THE SILVA OF COLORADO 



193 



3. Populus tremuloides Michx. Aspen, Quaking Asp 



Rydberg, Flora Colo. 91; Coulter, Manual 339; Sargent, Manual Trees of 



N. A. 154; Nelson, Key to Rocky Mountain Flora 15. 



Leaf-blades broadly ovate, often nearly circular in outline, short -acuminate at 



the apex, finely crenulate all 



around except at the base, 2 to 



4 cm. (J to i^ in.) broad, except 



those at the ends of twigs which 



are much larger; dark green 



above, grayish green below. 



Leaf-stalks about the same 



length as the blades; slender 



and laterally compressed. 



A small or medium-sized 



tree, bearing slender, remote, 



crooked, horizontal branches. 



Bark thin, rather smooth, or 



with wart -like excrescences, 



white or yellowish green; near 



the base of old trees the bark is 



thick, ridged and almost black. Heart-wood yellowish brown; sap-wood nearly 



white; specific gravity 0.4032. Wood soft, light, 

 close grained, consisting of fibers and small 

 ducts; on account of its structure admirably 

 adapted for the manufacture of wood pulp. 

 Thus far no pulp mills have been established in 

 Colorado to use this wood. 



Widely distributed throughout the northern 

 United States and Canada, south to Pennsylvania, 

 Missouri, northern Nebraska, and in the mountain 

 ranges of the West, extending to Arizona, New 

 Mexico and northern Mexico. The light, cottony 

 seeds easily distributed by the wind. Like the 

 lodgepole pine it forms pure growths in places 

 formerly covered with a mixed coniferous forest. 

 It does not extend to the plains region. This 

 tree is sometimes planted for shade in the mining 

 camps of Colorado. 



4. Populus wislizeni (Wats.) Sargent. 

 Cottonwood (Fig. 2) 



Rydberg, Flora Colo. 91; Sargent, Manual Trees 0} N. A. 165. 

 Leaf-blades broadly deltoid or ovate -orbicular, about 5 to 7 cm. (2 in. to 2| in.) 



Fig. 2. 



