40 



WILD FLOWERS AND TREES OF COLORADO 



be distinguished as mesophytic and xerophytic. The former are those 

 made up of trees requiring a moderate amount of moisture, the latter 

 are dry-country forests capable of enduring drought; they occur in 

 regions of slight rainfall or in situations where the run-off is rapid. 

 Mesophytic trees in Colorado are confined to areas where there is con- 

 siderable seepage water, except in the case of Engelmann spruces in the 

 sub-alpine zone where there is much more rainfall than at lower altitudes. 

 It may be convenient to list the principal forest formations of Colo- 

 rado in tabular form. 



* The pinyon pine is found only in the warmer and drier parts of the state. Oaks, chiefly shrubby species, 

 enter into the foothill forest formation in certain places. They are entirely absent from the foothills north 

 of Denver. 



Distribution of species. — The tree flora of the state embraces fifty- 

 five species belonging to twenty genera. Most of the individuals, as 

 well as the species in general, are found in the foothill, montane and 

 sub-alpine zones of plant life. Timberline, at about n,ooo or 12,000 



