320 



associated with it Myriophylliun verticillatiim. In little streams of 

 running water the dominant species is usually Potamogeton foliosiis 

 niagarensis, and associated with it are Myriophyllum verticillatnm and 

 Hlodea canadensis. In one such little stream MyriophyUiun and Blodea 

 occur almost to the exclusion of the Potamogeton. This association 

 is developed to such a limited extent that in a description of this 

 region no adequate idea can be given of it. A more detailed account 

 may be found in Jennings (1909). 



the: castaIvIA-nymphaka association 



In shallower water than that occupied by the Potamogetons is 

 the Castalia-Nymphaea association. The water is quiet and a layer 

 of mud covers the bottom. The plants of this association are essen- 

 tially submerged, but they frequently have their leaves at or above 

 the surface of the water. They may mature their flowers and fruits 

 under water, at the surface, or above the water. This association is 

 verv^ effective in accumulating matter which builds up the bottom. 

 This work is furthered not only by the petioles of the water-lilies, 

 which serve to catch materials, but also by the semi-floating secondary 

 species when they occur. The large leaves of the water-lilies, spreading 

 out on the surface, serve to keep the water calm, and this permits a 

 deposition of the matter brought there in suspension. The very 

 noticeable accumulation of organic matter on the bottom is correlated 

 with slow subaqueous oxidation. 



The Association. — This association is not well represented in the 

 area. In only one pond do both the species which give the name 

 to the association occur. When this happens, the white water-lily 

 (Castalia tuherosa) appears to prefer deeper water than the yellow 

 water-lily {Nyiuphaca advena). Castalia is not usually emersed, 

 while Nymphaea frequently grows above the water. In this particu- 

 lar ponci, associated with the water-lilies are CeratophyUuin dcmer- 

 sum, Chara, Potamogeton sp. ?, and Blodea canadensis. In all the 

 other places in this region where this association occurs, it is repre- 

 sented by the dominant species, Nymphaea advena, and there are 

 seldom any secondary species with it. (See PI. LUI, Fig. 2.) Not 

 only does Nymphaea occur along the ponds in the swales, but 

 it also grows in a good manv of the ditches and holes that have 

 been dug in the right of way of the Chicago and North Western rail- 

 way. Only one case is at hand to give an idea of how long it takes 

 for the Nymphaea to appear in a ditch after it has been dug. In an 

 excavation made during the summer of 1906 Nymphaea appeared in 



