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A tree attaining 30 feet in height and 2 feet in girth. Bark bright red-brown, broken on the surface into small scales. Young branchlets glabrous. Leaves (Plate 268, Fig. 15) about 3 inches long by 2 inches wide, ovate, broad and rounded at the base, acute or shortly acuminate at the apex; nerves, nine or ten pairs, running parallel and straight to the margin, and ending in acute triangular lobes, which are finely serrate ; upper surface dark green, pubescent on the midrib and nerves ; lower surface yellowish green, glandular, pubescent on the midrib with slight axil-tufts ; petioles pubescent, ¾ to 1 inch long. Buds stalked, pubescent at the base. Stipules deciduous, lanceolate, acute, pubescent. Staminate catkins, three to four, in slender- stemmed racemes; stamens four. Cones, ovoid-oblong, ⅓ to ½ inch long, three to four in a raceme; scales thickened, three-lobed and truncate at the apex; nutlets nearly circular, surrounded by a thin membranous border.

This species, distributed over a wide area, shows two well-marked geographical varieties :—

1. Var. virescens, Callier. This is the commonest form, and has been described above (Plate 268, Fig. 15).

2. Var. occidentalis, Callier. Alnus occidentalis, Dippel. Leaves (Plate 268, Fig. 14) larger, 4 to 5 inches long and 3 to 4 inches wide; nerves, twelve pairs ; slightly bluish-green and pubescent throughout beneath ; stipules ovate, broad, obtuse. This variety is rare, and has only been observed in British Columbia and Oregon.

This species is widely distributed in western North America. It occurs in British Columbia, from Francis Lake in lat. 61° to the valley of the Lower Fraser River, and extends eastward along the Saskatchewan River to Prince Albert. It extends southwards along the Rocky Mountains to northern New Mexico, and is the common species in the northern interior region, east of the divide of the Cascade Mountains, in eastern Washington, Oregon, Idaho, and Montana. It is very abundant on the southern California Sierra, forming great thickets at 6000 to 7000 feet above the sea, along the head-waters of the rivers of southern California flowing to the Pacific Ocean. It is equally abundant and attains its largest size in Colorado and northern New Mexico, and is met with in Nevada and Utah.

This species is rare in cultivation. There are two or three trees of each variety in the alder collection at Kew, which are about 15 feet in height, and show no special beauty or vigour. Var. virescens is thriving at Aldenham ; and a fine specimen