Methods of Meeting Osmotic Problems 95 



The salt content of the water in alkaU soils may be even greater. 

 Animals and plants living in sea water or in saline soils are therefore 

 smTOunded by liquids having a very high osmotic pressure. 



Methods of Meeting Osmotic Problem 



If the osmotic pressure inside the organism is the same as that out- 

 side, relatively little difficulty is experienced in adjusting water 

 equilibrium. If the osmotic pressure of the fluids of an animal or 

 plant departs greatly from that of the outside medium, a problem re- 

 sults in maintaining the proper water balance. If an animal moves 

 into an area of very different salinity, a further adjustment will be 

 required. 



Sample values for the depression of the freezing point (Table 6) 

 reveal the fact that because of the higher salinity the osmotic pressure 

 of Mediterranean water is higher than that of Atlantic water. The 

 latter is very much higher than a typical value for fresh water. The 

 osmotic pressure of the internal fluids of plants and of invertebrates is 

 generally equal to, or higher than, the medium in which they are 

 living. Among elasmobranch fishes the osmotic pressure is main- 

 tained at different levels but is always higher than the surrounding 

 water due to the retention of urea in the blood. 



The osmotic pressure of teleost fishes tends to remain within a 

 relatively narrow range for both fresh-water and marine species. In 

 fresh water the teleost is osmotically superior to its medium, whereas 

 it is osmotically inferior in salt water (Fig. 4.2). In the sea a strong 

 tendency exists for the fish to lose water from its tissues to the sur- 

 roundings. A brief calculation will indicate the magnitude of the 

 osmotic force with which such an animal must contend in order to 

 maintain its water balance. If the osmotic pressure of the fish is 

 represented by A = 0.7 and that of the surrounding medium 

 by A = 1.8, the magnitude of the pressure corresponding to this 

 difference of A = 1.1 is: 



z^T X 1700 = 1015 cm Hg 

 l.o4 



An osmotic pressure of 1015 cm Hg, or 13 atmospheres, is tending to 

 extract water from the fish's tissues! For the teleost, therefore, and 

 for other animals and plants with low internal osmotic pressures sea 

 water has the effect of being physiologically dry. Fresh-water ani- 

 mals and plants that migrate, or are carried, into the sea are indeed 

 entering an arid climate. 



