428 BOTANICAL GAZETTE [MAY 
and there. In other places, Ranunculus is replaced by Myrio- 
phyllum humile or by young plants (growing chiefly from detached 
leaves) of Radicula aquatica, while Polygonum is replaced by 
Veronica Anagallis-aquatica, and Sium by Rumex verticillatus. 
And while it is true that Nymphaea and Castalia, or Typha and 
Sparganium and Scirpus, or Ranunculus and Myriophyllum and 
Radicula, or Polygonum and Veronica, or Sium and Rumex are 
mutually competitive, yet a complete community (as shown, e.g., 
in fig. 6) is complementary; the basal parts chiefly because of 
different depths, and the upper parts chiefly because of different 
growth-forms. 
An inspection of the Nymphaeetum shows that only where 
Nymphaea is nearly or quite absent does Sagittaria latifolia success- 
fully invade from the Scirpo-typhetum. As is commonly known, 
the rhizomes of Nymphaea in many habitats are usually decayed 
to within a short distance of the growing apex. An investigation 
during August 1911 showed that generally where the rhizomes of 
Sagittaria had penetrated these decayed parts, they themselves had 
started to decay. Frequent cases were found where the decayed 
Nymphaea rhizomes lay nearer the surface and the Sagittaria 
rhizomes had proceeded underneath, unharmed. Speaking in a 
general way, while Nymphaea and Sagittaria thrive better in the 
Nymphaeetum and Scirpo-typhetum respectively, yet along the 
line of tension between these two associations the injury done by 
the decayed Nymphaea rhizomes to the rhizomes of Sagittaria is a 
factor that appears to be decisively in favor of Nymphaea. The 
inverted rhizome arch of Sagittaria, useful in the Scirpo-typhetum, 
is here more often harmful. 
In many parts of the Irido-acoretum, Polygonum Muhlenbergit 
and Galium Claytoni abound, and these form with Acorus a comple- 
mentary community (fig. 7). The creeping stems of Galium root 
upon the soil surface, the rhizomes of Acorus lie just beneath, and 
those of Polygonum are deepest of all. The bushy shoot of Galiwm 
appears not to harm the slender, ensiform leaves of Acorus, and 
§ Many litmus tests uniformly showed the decayed parts of the Nymphaea rhizomes 
to be strongly acid. Enough cultural experiments have not been performed, however, 
to determine whether the effect upon the Sagittaria rhizomes, as above noted, is due 
to acid or to other causes. 
