286 ECOLOGY [CH. 



and of the degree of aeration of the water in determining the 

 distribution of aquatics 1 . It might have been supposed that the 

 nature of the soil, underlying the water in which hydrophytes 

 grow, would be relatively unimportant, but, on investigation, it 

 proves to be a factor of almost as much significance as in the 

 case of terrestrial plants. It is true that there are certain ex- 

 ceptions, such as the Podostemaceae, which seem indifferent 

 to the chemical composition of the naked rocks on which they 

 live 2 , but this case may perhaps be explained by the fact that 

 the rapidly flowing waters, to which they are confined, probably 

 owe little of their dissolved constituents to the particular rocks 

 over which they are passing at any given moment. The majority 

 of hydrophytes, however, show definite preferences and aver- 

 sions in the matter of the soil underlying the water in which 

 they grow, and of the resulting differences in the nature of the 

 solution in which they are immersed. 



A case has been described in America, in which the depen- 

 dence of water plants upon the substratum is shown with dia- 

 grammatic lucidity 3 . Lake Ellis in North Carolina is an area 

 of shallow water, 2 J by 3 miles across, and seldom more than 

 two feet in depth; the entire floor is clothed with plants. Three 

 distinct assemblages of vegetation occur in the Lake, the differ- 

 entiation apparently depending wholly on the nature of the soil. 

 The central region, where the soil is sandiest, is characterised 

 by Eriocaulon, E/eocharisand Myriophyllum ; a number of different 

 plants, including one or two Waterlilies, frequent the inter- 

 mediate muddy belt, while the marginal area of muddiest soil 

 is chiefly clothed with Grasses and Sedges. The observation, 

 made long ago by a German writer 4 , that the variety of Hydrilla 

 verticillata found in Pomerania is intolerant of sandy soil and 

 is confined to muddy clay, is comparable with the facts just 

 cited concerning Lake Ellis. 



The two classes of substratum which offer the most marked 

 contrast, as regards the flora which they support, are the cal- 



1 Tansley, A. G. (1911). 2 Willis, J. C. (iQH 1 )- 



3 Brown, W. H. (191 1). * Seehaus, C. (1860). 



