16 ( J 



1. Cassia— Ambrosia— Cycloloma (Blow-sand association) 



2. Stenophijllus—Panicum—Sorahastruut \ /U1 



(2a. Populus—Acer) f (Blowout association) 



3. Cladonia — Antennaria ) 



4. Helianthus—Hieracium, ( Prairie association) 



It is in only a small proportion of the numerous blowouts 

 that this succession is followed to the culmination in a prairie. 

 In fact, only one was observed that had reached the stage num- 

 bered 4. Probably not more than a tenth ever reach stage 2. 



Although the wind alone is instrumental in the excavation 

 of the blowouts, it has in general a leveling action. The sand 

 removed from each blowout is distributed over a larger area 

 than that from which it was taken, so that the tendency of the 

 wind action is toward a level surface of sand. 



Reversion to Bunch-grass. 



It has just been shown that in certain cases the blow- 

 out association may develop into one of the prairie associations- 

 Similarly, if for any reason the blowing of the sand ceases, recol- 

 onization is begun by the bunch-grass association. Naturally 

 the species most active in this are those members of the bunch- 

 grass association that are able to live on blow-sand, and partic- 

 ularly the sand-binder Sporobolus cryptandrus (PI. XVIII., Fig. 

 2). The bunch-grasses themselves are among the last to ap- 

 pear, and their place is previously filled by Callirrhoe triangu- 

 lata, Rhus aromatica, Chrysopsis camporum, and other sand-bind- 

 ers, or sometimes even by the Kentu cky blue-grass. Poa pra- 

 tensis. Two stages in the regenera tion of the bunch-grass are 

 shown in the following tables. 



Even after the sand is completely fixed, the bunch-forming 

 grasses are much less abundant than in the normal association, 

 their places being rilled by Chrysopsis camporum, Opuntia humi- 

 fusa, Callirrhoe triangulata, and Rhus aromatica. 



The blowouts may be fixed without first filling up with 

 blow-sand, and in this case they are occupied directly by the 

 species of the bunch-grass association, which extend down into 

 it, and ultimately occupy it completely. The stools growing 



