240 MINNESOTA BOTANICAL STUDIES 



Sieglingia purpurea Tradescantia occidentalis 



Stipa comata 



Blow-outs originate on an exposed upper slope when the cover 

 becomes broken or seriously depleted from any cause. As the wind 

 sweeps up and over such slopes it rolls the heavier sand grains along 

 on the ground but catches up the finer particles and carries them 

 over the crest of the hill a few yards farther away and may drop 

 them upon the leeward face of the hill. As more and more sand is 

 carried away in this manner and the uprooted plants are swept on 

 with the gale the embryonic blow-out comes into existence. During 

 these early stages the blow-out appears as an area of bare sand a 

 few feet or yards across and a few inches in depth over which the 

 wind sweeps and continues to eat its way downward. The roots of 

 deeper-rooted species are exposed and soon the whole plant is 

 blown away. At this time the woody root-remnants of certain 

 perennial bunch-grass associates frequently appear strewn over the 

 surface of the depressions which may now be hollowed out to a 

 depth of 20 to 40 inches. The dead roots become stripped of their 

 cortical tissues. Finally the wind eats its way far beneath the point 

 of penetration of the deepest-rooted species (except possibly Yucca) 

 and all these are then added to the accumulating debris on the lee 

 slope or elsewhere. 



The two principal factors that enable the wind to begin this 

 work of destruction are fires and overgrazing. Either factor may 

 result in reducing the plant cover to a point below effective wind 

 resistance and, as soon as this occurs, destructive wind erosion is 

 sure to begin upon exposed slopes. Nothing is quite so eft'ective as 

 a prairie fire in clearing the vegetation and exposing the soil to wind 

 action. A severe fire or repeated burning destroys absolutely every- 

 thing above the surface of the ground in so far as the plant cover is 

 concerned. The old practice of burning the range to increase and 

 improve the yield of forage has practically ceased since the great 

 majority of stockmen and homesteaders have learned the folly of 

 such a method. A recently burned range is a truly beautiful sight 

 in early summer when the wealth of new growth appears with all its 

 freshness, often in sufficient quantity to mask the charred surface. 

 This freshness is especially notable in such places because it is not at 

 all diminished by the presence of old stems, leaves, and other litter 

 of bygone generations of plants. The perennial roots of Andropo- 



