Michigan Trees: A Handbook of the Native and Most Important Introduced Species
dd. Margin of leaves finely serrate; leaves less than 3 inches
long. _P. tremuloides_, p. 49.
+WINTER KEY TO THE SPECIES OF POPULUS+
a. Branches erect, more or less appressed to the main stem, forming a narrow, spire-like crown. _P. nigra italica_, p. 59.
aa. Branches spreading, forming a broad crown.
b. Terminal buds 1/8-1/4 inch long, not resinous.