424 BOTANICAL GAZETTE [MAY 
PIETERS (13) found among the plants of western Lake Erie 
that even where Sagittaria latifolia was most abundant, Sparganium 
(and Zizania) had secured a foothold. On the other hand, through- 
out all the broad “zones” of Sparganium, Scirpus validus (“‘S. 
lacustris”), and S. fluviatilis that he describes, he says Sagittaria 
latifolia was common. 
A study of the subterranean organs of Sagittaria, Sparganium 
(or Typha or Scirpus), and Polygonum shows that because of 
differences in direction or in depth they conflict but little. Again, 
because of differences in growth-form, their aerial parts do not 
conflict seriously. Thus a given area can usually support a greater _ 
mass of vegetation if these three growth-forms be present in fair 
mixture than if only one be present. SPALDING (15) has described 
the mutual relationships of Cereus giganteus and Parkinsonia 
microphylla, two desert species which thrive together because the 
occupation of different depths by their root systems enables them 
“to utilize to the utmost the scanty rainfall.” WoopHeap (18) 
found Holcus, Pteris, and Scilla forming a non-combative “society 
or sub-association.”’ For a group of plants mutually competitive, 
WoopHEAD uses the term “‘competitive association.’’ Recently 
WItson (17) likewise speaks of a ‘‘complementary association” 
or “‘society.’’ But the use of the words “‘association”’ and “‘society”’ 
in this connection is unfortunate. These words have already been 
used by Cow tes (4) and others (see WARMING 16, p. 144) to denote 
a primary subdivision of a formation. As will be seen later (and 
in fact as WOODHEAD’s use interchangeably of ‘‘sub-association”’ 
and ‘‘association’”’ would imply), not all complementary or com- 
petitive groups are coextensive with a true association. We shall 
here substitute the word community, which is of less restricted 
application. Thus, Sagittaria and Polygonum, where occurring in 
the Scirpo-typhetum with either Typha or Scirpus fluviatilis or S. 
validus, constitute a complementary community. But Sparganium, 
Typha, Scirpus fluviatilis, and S. validus, where they occur inter- 
mixed, form a competitive community. 
Species that are complementary in one association may be less 
so in another. Thus, Polygonum Muhlenbergii and Sparganium 
are complementary in the Scirpo-typhetum; but in the Sio- 
