THE PLAINS ASSOCIATION. 



47 



e\ddently been open cracks and the soil on the sides of them is much darker 

 in color, having been washed down from the sm'face. It is in these crevices, 

 where the moisture-content is somewhat higher, that grasses and other plants 

 branch and rebranch so profusely. 



Fig. 9. — Aristida purpurea. Fig. 10. — Artemisia frigida. 



MuWenbergia gracilliina. — This grass is also a dominant of the short-grass 

 plains; it is less widely spread and less abundant than Bouteloua gracilis, 

 though in some places it ranks close to the latter. Superficially, it appears like 

 Bulbilis dadyloides, because of its mat-like growth in small areas and the short, 

 curled leaves. Frequently the center of the mat is dead and only the 

 peripheral portions produce flowers. 



A trench over 4 feet long was dug with, one face cutting through a pure 

 stand of this grass to a depth of about 5 feet. Great clusters of roots, only 

 about 0.5 mm. or less in diameter, ran off in all directions from the very sur- 



