[Chap. XLVI UNDER-WATER ENVIRONMENTS 599 



Our largest bodies of water are the oceans, with a combined surface 

 area of nearly 140 million square miles, or about 70 per cent of the 

 earth's surface. In contrast to this, the surface of all bodies of fresh 

 water taken together is scarcely one million square miles. The vast ex- 

 panse and diverse conditions of oceans constitute enormous possibilities 

 of development of marine plant and animal populations. Ocean water 

 is practicallv a continuous medium, in sharp contrast to the more isolated 

 bodies of fresh water. 



The oceans vary in depth from a few feet of water along the coasts 

 to nearly 6 miles in the so-called "deeps." On the ocean floor are plains, 

 hills, valleys, and mountain ranges. 



Pressure varies from 15 pounds to the square inch at the surface to 

 nearly 8 tons at the greatest depths. Such enormous pressures would 

 seem to preclude the possibility of living organims, but the effects of 

 this pressure are annulled by an equivalent pressure inside the body. 

 Equalization of pressure within the body, however, does not take place 

 rapidly; consequently, vertical migration of living plants and animals 

 under these conditions is somewhat restricted. Except along the coast, 

 the influence of the land on the ocean is much less than it is on bodies 

 of fresh water. 



The specific gravity of sea water with a salinity of 3.5 per cent is about 

 1.028 at 0° C. The density of the aerial environment of land plants is 

 much less than that of their protoplasm, while the density of the living 

 parts of marine organisms is about the same as that of the medium in 

 which they live. Most marine plants are slightly heavier than the water, 

 but their small size and proportionally large surface areas, as well as 

 occasional gas bladders and fat globules, contribute to their floating 

 capacity. Dead bodies of marine plants and animals eventually sink to 

 the bottom, unless devoured or dissolved, although the rate of sinking 

 is extremely low. Below depths of 5000 meters none of the parts of 

 most of the smaller plants and animals can be found, for the whole body 

 has gone into solution. 



The solar energy actually available to plants is decreased in the ocean 

 by the same factors that decrease it in fresh water: absorption, reflec- 

 tion, suspended matter, wave action. Where suspended matter and 

 minute organisms are abundant, the water is apparently green because 

 of tlie scattering of the short blue and violet rays and the absorption of 

 the red and yellow. Blue water contains little or no suspended matter. 



Photosynthesis in plants of the ocean generally occurs above a depth 



