SUMMER KEY TO THE SPECIES OF POPULUS 
a. Leaf-petioles essentially terete. 
bD. Petioles and lower sides of leaves pubescent; leaves heart- 
shapedsvesseacscssevgauinwsees sees e ees P. candicans, p. 55. 
‘bb. Petioles and lower sides of leaves glabrous; leaves ovate- 
lancedlatess: 24 cyevkascarssaatieeess P. balsamifera, p. 53: 
aa. Leaf-petioles strongly flattened. 
b. Petioles and lower sides of leaves tomentose; twigs 
pubescent: .. san qeaeecse ede es esss ie Beales ...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.. 
Sn aE NG 0 Nd Hioedtbaeatie sts SAG 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 
ClOW ties de reavc coi Mer adewausen P. deltoides, p. 57. 
cc. Leaves ovate to nearly orbicular in shape. 
d. Margin of leaves coarsely sinuate-toothed; leaves 3-5 
inches long...........0..-065 P. grandidentata, p. 51. 
dd. Margin of leaves finely serrate; leaves less than 3 
inches long.................465 P. tremuloides, p. 49. 
