248 MINNESOTA BOTANICAL STUDIES 



pearance of the pioneers, already far advanced, is soon completed 

 and the transformation from blow-out association to bunch-grass 

 association becomes perfected. Certain of the pioneers linger for a 

 time as relicts in the more sandy areas between the bunch-grasses 

 upon the most exposed sites. 



With the development of the bunch-grasses and their numerous 

 interstitial species all traces of the blow-out association ultimately 

 disappear from large areas of the uplands. The change from the 

 one association to the other is often so complete that the only indi- 

 cations of the former history of the habitat are seen in the crater- 

 form depression which persists with its acquired grassy covering. 



Frequently various stages in the movement of this floral wave 

 that sweeps over the blow-out may be seen in a single blow-out. 

 These stages are especially noticeable when the original point of 

 colonization lies near the lowest point in the blow-out. From this 

 starting point the succession moves up and over the inner face until 

 the upper rim is reached. 



Next to blow-outs, the sand-draws present the most crucial 

 physical environment to be found within the region. The sand- 

 draw may even become more severe than the blow-out because of 

 the rushing torrents that occasionally pour through these drainage 

 lines. Sand-draws as a rule are somewhat like dry stream beds. 

 The course of the draw is usually irregular; the head of the draw 

 may be far back in the hills and the course may continue for a mile 

 or more as a regular, rather steep linear depression. As the draw 

 leads farther away from its point of origin the gradient becomes less 

 and as a rule the bottom wider. Such draws vary in length from a 

 few hundred yards to several miles and in width from 10 to 45 feet 

 or more. The flanks are usually steep and inclined to slip. The 

 general form of the draw is that of a trough with a relatively flat 

 bottom. The floor of such draws is covered with gravel or coarse 

 sand several inches in thickness. The texture of the soil varies 

 considerably at different places in the same draw because of the 

 influence of wind and water in sorting the particles. 



Sand-draws are few in number in our sandhills as compared 

 with blow-outs. Such habitats are found in greatest number and in 

 best development toward the extreme southwestern border of the 

 main sandhill region in Deuel, Garden, Cheyenne, and Morrill 

 counties where the largest draws lead from the sandhills toward the 

 North Platte River. 



