236 MINNESOTA BOTANICAL STUDIES 



plants localized towards the crest becomes excessively eroded on 

 the flanks in such a manner that the surrounding;- sand with its open 

 vegetation is blown away to considerable depth bordering the sand 

 volume actually penetrated and held in position by the roots of the 

 Yuccas. Columns or cylinders are thus produced that vary from 

 10 to 30 feet in diameter and from 6 to 10 feet in height. These 

 may resist for a long time the destructive agencies raging about 

 them. The wind sometimes succeeds in tearing its way into these 

 columns and then the long, brown, thick woody roots become ex- 

 posed. If such action is continued the columns become thoroughly 

 undermined and many of the plants fall outward and are blown 

 away. A more vivid picture of the severity of the action of the 

 wind can not be seen anywhere in the state than here where one of 

 the Yucca columns has been torn and twisted until many of the 

 plants lie upon the bright sand, a tangled mass of large, dark brown 

 roots crowned with the green rosettes of dagger-like leaves. 



The presence of societies of Pniiins besseyi only within pro- 

 tected areas of this association shows in a convincing manner the 

 degree of wand-avoidance typically possessed by this species. In 

 the bunch-grass association this species is found upon hilltops and 

 ridges under all exposures but there the necessary protection from 

 the full fury of the wind is afforded by the bunch-grasses. The 

 even more wind-sensitive species, Ccanothus ovatus, was not found 

 at a single station in the Aluhlenbergia association. 



Further notable structural characteristics of this association are 

 seen in the number and abundance of the principal and secondary 

 species. The few interstitials are especially contrasted to the condi- 

 tion already described for the bunch-grass association. All of these 

 species are either very wind-tolerant or they occur only upon pro- 

 tected slopes within this association. The latter is especially true 

 for Primus besseyi, Boiiteloua hirsuta, Rosa arkansana, Senecio 

 plattensis, and Lathyrus ornatus. The extensive and efficient rhi- 

 zome systems of PJiaca longifoUa, Psoralea lanceolata, and the 

 naked, extremely xerophilous nature of Lygodesmia juncea are 

 characters that enable these species to assume an importance here 

 that is seldom seen elsewhere. The painted-pod, Phaca longifoUa, 

 for instance, reaches a frequency and an abundance in this associa- 

 tion that is second only to its distribution in the blow-out association. 

 The presence of Calamovilfa longifoUa, Andropogon scoparius, and 



