282 NEBRASKA STATE HORTICULTURAL SOCIETY. 



interior surface of the blow-out and out over the rim. This action is 

 quite appropriately called the "sand mill." The action of these spiral 

 currents are conspicuous during rather low v/inds as well as on very- 

 windy days. Such activity is a very important factor in hollowing the 

 blow-outs to the greater depths. 



After many years of this sort of growth, blow-outs at the end of 

 their maximum activity become enormous depressions with a rim some- 

 times 300 to 900 feet in circumference with sides of bare sand sloping 

 inward at an angle of about 30 degrees to the bottom, which may be 

 from 20 to 75 feet or more beneath the rim. In the western portion of 

 the region where blow-outs are formed in rather low hills among the 

 lakes the sand is removed from the interior until the water table is 

 reached. 



During the years of greatest blow-out activity plants fail absolutely 

 to gain a foothold and establish themselves in the blow-out because oE 

 the great exposure to wind and shifting sand. The combined action of 

 a high wind, high soil temperature, excessivb evaporation, and an un- 

 stable soil in the active !:ilow-out, is a condition that plants can not sur- 

 vive. Sooner or later, however, because the blow-out has reached such 

 a depth that the "sand mill" becomes ineffective and the sliding sand 

 fails to reach the bottom, certain plants appear in the bare sand of the 

 blow-out. From this time the terrible physical conditions begin to wane 

 and the vegetation gradually creeps up from the bottom of the blow-out 

 and slowly becomes the master of the situation. The decadence of the 

 blow-out is traced in the development of the vegetation from these first 

 successful invasions until the whole crater-like depression Is claimed by 

 the bunch-grasses and their common neighbors. 



The first plants to become established in such places are certain 

 grasses commonly called "blow-out grasses." The most important of 

 these is Redfield's gi-ass (Redfieldia flexuosa) which is almost always 

 the very first pioneer in the reclamation of the blow-out. Redfieldia may 

 be the only plaint in such situations for many years. All during this time 

 it is extending its area by undermining and binding the soil v/ith its 

 network of slender rhizomes. From these rhizomes there arise tufts of 

 long, flexuous, narrow leaves gracefully nodding in the gentle breeze or 

 lashing about like so many slender wires in the higher winds. Some- 

 times in a single windstorm the sand level about these tufts may be re- 

 duced two inches or more, but seldom are the plants uprooted. It is to 

 the rhizome habit of propagation that Redfieldia owes its success in 

 thus so completely capturing the blow-out. The later invaders are also 

 provided with this device, which certainly is the key to the whole situa- 

 tion. 



After Redfieldia has once taken charge of the habitat other species 

 soon begin to wander over the rim of the blow-out and to invade the 

 area occupied by the first blow-out pioneer. Among the first of these 

 early invaders we must number the spiny blow-out grass (Muhlen- 

 bergia pungens), sand grass (Calamovilfa longifolia), and the hair-like 



