A third species--the Aspen (_P. tremula_) receives its specific name from the tremulous movements of its leaves, which swing with a rotary movement when disturbed even by the slightest breeze. This characteristic is common, to a greater or lesser degree, to all the species of this genus, and is due to the peculiar nature of the leafstalks, which are long, and flattened in a plane at right angles to that of the blade of the leaf. The Aspen has a grey bark, spreading branches, and downy shoots. The leaves are nearly round, with a sharp point and a serrate margin. When young they are downy above and beneath, but become smooth later. The catkins are very dense, and the flowers of the female tree have two divided stigmas. The Black Poplar (_P. nigra_) and the Lombardy Poplar (_P. fastigiata_), though very common, are not natives of this country. The former is a large, spreading tree, and the latter is readily distinguished by its tall, pyramidal form, with all its branches directed upward. Although these two trees are so very unlike in general appearance, yet they resemble one another so closely in the form of the leaves and the character of the flowers that they are sometimes regarded as two varieties of the same species. In both the leaves are very variable in form, being either triangular, rhombic, or nearly circular, with rounded teeth. Both have smooth shoots, and sticky buds; and their catkins are not so dense as in the other members of the genus. The leaves also are smooth on both surfaces except when young, at which stage they are slightly downy beneath. The male catkins are two or three inches long, of a deep red colour; and, since they appear before the leaves, are very conspicuous. The female catkins are much shorter and erect, and the ripe capsular fruits burst in June, setting free seeds which are covered with a cottony down. _P. nigra_ has a furrowed grey bark, rendered still more irregular by prominent swellings, and it rarely produces suckers. _P. fastigiata_, on the other hand, often produces numerous suckers, and its trunk generally has a rough, furrowed, and twisted appearance. It is interesting to note that the female of the latter does not occur in our country. The tree was introduced by means of suckers, and it appears certain that suckers of the male tree only were brought over for this purpose.