FOREST FORMATIONS AND FOREST TREES 59 



2a. Leaves with minute teeth near the apex; fruit generally one-seeded. 



Sabina monosperma (Engelm.) Rydb. 



2b. Leaves entire at apex; fruit generally several-seeded. The common cedar of the 



eastern slope. Sabina scopulorum (Sarg.) Rydb. 



Fig. 46. — -Rocta Mountain Red Cedar (Sabina scopulorum). X £ 



Genus 6. POPULUS, Poplars, Cottonwoods, Aspen 

 These are the most important trees of the river-bottom and canyon 



forests. The broadleaf cottonwoods seldom extend far up the canyons 



and the narrowleaf trees seldom get down to the lower altitudes of the 



plains region. 



1 a. Leaf-stalk much flattened laterally where attached to blade of leaf. Leaves easily 

 rustled by the wind. (2) 



lb. Leaf-stalk nearly cylindrical where attached to blade. (3) 



2a. Leaf-blade somewhat circular in outline with short-pointed apex. Bark very white. 



Populus tremuloides Michx. 

 "Quaking Aspen" 

 2b. Leaf-blade broadly triangular or heart-shaped; marginal teeth less than ten on each 

 side of blade (tree of southern Colorado). Populus wislizeni (Wats.) Sarg. 



"Broadleaf Cottonwood" 

 2C. Leaf-blade broadly triangular or heart-shaped; marginal teeth more than ten on 

 each side of blade. (This is the common broadleaf cottonwood at Denver and in 

 northern Colorado.) Populus sargentii Dode. 



"Broadleaf Cottonwood " 



