bb. Leaf-sinuses rounded at the base. 



c. Lower sides of leaves and petioles distinctly downy, the 

 lobes undulate or entire; leaves very thick, drooping at 



the sides A. saccharum nigrum, p. 183. 



cc. Lower sides of leaves and petioles essentially glabrous, 

 the lobes serrate ; leaves not thick, not drooping at the 

 sides, 

 d. Leaves coarsely and sparsely toothed or notched; 



bark not longitudinally white-striped; large trees, 

 e. Twigs coarse; petioles exuding a milky juice when 

 cut; wings of fruit making an angle greater than 

 90; bark of the trunk closely fissured, not scaly.. 



A. platanoides, p. 189. 



ee. Twigs slender; petioles not exuding a milky juice 

 when cut ; wings of fruit diverging only slig'htly ; 

 bark of the trunk deeply furrowed, often cleaving 



in long, thick plates A. saccharum, p. 181. 



dd. Leaves finely and abundantly toothed; bark longi- 

 tudinally white-striped; a bushy tree or shrub 



A. pennsylvanicum, p. 177. 



aa. Leaves compound; twigs usually with whitish bloom 



A. negundo, p. 193. 



173 



