Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 243 



The combined action of desiccating winds of high velocity, 

 great extremes of air and soil temperature, excessive transpiration, 

 and the unstable soil of the active blow-out is a condition that few 

 plants can endure. Consequently from the period of early youth, 

 and continuing throughout the years of greatest blow-out activity, 

 plants fail absolutely to gain a lasting foothold within these depres- 

 sions. Sooner or later, however, the blow-out reaches a period that 

 may properly be termed maturity. At this time a depth has been 

 reached such that even the most vigorous wind action fails to in- 

 crease it further, the sand mill becomes ineffective, and the sand that 

 slides into the bottom of the blow-out tends to remain there. In 

 some cases, especially during quiet periods in early spring or during 

 wet seasons, landslides reach the bottom of the depression and at 

 such times a much less agitated condition makes possible the germ- 

 ination of numerous seeds that have been carried into the depths. 

 If the relatively quiet conditions persist for many days some of the 

 more wind-tolerant species become established and are enabled to 

 resist the returning activities of the wind. Henceforth the extreme 

 physical conditions wane and the sparse plant cover, gradually 

 creeping upward, slowly becomes the master of the situation. Old 

 age, or the period of blow-out decadence, is traced from this time in 

 the development of the vegetation from these first successfully 

 established centers until the whole crater-like depression is claimed, 

 first by the pioneers of the blow-out association, but ultimately by 

 the bunch-grasses and their common interstitial species. 



The position most commonly captured by the first invaders 

 usually lies far down upon the slope of the blow-out. Quite fre- 

 quently the first plants to become established are found over the very 

 lowest points of the depression. Points of establishment are some- 

 times seen higher on the inner slopes. These are commonly made 

 possible by the sliding in of bunch-grasses which then present a 

 relatively stable point about which blow-out pioneers may become 

 established. Tufts of Andropogon scoparius and of A. hallii have 

 often been seen far down upon the slopes, having been carried there 

 in a slide that followed a heavy rain. These resistant spots are 

 frequently centers of growth and they usually lead to a more certain 

 and rapid reclamation of the bare slopes. 



The first plants to become established at this stage in the life of 

 the blow-out are certain grasses and legumes. The pioneer grasses 

 are known as "blow-out grasses." The most important of these is 



