Here's one way to determine whether the narrower leaf pictured on the preceding page is from a narrow leaf (angustifolia) or ablack 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 Fallsof the Missouri, Meriwether Lewis wrote his own brief description of the species previously unknown to science: