Pool : THE VEGETATION OF THE SANDHILLS OF NEBRASKA 247 



species of this nature are indicated in the Hst of secondary species of 

 this association upon an earlier page. The structure of the associa- 

 tion at this time resembles somewhat the "sand-hills mixed associa- 

 tion" of Shantz (60), especially when, as frequently happens, Cala- 

 movilfa longifolia is present in considerable abundance. 



Fig. 13. Sand-grass, Calamovilfa longifolia. Clump, showing 

 habit, o; single plant, b; spikelet, c; spikelet with 

 empty glumes removed, d. (From Shantz.) 



With the invasion and multiplication of these forms the blow- 

 out becomes in the course of years completely covered. In this 

 manner blow-out conditions finally become far removed and the 

 bunch-grasses with many of their common associates that have been 

 kept out for so long a time now take possession of the areas so well 

 prepared by the members of the blow-out association. The disap- 



