587 



communities of Typha and Teuo^iuui, Typha and Scutellaria, Spar- 

 ganiuin and Teucriujii, 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 Calamagvostis canadensis gains noticeable en- 

 trance, and then imperfectly. The dead Phragmites, the 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 (PI. LXXXIX, Fig. 8) shows another fact which 

 probably is more important. They do not occupy one particular level, 

 Ixit rather several dift'erent levels of soil. As a result, there is 

 formed a dense mat of rhizomes and roots, about 2.5 dm. deep. Ob- 

 viously, the subterranean organs of other species which might 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 be decisive in favor of Phrag- 

 mites. 



No cases were found where Phragtiiites had re-gularly produced 

 rhizomes (or stolons) upon the surface of the ground. Frequent in- 

 stances were met with, however, in which the entire aerial shoot 

 had fallen over upon wet, mostly nude soil and, having produced 

 numerous roots, had elongated at a much more rapid rate than be- 

 fore. 



The Nymphaeetum displays many complementary communities. 

 The rhizomes of Nyniphaea advena (PI. XC, Fig. 9) are usually 5-10 

 cm. thick and lie mostly at a depth of 8-25 cm. below the soil sur- 

 face. The rhizomes of Castalia odorata, while smaller, lie at a simi- 

 lar depth. Where the Nymphaeetum intergrades with the Scirpo- 

 typhetimi, as is commonly the case, the rhizomes of Typha, Sparga- 

 nitim, and Scirpus validus lie higher in the soil. In many places the 

 soil surface itself is occupied by the stolons of Ranunculus delphini- 

 folius and the creeping stems of Polygonum hydropiperoides, with 

 a large, upright stem base of Siuni ciciitaefoliunv present here and 

 there. In other places. Ranunculus is replaced by Myriophyllmn 

 humile or by young plants (growing chiefly from detached leaves) 

 of Radicula aqiiatica, while Polygonum is replaced by Veronica Ana- 

 gaUis-aquatica, and Sium by Rumex verticillatus. And while it is 

 true that Nymphaea and Castalia, or Typha and Sparganiuin and 



