FLOATING PLANTS 486 



than 150 feet, and accordingly are coastal forms. The 

 green species thrive best in yellow light, and as this is 

 most plentiful in shallow water they fringe the immediate 

 shore line. Brown and red species flourish in blue light, 

 and therefore are in deeper water. While light thus con- 

 trols the distribution of these plants in a given locality, 

 temperature is a prime factor in determining their distri- 

 bution over wider areas. Cold water species extend well 

 to the southward but stop short of temperate waters 

 whose characteristic forms are in turn replaced by warm 

 water species in the tropics. 



Sea weeds are not only commercially valuable for the 

 potash and iodine they contain, but they are extensively 

 eaten by many species of crabs, snails, sea urchins, etc., 

 whose distribution is thus determined. Of greater im- 

 portance is the fact that the decomposition of their 

 bodies, together with that of organic matter swept from 

 the soil, supplies the ocean with nitrogen and nitrogenous 

 compounds upon which the growth of a vast number of 

 other plants as well as animals depends. 



Floating Plants. — Since the discovery of the micro- 

 scope it has been found that the superficial waters of 

 the ocean support a wealth of plant life that in point of 

 numbers of individuals and aggregate bulk far outstrips 

 the shore forms. These floating species agree in being 

 unicellular and in containing chlorophyll and other pig- 

 ments that enable them to utilize the energy of sunlight 

 after the fashion of land plants. Hence, directly or in- 

 directly, all of the animal life in the sea depends upon 

 marine plants for their primary food supply. 



Among the common floating plants are the diatoms 

 (Fig. 139) that are at once distinguished by being en- 

 closed with a shell of flint, exquisitely sculptured and 

 with shapes ranging from discs and ellipses to rectangles, 

 which may form threads where many are attached to- 

 gether. These plants are extraordinarily abundant in 

 cold water, but in the tropics are largely replaced by other 



