This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
Deciduous trees whose scaly buds are often covered with a clammy resinous exudation. Catkins pendulous, appearing before the leaves; scales irregularly lobed or cut. Leaves usually broad, rounded or angular. The species of this genus are confined to the northern hemisphere. The generic name is of classical origin.
1. P. tremula. Aspen. - An indigenous tree with glabrous buds, pubescent shoots, orbicular-cordate entire or angularly toothed leaves glabrous or pubescent beneath, on long slender laterally compressed petioles. Scales of the rather small catkins ciliate and deeply cut. Stamens about 8. This tree rarely exceeds 50 feet in height, and is remarkable for the almost perpetual quivering motion of the leaves. There is a good weeping variety.
P. tremuloides, the American Aspen, is an allied species with roundish-cordate sharp-pointed minutely regularly-toothed leaves. P. Graeca, a slight variety of the foregoing, or perhaps the selfsame thing, is represented in gardens by a weeping form.
2. P. alba, syn. P. nivea, etc. Abele. - A large fast-growing tree with glabrous buds, pubescent shoots, ovate-cordate or deltoid lobed and toothed leaves densely clothed with a cottony down on the lower surface, and long slender petioles. Scales of the catkins ciliate. This is very rare in a wild state, and perhaps not truly indigenous. P. canescens, the Grey Poplar, is a variety with smaller rarely lobed leaves having a greyish tomentum.
3. P. nigra. Black Poplar. - A fast-growing spreading tree from 50 to 80 feet high with glabrous shoots and glutinous buds. Leaves triangular-ovate, acuminate, serrate, rounded at the base, silky beneath when young; petiole slender, compressed. A native of Europe and North Asia, now much planted in this country. There is a narrow-leaved variety called salicifolia.
4. P.pyramidalis, syn. P.fastigiata, and P. dilatata. Lom-bardy Poplar. - This is very distinct in habit, and easily recognised from all other Poplars by its slender erect branches; but it is nevertheless considered to be a form of the same species as the last. In aspect it is distinct from all other deciduous trees, approaching the close perpendicular growth of the Cypresses. It attains a height of 100 to 150 feet, and is found wild in Southern Europe and the Himalaya mountains.
5. P. balsamifera. Balsam Poplar, Tacamahac. - This is the most commonly planted of the North American species, and resembles P. nigra in foliage, but the branches are round, not angular, and the bark more furrowed. The leaves too are quite glabrous, strictly ovate-acuminate, with a rounded base, serrate margin, and paler reticulated under-surface. The buds are covered with a fragrant resin, whence the specific name. This species is much subject to canker in some soils. P. suaveolens is a Siberian form of this species, and there is a variety called candicans, syn. P. Ontariensis, Balm of Gilead Poplar, in which the leaves are broader and cordate at the base. This again is found in nurseries under the names of macrophylla and cordifolia.
6. P. monilifera, syn. P. Acladesca, P. Canadensis, etc. Necklace Poplar, Cotton Wood. - A tall tree with the young branches slightly angular, and broadly deltoid glabrous shining serrate leaves with spreading prominent nerves, rounded or slightly cordate at the base. The female catkins are very long and pendulous, which suggested the specific name. A native of North America.
7. P. angulata. Carolina Poplar. - This large tree is remarkable for its ample ovate-cordate or deltoid entire or obtusely serrate glabrous bright green leaves, which on vigorous trees are from 6 to 9 inches long, and proportionately broad. The branches are angular or almost winged. A native of North America, and one of the most ornamental and desirable of the genus.
8. P. grandidentata. - A moderate-sized tree with ovate-orbicular coarsely sinuately toothed leaves clothed with a dense white pubescence when young. A weeping variety of this is more frequently seen than the erect one. North America.
There are several other species or forms occasionally seen in gardens, but none of them superior to those enumerated.