Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 235 



one of the most effective sand-binding, sand-collecting', or dune- 

 forming species of the region. The dune-forming relation is most 

 often seen upon the more exposed slopes and within large shallow 

 blow-outs. The peculiar ability of this species enables it to grow 

 into the light when buried by the sand. Thus as the radial distri- 

 bution of the colonies is increased the plants are also raised some- 

 what above the level occupied before the last sand collection. The 

 combination of these two directions of growth maintains a constant 

 obstacle in the face of the wind and because especially of height in- 

 crement and continued sand collection the small or embryonic dune 

 increases in height. Cowles (30) has described the similar dune- 

 forming ability of Ammophila anmdinacea in tracing the life-history 

 of an Ammophila dune on the shore of Lake Michigan. The equip- 

 ment of xerophytic adaptations is perhaps even more perfect in 

 Muhlenhergia pungens than is the case with Ammophila arundin- 

 acea. The former species within the region under study appar- 

 ently swings from the dune-forming habit to the broad carpet- 

 forming nature according as the exposure is greater or less severe. 

 Both tendencies are abundantly represented in that portion of the 

 sandhills already noted wherein this association is seen in its most 

 typical and widespread expression. After midsummer when nearly 

 every glaucous plant of Muhlenhergia is provided with a narrow 

 brushlike panicle of bright reddish-purple or rose-colored spikelets 

 the areas or patches, or embryonic-dunes, controlled by this species 

 become veritable blazes of color. 



We have already seen that Yucca glaiica is one of the typical 

 widespread principal species of the bunch-grass association. This 

 species is even more pronounced in the Muhlenhergia association. 

 The Muhlenhergia-Yucca combination is much more successful in 

 the face of extreme dune conditions than is Andropogon-Yucca. 

 The soap-weed appears always to enter such wind-swept areas long 

 before the bunch-grasses and to leave after they are gone. Within 

 the Muhlenhergia maximum the highest hills are frequently studded 

 with the Yucca hosts whose dark individuals and segregated rosettes 

 stand out in vivid contrast against the glaucous background of 

 Muhlenhergia pungens or bare sand. It is here that Yucca often 

 exercises an important role in holding the uplands against the fury 

 of the wind and sand-blast. I have often seen hilltops and ridges 

 firmly resisting destruction because of the presence of myriads of 

 Yucca plants. Sometimes a knoll with a great many of these 



