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crop of acorns, but comparatively few of them germinate. This 

 association is not peculiar to the river dune, but is widely dis- 

 tributed over all the sand deposits of the state. 



The presence of these trees, together with certain of their life 

 processes, makes a gradual change in the environmental con- 

 ditions for the plant. Chief among these is the annual leaf- 

 fall, covering the sand with a thick layer of vegetable material 

 rich in organic matter. This, and the shade of the trees during 

 the summer and autumn, also> tend to check the evaporation of 

 water fromi the sand and to aid in retaining water in the su- 

 perficial layers. Naturally the shade also tends to reduce the trans- 

 piration from the herbs and shrubs beneath the trees. By the 

 combined and long-continued action of these two agents, shade 

 and ground cover, the association is prepared for invasion by 

 moisture-loving species ; or, in other words, it changes from 

 xerophytic to mesophytic. Moreover, their effect is cumulative 

 and proceeds with ever increasing rapidity, so that while the 

 first mesophytes appear only after a long period, the remainder 

 follow them at shorter intervals. 



A hillside north of Oquawka shows the order of appearance 

 of the mesophytes very clearly. The river dune here has a 

 maximum height of about one hundred feet, indicating a very 

 strong and continued wind action at some time in the past. 

 Now it is covered with trees to the very base and the upper 

 layers of sand are well mixed with organic matter. Just off 

 shore lies a series of wooded islands, which serve to shelter 

 the dune from erosion by wind or water. At the north end 

 the forest is composed exclusively of black oak and blackjack 

 oak, which are found over the whole length of the dune. Farther 

 towards the south, trees of scarlet oak, Quercus coccinea Muench., 

 are soon noticed near the base of the hill not far above water- 

 level ; farther down they extend higher, and soon appear even 

 at the very crest of the dune. The black walnut, Juglans nigra 

 L., next appears near the base, and in the same way climbs 

 towards the top. It is usually accompained by the redbud. Cer- 

 cis canadensis L. In a similar way there appear in order the 

 river birch, Bctula nigra L., /Vmerican elm, Ulmus americana 



