96 



Water 



The easiest way for an organism to deal with the osmotic pressure 

 of its environment is to estabhsh an internal osmotic concentration of 

 the same magnitude. Even better, if the tissue fluids can be main- 

 tained at a slightly higher pressure, then there will be a tendency for 

 the needed water to enter the organism from the surrounding medium. 

 This is the general method followed by plants and by invertebrate 

 animals. 



A Water 

 Fig. 4.2. Schematic diagram of osmotic pressure differences between elasmo- 

 branch and teleost fishes and their fresh-water (left) and marine (right) media. 

 The degree of shading indicates the relative values of osmotic pressure. (Modi- 

 fied from H. W. Smith, 1936). 



Most fresh-water and terrestrial plants can maintain an internal 

 osmotic pressure ranging up to 2 atmospheres but they find saline 

 habitats with a higher osmotic pressure physiologically too dry and 

 uninhabitable. However, specially adapted plants, the halophytes, 



