laterally, 1½′—3′ long. Flowers: aments 1½′—2½′ long, the pistillate becoming 4′ in length at maturity; scales deeply divided into 3—5 linear acute lobes fringed with long soft gray hairs; disk oblique, the staminate entire, the pistillate slightly crenate; stamens 6—12; ovary conic, with a short thick style and erect stigmas thickened and club-shaped below and divided into linear diverging lobes. Fruit maturing in May and June, oblong-conic, light green, thin-walled, nearly ¼′ long; seeds obovoid, light brown, about 1/32′ in length.
A tree, 20°—40° high, with a trunk 18′—20′ in diameter, slender remote and often contorted branches somewhat pendulous toward the ends, forming a narrow symmetrical round-topped head, and slender branchlets covered with scattered oblong orange-colored lenticels, bright red-brown and very lustrous during their first season, gradually turning light gray tinged with red, ultimately dark gray, and much roughened for two or three years by the elevated leaf-scars. Winter-buds slightly resinous, conic, acute, often incurved, about ¼′ long, narrower than the more obtuse flower-buds, with 6 or 7 lustrous glabrous red-brown scales scarious on the margins. Bark thin, pale yellow-brown or orange-green, often roughened by horizontal bands of circular wart-like excrescences, frequently marked below the branches by large rows of lunate dark scars. Wood light brown, with nearly white sapwood of 25—30 layers of annual growth.
Distribution. Southern Labrador to the southern shores of Hudson’s Bay and northwesterly to the mouth of the Mackenzie River, through the northern states to the mountains of Pennsylvania, northern Ohio, Indiana and Illinois, eastern and central Iowa and northeastern Missouri; common and generally distributed usually on moist sandy soil and gravelly hillsides; most valuable in the power of its seeds to germinate quickly in soil made infertile by fire and of its seedlings to grow rapidly in exposed situations; westward passing into the var. _aurea_ Daniels, with thicker rhombic to semiorbicular or broad-ovate generally smaller leaves, usually pale on the lower surface, rounded or acute and minutely short-pointed at apex, rounded or cuneate at base, often entire with slightly thickened margins, or occasionally coarsely crenately serrate, with inconspicuous reticulate veinlets, turning bright golden yellow in the autumn before falling.