Trees or shrubs with light wood, brittle twigs, and simple alternate leaves; flowers borne in catkins; the staminate and pistillate on separate plants, the seed provided with a covering of long, white, silky, hairs.

A large tree with nearly smooth gray bark, reaching a maximum height of 80 feet; branches stout, ascending, the larger buds very resinous. Leaves smooth, broadly ovate or ovate-lanceolate, entire, dark green and shining above, pale beneath, acute or acuminate at the apex, rounded or acute at the base, petioles round. Flowers in slender catkins, the staminate and pistillate on separate trees.

Throughout the region in the river valleys and on the surrounding slopes, usually a tree not over 20—30 feet high but sometimes attaining an immense size.

A slender tree with smooth, light green bark, seldom more than 40—50 feet high, and less than half that in our region. Leaves smooth when young except on the margins, ovate, short-acuminate at the apex, rounded at the base, finely crenulate all around; petioles flattened laterally, very slender, causing the leaves to quiver with the slightest breeze. Flowers in rather stout catkins.

Frequent in the low valleys and slopes through the Rockies, forming groves, or singly.

The willows which are very largely represented throughout the region, in the low or moist ground and banks of streams, as shrubs or small trees; or on the drier slopes, and in alpine meadows and summits, frequently as very diminutive shrubs with stems less than an inch high, have been omitted entirely, owing to the extreme difficulty of distinguishing between them in a work of this kind.