Here's one way to determine whether the narrower leaf pictured on the preceding page is from a narrow leaf (angustifolia) or a black cottonwood. First, the leafstalk, also called the petiole (Latin for "leafstalk"), of an angustifolia leaf is less than one-third the length of the midrib. Second, the angustifolia leaf is the same shade of green on both sides. Most likely the leaf pictured above is from a Populus balsamifera, or black cottonwood, whose own seed was pollinated by a Populus angustifolia, giving it narrower leaves than a full-blooded black cottonwood.
Further comparison makes the identification process easier. At left is a true Populus angusti-folia. Notice that the petiole is less than one-third the length of the midrib. The leaf on the right, with the long petiole, is from a Populus deltoides, or Plains and eastern cottonwood. Also, the edge (margin) of the latter is deeply notched (crenelated), while that of the angustifolia is less so, and the balsamifera's is smooth. Also, the deltoides leaf is the same deep green on both sides, while the hybrid, like the angustifolia, is lighter on the underside.
On June 12, 1805, the day before he reached the Great Falls of the Missouri, Meriwether Lewis wrote his own brief description of the species previously unknown to science:
The narrow leafed cottonwood grows here in common with the other species of the same tree with a broad leaf or that which has constituted the major part of the timber of the Missouri from it's junction with the Mississippi to this place. The narrow leafed cottonwood differs only from the other in the shape of it's leaf and greater thickness of it's bark. The leaf is a long oval acutely pointed, about 2-1/2 or 3 Inches long and from 3/4 to an inch in width; it is thick, sometimes slightly grooved or channeled; margin slightly serrate; the upper disk of a common green while the under disk is of a whiteish green; the leaf is smooth. The beaver appear to be extremely fond of this tree and even seem to scelect it from among the other species of Cottonwood, probably from it's affording a deeper and softer bark than the other species.
On July 26, 1806, just after departing from "Camp Disappointment" in the upper Marias River basin, Lewis astutely noted,
here it is that we find the three species of cottonwood which I have remarked in my voyage assembled together. That species common to the Columbia [balsamifera] I have never before seen on the waters of the Missouri, also the narrow [angustifolia] and broad leafed [deltoides] species