THE SILVA OF COLORADO 



195 



deltoides which is found in the Mississippi Valley. In fact our tree was long con- 

 sidered to be merely a western form of that species. 



6. Populus deltoides Marsh. Cottonwood, Carolina Poplar 



It is not necessary to give a full account of this species since it is much like our 

 western cottonwood. Certain differences may be pointed out: the leaves do not 

 have the long narrow point seen in the western cottonwood, they are longer in pro- 

 portion to the width, and are more finely toothed, and the young twigs are usually 

 ridged longitudinally with corky thickenings. 



The name "Carolina poplar" applies to a form of this species which has a sym- 

 metrical shape. There is one main trunk from which the branches pass off in a 

 somewhat ascending direction.' 



This species ranges through the eastern United States and the Mississippi Valley. 



7. Populus balsamifera Linn. Balm of Gilead 



Coulter, Manual 339; Sargent, Manual Trees of N. A. 157; (not reported in 

 Rydberg's Flora 0} Colorado). 



Leaf-blades ovate, serrate, dark green above, pale 

 beneath, very distinctly veined below; abruptly pointed 

 at the apex; rounded or cordate at base, about 6 cm. 

 (2I in.) broad, nearly cylindrical, the stalks at least half as 

 long as the blades. 



A large or medium-sized tree resembling in general 

 appearance the lanceleaf cottonwood. Winter buds large, 

 coated with a sticky varnish. Bark gray-brown, in old 

 trunks broken into rounded ridges. 



Widely distributed in the northern part of North 

 America. Common along the northern boundary of the 

 United States from Maine to Minnesota; extending from 

 Alaska to Oregon, Nevada and Colorado. In sub-arctic 

 regions of northwest America it forms the most char- 

 acteristic feature of the vegetation; in Colorado confined 

 to canyons and draws in the higher foothills and 

 mountains.' The writer has collected specimens at Eldora, 

 Boulder County, Colo., altitude 8,400 feet. Distinguished 

 from the lanceleaf cottonwood chiefly by the whitish under surface of the leaves and 

 prominent reticulate venation. 



' Mr. George B. Sudworth of the U. S. Forest Service informs me that the diflEerences between the " Caro- 

 lina poplar" and the typical P. deltoides disappear as the trees reach fuU growth. 



' The writer is indebted to Professor E. Bethel, of Denver, for information in regard to the distribution 

 I f this species and also for a specimen collected near Georgetown. 



Fig. 4. 



