250 MINNESOTA BOTANICAL STUDIES 



serve to modify the older ideas in regard to "sand-draw formation" 

 and to place the sand-draw perhaps somewhat nearer its true rela- 

 tionship with the rest of sandhill uplands. 



Polanisia and Cristatella are both annuals and hence have little 

 or no direct influence in the production of ultimate stability over the 

 dynamic sand-draw floor. If left to the capers alone, sand-draws 

 would probably always retain their extreme dynamism. However, 

 these plants aid in an indirect manner in facilitating vegetative en- 

 croachment by presenting local spots of relatively quiet sand within 

 which one or another of the perennial sand-holders may become es- 

 tablished. The presence of seedlings of Psoralea lanceolata in early 

 spring near similar rather slight obstacles indicates that the some- 

 times persistent patches of the two capers might serve as foci for the 

 establishment of sand-binding species. 



A certain degree of stability produced by these early invaders 

 makes possible the invasion of sand-draws by numerous secondary 

 species that migrate from neighboring associations. As a result of 

 this activity the sand-draw begins to lose its emphatically nude char- 

 acter and to assume a vegetative cover resembling the bunch-grass 

 association. The more common species that share in this earlier 

 invasion after the regular blow-out pioneers and Cristatella and 

 Polanisia have prepared the way are : Munroa squarrosa, MerioUx 

 serrulata, Sieglingia purpurea, Euphorbia petaloidea, E. geyeri, 

 Ipomoea leptophylla, Collomia linearis, Chrysopsis villosa, Arge- 

 mone intermedia, and a number of ruderal species such as those of 

 Gaertneria, Ambrosia, and Artemisia. 



The fixation of the sand-draw may proceed from both ends. At 

 the upper extremity the culminating type is that of the bunch-grass 

 association, while below various ruderal species appear to hold the 

 ground for a much longer period, probably because of the presence 

 of considerable silt and a higher water content at such places. The 

 encroachment also commonly progresses from the sides of the draw. 

 Ultimately these irregular belts more or less completely cover the 

 depression and for the greater portion of its length the sand-draw 

 becomes completely merged with the bunch-grass association. 



The discovery of these additional life-history relations leads to 

 the conclusion that the "sand-draw formation" of Pound and Clem- 

 ents, at least as it occurs within the sandhill region, is but a peculiar 

 modification or variation of the blow-out association as described in 

 this paper. Accordingly sand-draws are destined to become assimi- 



