585 



low, inverted arch.* Thus, interference from surface rhizomes and 

 roots is to a great ex,tent avoided. In this case, then, while it is not 

 certain that the inverted arch of the Sagittaria rhizome is a direct 

 adaptation to this particular struggle, it is certain that it is here of 

 the greatest value, however induced originally. 



Pieters ('oi) found among the plants of western Lake Erie that 

 even where Sagittaria latlfolia was most abundant, Sparganiitin (and 

 Zizania) had secured a foothold. On the other hand, throughout 

 all the broad "zones" of Sparganiiun, Scirpiis validus ("S. lacustris"), 

 and S. liiiviatilis that he describes, he says Sagittaria latifolia was 

 common. Thus, in these cases, Sagittaria was found able to asso- 

 ciate successfully with Sparganium and other species having a Spar- 

 ganium growth-form, even where these species foniied dense "zones". 

 A study of the subterranean organs of Sagittaria, Sparganiiini (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 ('09) has described the mutual relation- 

 ships of Ccreiis giganteus and Parkinsonia microphyila, twO' desert 

 species which thrive together because the occupation of different 

 depths by their root systems enables them "to utilize to the ytmost 

 the scanty rainfall." Woodhead ('06) found Holcus, Pteris, and 

 Scilla forming a noncombative "society or sub-association." For a 

 group of plants mutually competitive, Woodhead uses the term "com- 

 petitive association." Recently Wilson ('11) 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 been used already by Cowles ('01) and others (see 

 Warming '09, p. 144) to denote a primary subdivision of a forma- 

 tion. As will be seen later (and in fact as Woodhead's interchange- 

 able use of, "sub-association" and "association" might imply), not 

 all complementary or competitive groups are coextensive with a true 

 association. We shall here substitute the word community, which is 

 of less restricted application. Thus Sagittaria and Polygomim, 

 where occurring in the vScirpo-typhetum with either Typha or Scirpiis 

 Huziatilis or S. validus, constitute a complementary community; but 

 Sparganium, Typha, Scirpus Unviatilis , and 6". validus, where they 

 occur intermixed, form a competitive community. 



*For illustrations of the similar rhizomes of Sagittaria sagittifolia see Gliick 

 ('05, pi. 6 and figs. 35, 39). 



