1914] 



VES TA L—PRA IRIE VEGE TA TION 



387 



striking than the differences. The Bouteloua mixed consocies, 

 presenting the most radical departure from the typical short-grass 

 growth, is different mainly in the possession of a derived element. 



he meso- 

 moisture 



mixture 



stituents. 



Fig. 4. — Wheat-grass, with Artemisia aromatica (the dark plant) in fine-grained 

 soil of the mesa-terrace just south of Boulder; June 1913. 



THE WHEAT-GRASS ASSOCIATION 



Wheat -grass association. — Shaxtz 18, pp. 21, 48. 

 Agropyron formation. — Pound and Clements 13, p. 383. 

 Agropyron occidentale consocies. — Shantz 17, p. 36. 

 Agropyron growth of prairie crests. — Harvey 8, p. 279. 



The wheat-grass {Agropyron Smithii Ryclb.; Agropyron occi- 

 dentale Scribn.) is established in rather looser clay soil than that in 

 which the short-grasses dominate. The typical habitat is the depo- 

 sition area at the base of side-slopes of mesas, with fine soil washed 

 down from above. Recently disturbed clay and what appear 

 to be areas of wind deposition are also occupied by wheat-grass. 

 Invasion by grama-grass is often initiated, and the intermediate 



