26 The Medium 



heat of evaporation than any other common substance. These facts 

 play a very important role in the heat regulation of organisms them- 

 selves and in the resistance of natural environments to temperature 

 change. 



Another characteristic of water having crucial ecological significance 

 is its relatively high freezing point. Because of the large amount of 

 heat which must be given up before water can turn to ice and because 

 of restricted stirring, oceans and lakes freeze only at the surface. 

 Even ponds rarely freeze to the bottom. The temperature of the 

 medium, therefore, can drop only to 0°C in fresh-water environments, 

 or to a few degrees lower in the ocean. The biological reactions of 

 a great many plants and animals can still go on perfectly well at 

 temperatures down to the freezing point of water. 



Another unusual quality of water is its power as a solvent; no other 

 common substance compares with water in this respect. Many kinds 

 of material can pass into, through, and out of the body of an organism 

 in aqueous solution. Water provides a transporting medium that is 

 versatile as a solvent but not too active chemically. Very consid- 

 erable solution would take place if sulphuric acid ran in our rivers or 

 coursed through our veins, but such a solvent would profoundly 

 alter the materials that it carried. Furthermore, the extent of ioniza- 

 tion of solutes in water is extremely high, providing the possibility 

 of a great variety of radicals and of chemical recombination. Water 

 has the highest surface tension of any common substance except 

 mercury. This high surface tension has many ecological influences, 

 involving the movement of water into and through organisms as well 

 as the rise of ground water in the soil. 



Many of the foregoing differences and special qualities of air and 

 water will be referred to again in connection with other factors of 

 the environment. Confining ourselves for the moment to the simple 

 physical differences between the two fundamental media, let us ex- 

 plore the ecological effect of the difference in density of air and 

 water. The densities of representative natural waters and of air, 

 and the approximate average density of protoplasm are as tabulated. 



The density of protoplasm is closely similar to that of sea water and 

 only slightly greater than that of fresh water, but it is more than 850 

 times greater than that of air. Associated with this difference in 



