This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Trees with scaly resinous buds, terete or angled twigs and broad or narrow, usually long-petioled leaves, the stipules minute, fugacious. Bracts of the aments fimbriate or incised. Disk cup-shaped, oblique, lobed or entire. Staminate aments dense, pendulous. Staminate flowers with from 4-60 stamens, their filaments distinct. Pistillate aments sometimes like through the elongation of the pedicels, pendulous, erect or spreading. Ovary sessile; style short, stigmas 2-4, entire or 4-lobed. [Name ancient, used for these trees by Pliny.]
* Petioles terete or channeled, scarcely or not at all flattened laterally. (Poplars.)
Leaves persistently and densely white-tomentose beneath.
Leaves glabrous or very nearly so when mature, crenulate.
Foliage glabrous or nearly so; capsule very short-pedicelled.
Leaves broadly ovate, rounded or cordate at the base.
Petioles glabrous; leaves rounded or truncate at the base.
Petioles ciliate; leaves mostly cordate.
Leaves lanceolate or ovate-lanceolate, mostly narrowed at the base.
Leaves acute, short-petioled.
Leaves acuminate, long-petioled.
Foliage densely tomentose when young; capsules slender-pedicelkd.
** Petioles strongly flattened laterally. (Aspens.)
Leaves coarsely undulate-dentate.
Leaves ovate or suborbicular, short-pointed.
Leaves broadly deltoid, abruptly acuminate.
Leaves obtuse at the base; capsules nearly sessile.
Leaves truncate at the base; capsules slender-pedicelled.
Pedicels as long as the capsules or longer.
Pedicels shorter than the capsules.