95 



SUMMER KEY TO THE SPECIES. 



Page. 



Leaves with round or channeled petioles; twigs with orange colored pith, 



' P. hetlerophylla 98 



1. Leaves with flattened petioles; twigs with white pith, 2 



2. Leaves broadly deltoid, abruptly acuminate; stigma-lobes expanded in all direc- 



tions P. deltoides 99 



2. Leaves brondly ovate to nearly round, usually acute at apex; stigma-lobes thread- 



like, ,3 



3. Leaves finely serrate on margin; bark greenish-white, P. tremuloides 96 



3. Leaves coarsely dentate on margin; bark yellowish-gray to black, P. grandidentata 97 



WINTER KEY TO THE SPECIES. 



I. Terminal buds 5 to I of an inch long, decidedly resinous; lateral branches with a tend- 

 ency to become vertical, P. deltoides 99 



1. Terminal buds about i of an inch long, devoid of resin or only slightly resinous; lateral 



branches without the vertical tendency, 2 



2. Twigs with orange-colored pith P. heterophylla 98 



2. Twigs with white pith, 3 



3. Buds smooth, glossy, conical, sharp-pointed; often incurved and closely appressed, 



barK greenish-white P. tremuloides 96 



3. Buds downy, dull, ovate, blunt-pointed, straight, divergent; bark yellowish-gray 



to black P. grandidentata 97 



