1 



1907] 



YOUNG— FOREST FORMATIONS OF COLORADO 



33^ 



permitting the entrance 1 

 most of the soecies from 



the adjoining formations. These 

 species are largely ruderal, owing to the presence of cattle and the 

 proximity of farms and highways. 



Of the weeds entering into the formation some of the commonest 

 are Medicago sativay Melilokis alba, M. officinalis^ Trijolium pratensey 

 T. repens^ Phleiim pratense, Poa pratense, Bromus brizaeformisy Poly- 



*^ 



Fig. 5.— The Populus occidentalis-Salix flmnatilis formation, two miles above the 

 mouth of Boulder Canon; to the left the control is shared by Populus occidentalis 

 and Rulac negimdo; to the right Salix fluviatilis dominates. 



mm lapathijoliiimj Verbascum thapsus, Ambrosia trifida, Solidago 



ladensiSj and Helianthus grosseserralus ( ?) . , 



In the more dense parts of the formation, the forms of the under 



more 



more 



individuals. Three of the 



Vagnera siellata, and Viola canadensis. 



The stream banks form a path along which some of the montane 

 species are invading the plains. Hence we find in the cottonwood 

 formation a few such forms as Viola canadensis and Apocynum 



