2. Pópulus tremuloìdes, Michx. (Quaking-asp. American Aspen.) Leaves roundish heart-shaped, with a short sharp point, and small, quite regular teeth; downy when young, but soon smooth on both sides; margins downy. Leafstalk long, slender, compressed, causing the leaves to tremble continually in the slightest breeze. Leaf with 2 glands at the base on the upper surface; buds varnished. A medium-sized tree, 30 to 60 ft. high; bark greenish-white outside, yellow within, quite brittle. Common both in forests and in cultivation.
P. grandidentàta.
3. Pópulus grandidentàta, Michx. (Large-toothed Aspen.) Leaves large, 3 to 5 in. long, roundish-ovate, with large, irregular, sinuate teeth; and when young densely covered with white, silky wool, but soon becoming smooth on both sides; leaf, when young, reddish-yellow; petiole compressed. A large tree, 60 to 80 ft. high, with rather smoothish gray bark. Woods; common northward, rare southward, except in the Alleghanies. Wood soft and extensively used for paper-making.
P. heterophýlla.
4. Pópulus heterophýlla, L. (Downy-leaved Poplar.) Leaves heart-shaped or roundish-ovate with small, obtuse, incurved teeth; white-woolly when young, but soon becoming smooth on both sides except on the veins beneath. Leafstalk slightly compressed. Shoots round, tomentose. Buds not glutinous. A large tree, 70 to 80 ft. high, not very common; found from western New England to Illinois, and southward.
P. dilatàta.
5. Pópulus dilatàta, L. (Lombardy Poplar.) Leaves deltoid, wider than long, crenulated all round, both sides smooth from the first; leafstalk compressed; buds glutinous. A tall tree, 80 to 120 ft. high; spire-like, of rapid growth, with all the branches erect; the trunk twisted and deeply furrowed. Frequently planted a century ago, but now quite rare in the eastern United States. From Europe. It is thought to be a variety of Populus nigra (No. 7).