SUMMER KEY TO THE SPECIES OF POPULUS 



a. Leaf-petioles essentially terete. 



b. Petioles and lower sides of leaves pubescent; leaves heart- 

 shaped P. candicans, p. 55. 



bb. Petioles and lower sides of leaves glabrous ; leaves ovate- 

 lanceolate P. balsamifcra, p. 53. 



aa. Leaf-petioles strongly flattened. 



b. Petioles and lower sides of leaves tomentose; twigs 



pubescent P. alba. p. 47. 



bb. Petioles and lower sides of leaves glabrous ; twigs glab- 

 rous, 

 c. Leaves distinctly deltoid in shape. 



d. Leaves broader than they are long, abruptly acuminate 

 at the apex ; marginal teeth not conspicuously in- 

 curved ; branches erect and more or less appressed to 

 the main stem, forming a narrow, spire-like crown. . 

 P. nigra italica, p. 59. 



dd. Leaves longer than they are broad, more or less taper- 

 pointed at the apex ; marginal teeth rather conspicu- 

 ously incurved ; branches spreading, forming a broad 

 crown P. dclt aides, p. 57. 



cc. Leaves ovate to nearly orbicular in shape. 



d. Margin of leaves coarsely sinuate-toothed; leaves 3-', 



inches long P. grandidentata, p. 51. 



dd. Margin of leaves finely serrate; leaves less than 3 



inches long P. tremuloides, p. 49. 



44 



