1. Cassia — Amhrosia — (']ifh>loiiia. (Blow-sand association) 



2. Stenop/njllus — Paniruin — Sorq/ias/ru/ii ) , u, . ■ .■ ^ 

 /,„,■',, •' ■ (rilowout association) 

 1 2a. J-'opt/lus — Acer) \ 



3. Cladonla — Antennaria ) 



. TT T ,1 u- • I ("rairie association) 



It is in onl}- a small proportion of the numerous blowouts 

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

 Ill 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. 



ErrcrsiDH fo Ji/n/cl/-(/n/>:>:. 



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 liy the Ijunch-grass association. Naturally 

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

 grass association that are aide to live on Idow-sand, and partic- 

 ularly the sand-binder SjjDroho/iis cnjitfanil nis (PI. XVIII., Fig. 

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

 pear, and their place is previously hlletl by ('(il/ii-rlmr ffi(iii(/ii- 

 hitii. Hints itrniitiitiiit, < 'lii-i/st)jis/s fini/ji(innn, and other sand-bind- 

 ers, or sometimes even l)y the Kentu cky l>lue-grass. Paa pni- 

 friisis. 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 C/irz/soiis/s (■(uiijinnini, Opunfia liniiii- 

 fiixd. ('(iHiri'lnir frl(ni(/ii/(if(i, and li'Inis in'oiiidf/ca. 



The blowouts may be Hxed without first tilling up with 

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

 species of the Ininch-grass association, wdiich extend down into 

 it, and ultimately occupy it completely. The stools growing 



