The Poplars (Populus).
Almost everybody who has an elementary acquaintance with trees knows a Poplar at sight, the foliage being so very distinct from that of other trees. But the distinctions between the several species are not so immediately obvious. Five kinds of Poplar are commonly grown in this country, of which only two are regarded as distinct indigenous species. These are the White Poplar (Populus alba), and the Aspen (P. tremula). A third indigenous form, the Grey Poplar (P. canescens), is thought to be either a sub-species of P. alba, or a hybrid between that species and P. tremula. Then of common introduced species we have the Black Poplar (Populus nigra), and the Lombardy Poplar (P. fastigiata).
The Poplars (Populus) and the Willows (Salix) together constitute the Natural Order Salicineæ. The two genera agree broadly in the construction and arrangement of their flowers in catkins, but whereas the Poplars have broad leaves and drooping catkins, the Willows, with few exceptions, have long slender leaves and erect catkins. The sexes are not only in distinct flowers, but on separate trees—what botanists describe by the term diœcious. The males appear to be far more numerous than the females. In the popular sense there are no flowers, for there are neither sepals nor petals, each set of sexual organs being protected merely by a scale. The catkins containing these flowers usually appear before the leaves. As there is nothing to attract insects to the work, the trees have to rely upon the wind for conveying the pollen to the female trees. The first three species described below have from four to twelve stamens; P. nigra and P. fastigiata have from twelve to twenty stamens. The Poplars share the love of the Willows for moist places. They are found more frequently in gardens and hedgerows than in woods. Their
growth is rapid, and their timber, consequently, is of little value, though its softness and lightness render it suitable for making boxes, and its whiteness and non-liability to splinter fit it for use as flooring. An additional point in favour of White Poplar for the latter purpose is its unreadiness to burn.
The White Poplar, or Abele (Populus alba), grows into a large tree, something between sixty and a hundred feet high, covered with smooth grey bark. Its branches spread horizontally, and its broad heart-shaped leaves, which vary from an inch to three inches long, are hung on long slender foot-stalks.
In most trees the leaf-stalks are flattened from above, but in the case of the Poplars they are flattened from the sides, so that when moved by the wind they flutter laterally. These leaves have a waved margin, a smooth upper surface, and are snowy white and cottony beneath. The leaf-buds are also invested by cottony filaments. The roots produce numerous suckers, even at a distance from the trunk, and the leaves on these sucker-shoots are very large—two to four inches broad—of a more triangular shape, the outline lobed and toothed. The catkins, which appear in March and April, are cylindrical; those of the male trees may be as much as four inches long, each flower containing from six to ten stamens with purple anthers. The female catkins are not nearly so long, the two yellow stigmas are slender with slit tips, and the ovaries develop into slender egg-shaped capsules, each with its fringed scale. This species is said not to produce flowers in Scotland. In July, when the seed capsules open, the surrounding vegetation and ground are thickly strewn with the long white cotton filaments attached to the seeds. The wood of this tree is softer and more spongy than that of other Poplars.