170 BOTANICAL GAZETTE | MARCH 
the humus is present in sufficient quantity. It is by reason of 
this last fact that the mesophytic forest can appear in all con- 
ditions in this climate, since the meeeeny tic forest is associated 
to a high degree with humus. 
2. THE BEACH-DUNE-SANDHILL SERIES. 
A. The beach.—The author has previously discussed in con- 
siderable detail the dynamics of the dune societies,?5 and it will 
not be necessary to do more here than to summarize the chief 
conclusions, and add 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 alter-’ 
nate 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 consequence a 
vegetation of xerophytic annuals, the most prominent of which 
is Cakile Americana. The upper beach is beyond present wave 
action, and is tenanted by biennials and perennials in addition to 
the annuals. /ig. 33 shows a beach of this type, the lower 
beach being smooth and even, the middle beach covered with 
débris, while the upper beach has a scattered perennial vegetation. 
The beach at the base of cliffs shows similar subdivisions, 
though the zones are much narrower as a rule. The vegeta- 
tion, too, is much the same, though some forms, as Strophostyles, 
have not been seen as yet on the beaches of the dune district. 
At the foot of cliffs there often occur alluvial fans of sand, 
which have been deposited by the torrents during and following 
rain storms, These fans have a comparatively rich vegetation 
and species sometimes occur here that are not found elsewhere 
on the beach. 
75 COWLES, H. C.: The ecological relations of the vegetation on the sand dunes 
of Lake Michigan. Bor. Gaz. 27: 95-117, 167-202, 281-308, 361-391. 1899. 

