39 2 



BOTANICAL GAZETTE 



[NOVEMBER 



association. It may perhaps intergrade with the prairie-grass of 

 the middle part of the prairie region. The growth on what 

 is called the Middle Mesa (fig. 7) is intermediate between short- 

 grass and prairie-grass; it is floristically quite like the South 

 Dakota prairie-grass described by Harvey (8). This similarity 

 is more evident in late summer, xerophytic prairie-grass species 







£ S^F* J J 





- 



■ 







* 



" 



>j 



r 





■ 





■■■ 



m 



■ 



■ 



* 



7 -t. 



*? 



• t 







#* 



** 





V 1 



-'. 



'* * ?■ 



if 







■ 



■*-—- 



:«£. 



i ■ 



i 





9 





:"4 



H 



■ 





Fig. 7. — Mixed growth of the Middle Mesa, on the outskirts of Boulder: Koeleria, 

 Stipa, Helianthus, Senecio (the flat-topped heads), Erysimum, Aragallus, Eriogonum 

 alatum (the tall dead stalk), Eriogonum umbellatum; in the middle ground, the valley 

 of Boulder Creek (the trees are planted in the town) ; above, the foothills, generally 

 grass covered, with scattered rock pines, which are more abundant on the rocky 

 ridges; June 1, 1913. 



like Solidago rigida L., Sideranthus spinulosus (Pursh) Sweet, 

 Helianthus, Petalostemon, and Aster spp. being very conspicuous. 

 The dominant grasses are the same in both Colorado and South 

 Dakota areas. The dry prairies in east and west show con- 

 siderable similarity; the more mesophytic prairies of east and 

 west show considerable divergence, so far as species composition 

 is concerned. 



