96 



by sand, (4) the ability to adapt the root to a stem environment or 

 the stem to a root environment, depending- upon burial by sand or 

 exposure by its removal, and (5) a good set of xerophytic structures, 

 v^hich enable the plant to withstand the extreme conditions of its en- 

 vironment. To these might be added a sixth requisite, the persistence 

 of the subaerial parts during the winter. In every dune region there 

 are some plants which fulfil all or some of these requirements, and 

 which are responsible for the construction of the local dunes. In 

 Illinois the principal ones are Rhus canadensis, var. illinoensis, 

 Ccanothiis ovatiis, Panicitni virgatuni, and Tephrosia znrginiana. 

 These are discussed in the order of their effectiveness. 



Tephrosia znrginiana (PI. IX, Fig. i) is a perennial herb with 

 very long, slender, tough roots. Several stems, each 1-1.5 feet (3-4 

 dm.) high, arise from a common base and are densely covered by 

 leaves. These serve to catch the sand and hold it during the sum- 

 mer, but they die in autumn and the dead stems are soon removed 

 by the winter storms. The sand is then held by the subterranean 

 root system only. Tephrosia endures covering by sand if it is not to 

 too great a depth, at least not exceeding half the height of the stems. 

 It does not possess the power of unlimited growth during the season, 

 and is consequently not able to keep above the sand indefinitely. 

 Neither is it a very efficient sand-binder, and it dies if the crown 

 and a few inches of the roots are exposed (PI. IX, Fig. 2). Such 

 cases are seldom seen, because the dead remains are soon blown 

 away. From both of these reasons it is clear that Tephrosia is not 

 a very efficient dune-former, and this is fully substantiated by field 

 observation. Tephrosia dunes are low and gently sloping (PI. VII, 

 Fig. 2), and are found mainly on blowouts where the rate of sand 

 movement is apparently very slow. This, of course, does not pro- 

 hibit the plant from growing on larger dunes in company with other 

 species. It is always associated with Aristida tuberculosa. 



Panicuin virgatuni is by all odds the most abundant dune-former 

 in the Hanover area (PI. IV, Fig. 2). While it does occur in the 

 bunch-grass association, it is much more abundant on the deposits, 

 and in the 'area mentioned even the smallest and youngest blowouts 

 are sometimes marked by a conspicuous growth of the plant on their 

 newly formed deposits (PI. VI, Fig. i; PI. VII, Fig. i). In the 

 Haxana and Oquawka areas it is infrequent, and the dunes are 

 usually fomied and held by some other species. Like Tephrosia, 

 it has a large number of veiT deep tough roots which help bind the 

 sand, and it also spreads slowly by rhizomes. The subaerial parts have 

 the typical bunch-grass structure, and the dense basal leaves act 



