56 



THE PLANT SOCIETIES OF 



climate, since the mesophytic forest is associated to a high degree with 

 humus. 



2. THE BEACH-DUNE-SANDHILL SERIES. 



A. The beach. — The author has previously discussed in consider- 

 able detail the dynamics of the dune societies/ and it will not be neces- 

 sary to do more here than to summarize the chief conclusions, and add 



Fig. 33. — Beach at Dune Park, showing the smooth and naked lower beach, the middle 

 beach with its line of debris, the upper beach with scattered shruDs, and the dunes. 



a few new data. Before long it is expected that a paper will appear 

 giving the changes that have taken place since the first observations 

 were made in 1896. 



The beach in the Chicago area is xerophytic throughout. There is 

 nothing analogous to the salt marshes of the Atlantic coast, nor to the 

 hydrophytic shores farther north along Lake Michigan. The lower 

 portion of the beach is exposed to alternate washing by the waves and 

 desiccation in the sun, and is devoid of life. The middle beach, which 

 is washed by winter waves, though not by those of summer, has in con- 

 sequence a vegetation of xerophytic annuals, the most prominent of 

 jvhich is Cakile Americana (Sea rocket). The upper beach is beyond 



' CowLES, H. C: The ecological relations of the vegetation on the sand dunes of Lake 

 Michigan. Bot. Gaz. 27 : 95-117, 1B7-202, 281-303, 3B1-391. 13H9. Also reprinted separately. 



