VEGETATIONAL TYPES OF FRESH WATERS 



485 



Fig. 1 64. ^Vascular plants floating freely on fresh water. A, a Bladderwort 

 {Utricularia), showing underwater branches and leaves bearing bladders which 

 trap minute aquatic organisms. B, another free-floating aquatic plant, Pistia 

 stratiotes, that has roots and is very widely distributed in the tropics and sub- 

 tropics. ( ;■; \.) C, a ' Batrachian ' Ranunculus, R. aquatilis s.l., floating in the 

 surface water of a pond near Babylon, Iraq. The finely dissected leaves are 

 immersed but the flowers rise slightly abo\'e the surface, being photographed 



from above. 



Europe. Such tendencies result in largely different communities at 

 different depths — even in the same column of water at a particular 

 time. This is illustrated by the diagrams A (representing Algae 

 other than Diatoms) and D (representing Diatoms) in Fig. 165, the 

 numbers being those of organisms per litre in a Wisconsin lake, and 

 the 6 component parts of the figure being taken at approximately 

 monthly intervals during May to October. 



The conditions bringing about these varied types of depth-distri- 

 bution in phytoplankton appear to be those whose cardinal points 

 limit physiological activity, the depth of greatest population-density 

 being that of optimum conditions (the resultant of the factors involved). 

 As we have seen, these optimum conditions vary greatly for different 

 organisms. The issue is, however, rendered uncertain by the 

 operation of mechanical factors such as the dynamics of water. 

 Indeed the most important agent affecting the distribution of plankton 



