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TI[K ANDROPOGON SCOPARIUS DUNE ASSOCIATION 



This grass normally grows on level ground, but it may come in 

 on the sides of dunes originated by sand-binders such as Caiamovilfa. 

 With the death of the Calainovilfa, Andropogoii scoparins is left in 

 full possession. It is efficient in holding the dune, but further growth 

 of the dune ceases. Such dunes are at most five decimeters high. 



Near Waukegan, in a place where sand has been freed from 

 gravel, there was left a gravel mound about two meters high. The 

 summit and nearly all of the sides are tenanted by Andropogan sco- 

 parins stools, in the interstices of which are several sand plants, as, 

 for example, Arabis lyrata, Petalostennim piirpurcuiu f. arenariwn, 

 Lithospcruiuin giuclini, etc. It has the general appearance of a devel- 

 oped dune, such as Jennings has described from Presc^ue Isle, but the 

 manner of its origin was evident. 



THE) POPULUS-SAUX-CORNUS THICKET DUNE ASSOCIATION 



This dunelike condition exists near the state line where the lake 

 is attacking the shore. It is not a developed dune, but the result of 

 sand being blown in upon the Po pitlus-Salix-C ornits thicket w^hich is 

 being cut into by the lake. The thicket reacts to the inblowing sand, 

 however, by becoming a dense mass of liana-entwined vegetation with 

 an advance-guard of Sali.v longifolia to check the advancing sand. 

 Such thickets are well nigh impassable on account of the network of 

 lianas, which in this area are wild grape ( V"itis vulpina) and Virginia 

 creeper {Pscdcra quinqucfolia). Sand-bar wdllow (Salix longifolia) 

 easily keeps pace with the blowing sand, but succumbs to the violence 

 of wave action as the shore is gradually washed away. With the 

 Sali.v longifolia are associated a few prairie plants, the roots of which 

 are in sod buried beneath the sand. A few of the commonest are 

 loosestrife (Lythntiii olatuni), Panicuiu capillarc, white clover {Tri- 

 folinm rcpens), blue vervain (Verbena hastata), mullen {J^erbascum 

 thapsns), Polygonum lapathifoliuni, sandbur (Cenchrus carolini- 

 anns), and Canada thistle {Cirsiiim arvense), which in this and other 

 places forms small dunes five to six centimeters in hight. 



THE BETUI.A ALBA PAPYRIEERA DUNE ASSOCIATION 



But two examples of this kind of a dune occur in this area. The 

 sides are very steep and are effectually protected by a small grove of 

 seedling trees of white birch. 



