Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 249 



The effects of wind erosion are pronounced in sand-draws dur- 

 ing the more droughty seasons. The level of the floor is shifted by 

 such a force as well as by water after sudden thunderstorms. The 

 combination of wind and water erosion results in the production of 

 a sand-draw floor of remarkable instability which offers a precarious 

 foothold for plants. Species that are successfully to populate 

 sand-draws must resist conditions imposed by flood waters in addi- 

 tion to those of a wind-shifted substratum. The slipping or sliding 

 flanks of the draw present conditions somewhat similar to those 

 already described for certain portions of blow-outs. On the whole, 

 then, it is seen that the life conditions in sand-draws are somewhat 

 like those of blow-outs, but there are additional rigors imposed by 

 conditions usually absent in the more common blow-out depressions. 

 These serve to dift'erentiate quite clearly sand-draws from blow- 

 outs. 



The plants that first invade sand-draws are not, as a rule, 

 grasses or legumes with a highly specialized rhizome mechanism as 

 we have seen for blow-out pioneers. As stated by Pound and 

 Clements (57) : "The regular and habitual inhabitants of these 

 situations are but two, the closely related Polanisia trachysperma 

 and Cristatella jamesii. These two species which appear very much 

 alike are almost entirely confined to such areas, and when found 

 elsewhere are in sandy situations of the same essential character. 

 They constitute the bulk of what little vegetation there is in the 

 sliding sand on the sides of dry canyons in the sandhill region and 

 in sand-draws." This statement must now be somewhat modified 

 since a number of sand-draws have been seen with many additional 

 species and some such areas which contained the usual blow-out 

 pioneers. Indeed the latter species are sometimes controlling in 

 sand-draws as in blow-outs. Near the lakes in Garden County, for 

 instance, there are many very large active sand-draws that are prac- 

 tically free from vegetation save small patches here and there of 

 Cristatella and Polanisia. There are other sand-draws within the 

 same area, however, that show different or additional phases of the 

 sand-draw succession. Some such draws are controlled by Psoralea 

 lanceolata and others by Redfieldia Uexuosa. Others show various 

 mixtures of these two regular blow-out pioneers, and indeed mix- 

 tures of these two species with Cristatella and Polanisia were found 

 in a number of places. These observations, made under the typical 

 sand-draw conditions as first described by Pound and Clements, 



