622 RELATION TO ENVIRONMENT. 



few photosynthetic microphytes, especially diatoms, grow. 3d. 

 The dark region, or aphotic region, only occupied by those 



Fig. 533<*. 



Macrophytes in the upper zone of the photic region. Ascophyllum and Fucui 

 at low tide, Hunter's Island, New York City. (Photograph by M. A. Howe.) 



organisms not capable of photosynthesis (certain bacteria for 

 example).* 



1110. Concentration of salt water. The degree of concentra- 

 tion of salt solutions in water affects the distribution of plants. 

 The concentration of salt water varies in the different inland salt 

 lakes and seas because of the varying rapidity of evaporation 

 and the different proportions of fresh water draining into them. 

 The concentration of the salt water of inland seas is different 

 from that of the ocean, and here it usually diminishes from the 

 open sea toward the coast. The greatest salt content, according 

 to Schimper, is found in the Red Sea | (4.3 per cent), in conse- 

 quence of rapid evaporation and the small amount of fresh water 

 flowing into it, while in the Baltic Sea it is as low as i per cent. 

 In the ocean the content of mineral matter is about 3.5 per cent, 

 of common salt about 2.6 per cent, and it is greater in tropical 



* In the Gulf of Naples living bacteria have been found at a depth of 

 250-1 loom (=800-3500 ft.). 



f The color of the Red Sea is due to the red schimmer given it by the 

 swimming alga Trichodesmium erythraeum, one of the Cyanophyceas. 



