426 BOTANICAL GAZETTE [MAY 
ability of nitrogenous foods in the soil and for the differences in 
food requirements, it becomes clear that the numerous communities 
of Typha and Teucrium, Typha and Scutellaria, Sparganium and 
Teucrium, etc., are complementary. 
The purity of the Phragmitetum has already been mentioned. 
Many species that flourish elsewhere in the reed swamp under a 
wide range of light, moisture, and other shelter conditions fail 
to thrive here. Only Calamagrostis canadensis gains noticeable 
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Fic. 5.—Phragmiies communis; July 
entrance, and then imperfectly. The dead Phragmites’ growth of 
previous years makes a considerable but loose covering near the 
soil, its decay not being facilitated as in the Scirpo-typhetum, 
where water is more abundant. This dead cover may perhaps act — 
as a partial check upon the invasion of other species. But a study 
of the rhizomes of Phragmites (fig. 5) shows another fact which 
probably is more important. They do not occupy one particular 
level, but rather several different levels of soil. As a result, there 
is formed a dense mat of rhizomes and roots, about 2.5 dm. deep. 
Obviously, the subterranean organs of other species which might 
