196 FRESH-WATER BIOLOGY 



In April they were able to detect light at 250 meters. Of course 

 the clearness of the water has much to do with the penetration of 

 light rays. One can see deeper into the water of Lake Superior or 

 of Lake Huron than into that of Lake Erie. The latter is shal- 

 lower with much of the bottom clay, so that the water always holds 

 minute particles in suspension which greatly interfere with the 

 penetration of light. The red and yellow rays contained in the 

 beams of sunlight are more readily absorbed than those of higher 

 refrangibility, as the blue and violet, but whether this is of any 

 importance in determining the vertical distribution cannot be 

 stated. 



Schimper gives 6 meters as the maximum depth for phanerogams, 

 although the lower forms, such as Char a and Nitella, are said to 

 have been found as deep as 30 meters. Temperature is of little 

 importance because the variations are so slight within the limits 

 established by light. This is not so much the case with the free- 

 swimming, lower forms, but only the larger plants are considered 

 here. 



It is impracticable to attempt an elaborate classification of water 

 plants according to their habitat or adaptation to environment. 

 In some localities distinct zones of vegetation may be observed 

 which are apparently determined by the depth of water. Magnin 

 was able to distinguish four zones in the lakes of the Jura. First 

 is the littoral, with a maximum depth of 3 meters, which may be 

 divided into Phragmitetum with Phragmites communis as typical 

 and Scirpetum with Scirpiis lacustris, sedges, Eqiiisetmn, etc., as 

 representative. A second zone, the Nupharetum, extending to 3 or 

 4 meters, is composed of plants with large floating leaves such as are 

 common among the Nymphaeaceae. The third zone, the Pota- 

 mogetonetum, in water 4 to 6 meters deep, is characterized by 

 presence of several species of Potamogeton, especially perjoliatus, 

 crispus, lucens, and others. The fourth zone, Characetum, occupies 

 deep water, 8 to 12 meters, where Chara, Naias, and some mosses 

 cover the bottom. 



The flora of almost any lake may be regarded as composed of 

 zones and even rivers and small streams show plant societies, but 

 a grouping for one locality is frequently inapplicable to some other, 



