The true balsam poplar, or tacamahac, P. balsamifera, abundant in most parts of Canada and the northern States, is a tree of rather large growth, often of somewhat fastigiate habit, with round shoots and oblong-ovate sharp-pointed leaves, the base never cordate, the petioles round, and the disk deep glossy green above but somewhat downy below.
In the latter case the leaf usually becomes oval, ovate or even cordate or sagittate, but these forms are found in sessile leaves also (Olyra, Panicum).
The Mount Vernon beds, which occur about the middle of the series, have as yet yielded only a small number of species, though these include the most interesting early Angiosperms. Among them are recorded a Casuarina, a leaf of Sagittaria (which however, as observed by Zeiller, may belong to Smilax), two species of poplar-like leaves with remarkably cordate bases, Menispermites (possibly a water-lily) and Celastrophyllum (perhaps allied to Celastrus).
Of the European kinds one of the most important and best marked forms is the white poplar or abele, P. alba, a tree of large size, with rounded spreading head and curved branches, which, like the trunk, are covered with a greyish white bark, becoming much furrowed on old stems. The leaves are ovate or nearly round in general outline, but with deeply waved, more or less lobed and indented margins and cordate base; the upper side is of a dark green tint, but the lower surface is clothed with a dense white down, which likewise covers the young shoots - giving, with the bark, a hoary aspect to the whole tree.