15] VEGETATIONAL TYPES OF FRESH WATERS 475 



little in evidence and Brown Algae are practically unknown. Thus 

 different species are differently adjusted as to both wave-length and 

 intensity of light, the green surface-forms flourishing where the 

 intensity is high and red rays plentiful, whereas they are hardly able 

 to utilize the green and blue rays which penetrate deeply into clear 

 waters. (The still shorter-length ultra-violet radiations, which may 

 be lethal, penetrate very little.) Photosynthetic activity of higher 

 plants also diminishes downwards, though the depth at which the 

 daily assimilation barely compensates for respiration varies greatly 

 with different species. This ' compensation-point ' for the Canadian 

 Water-weed {Elodea canadensis) is about lo metres, whereas for the 

 Moss Fontinalis it is about i8 metres under comparable conditions. 



Highly important is the physical nature of the substratum, benthic 

 vegetation assuming a very different character according to whether 

 the ' bottom ' is rocky, gravelly, sandy, or muddy — in particular, 

 whether it is hard or soft. Whereas rocky beds are often suitable 

 for attachment of Algae, soft substrata are favoured by most higher 

 plants that have to take root. In the intimacy of smaller bodies of 

 fresh water, the chemical nature of the substratum tends to assume 

 a greater importance than in large and deep lakes. This is because 

 the substances in solution exert an influence largely according to 

 their concentration. Thus, the flora and vegetation often differ 

 markedly according to whether the water is rich or poor in dissolved 

 calcium carbonate and some other salts, while the relative abundance 

 of organic materials or humus may also be important. Ice action 

 is significant on manv polar and other frigid lake-shores as well as 

 sea-shores, and even in temperate regions can profoundly affect the 

 nature of the surface and the composition of the marginal flora. 

 Moreover, extensive freezing of water may significantly alter the 

 aciditv and nutrient and other chemical content of the underlying 

 medium. 



As regards periodic phenomena, temperature in general exhibits 

 far smaller and less rapid fluctuations in water than in the air, and 

 is therefore less influential than on land. But whereas perennial 

 marine Algae even in cold regions may exhibit no period of winter 

 rest, seasonal differences in both quality and quantity of vegetation 

 tend to be well marked in small bodies of inland water. This 

 commonlv results from the considerable variation in temperature and 

 easy formation of ice, while especially in boreal and austral regions 

 seasonal variations in light may cause a distinct periodicity. More- 

 over, the amounts of nitrate and phosphate available in bodies of 



