THE SILVA OF COLORADO 1 89 



of the earliest trees to turn color. As a rule, the leaves of certain large 

 branches become quite yellow while those on other limbs are still green. 

 Later the color becomes general. In mountain districts the quaking 

 aspen becomes a conspicuous feature of the landscape because of the 

 bright yellow of the ripened leaves. Sometimes a single grove will show 

 an orange or pale-red tint. Such an occurrence is by no means common, 

 but when this color is developed a very striking appearance is produced. 



Different species of Populus in Colorado. — The species are, for the 

 most part, easily distinguished. However, it sometimes happens that 

 there are puzzling variations. It is also probable that natural hybrids 

 occur and these may not be recognized as such. Anyone wishing to 

 become familiar with the different species should first make sure of the 

 typical specimens in the locality and not until later attempt to determine 

 the occasional abnormal forms. Since some trees are found only in the 

 foothills, some in the plains region and some under cultivation, it follows 

 that one need not look through all the descriptions given below for the 

 particular tree which he is examining. Thus the determination of a 

 species by means of the key and descriptions becomes simplified. It 

 must be remembered that specimens of any of the native species may be 

 found planted as shade trees, but the introduced species are not foimd 

 except under cultivation. In attempting to distinguish the various 

 species the chief difficulty is to tell the western cottonwood^ from the 

 Cottonwood of the Mississippi VaUey.^* This latter tree is frequently 

 planted for shade. The narrowleaf cottonwood^ and the lanceleaf Cot- 

 tonwood'* are also easily confused unless careful attention be given to 

 all the points of the description. The balm-of-Gilead,s although belong- 

 ing to the group with round leaf-stalks, resembles, in form of leaf-blade, 

 the species with flattened stalks, such as the common western cotton- 

 wood and the Carolina poplar. 



Botanical characters. — The genus Populus is one of the two genera 

 of the Willow Family. Salix, the other genus, includes the willows. As 

 a rule there is no difficulty in telling a poplar from a willow. Poplars 

 generally have broad leaves and willows have narrow leaves. But the 



^Populus sargentii= P . occidenlalis. 



' Populus deUoides. * Populus acuminata. 



3 Populus angustijolia. s Populus balsamifera. 



