1912] SHERFF—SKOKIE MARSH 427 
start growth here must compete with the extraordinarily large 
number of Phragmites roots and rhizomes. Where other factors 
are suited equally to Phragmites and to competing species, this 
biotic factor in the subaerial struggle ought usually to decide in 
favor of Phragmites. 
The Nymphaeetum displays many complementary com- 
munities. The rhizomes of Nymphaea advena (fig. 6) are usually 
a 
Fic. 6.—a, Ranunculus delphinifolius; 6, Nymphaea advena; c, Sium cicutae- 
jing Fi T 'ypha latifolia; ¢, Polygonum hydropiperoides; drawn in ee July. 
5-10 cm. thick and lie mostly at a depth of 8-25 cm. below the soil 
surface. The rhizomes of Castalia odorata, while smaller, lie at a 
similar depth. Where the Nymphaeetum intergrades with the 
Scirpo-typhetum, as is commonly the case, the rhizomes of Typha, 
Sparganium, and Scirpus validus lie higher in the soil. In many 
places the soil surface itself is occupied by the stolons of Ranunculus 
del phinifolius and the creeping stems of Polygonum hydro pi | peroides, 
with a large, upright stem base of Sium cicutaefolium present here 
