154 THE OCEAN 



being formed ; animals may devour these 

 vegetable products, and raise the living sub- 

 stances to a still higher level of complexity, 

 animal proteins being formed. However, on 

 the death of all animals and plants these com- 

 plex organic substances are broken down, 

 through the agency of bacteria, to carbon 

 dioxide and nitrogen or its simplest com- 

 pounds, which have relatively little stored 

 potential energy. 



The nitrifying bacteria appear to require 

 no organic compounds for their nutrition, 

 and the bacteria of root-nodules, given the 

 most minute quantity of organic carbon, 

 can derive all their nitrogen from the 

 atmosphere. With these may be classed 

 those forms which are able, with a very small 

 supply of organic matter, to break up specific 

 inorganic bodies and derive energy from the 

 process (sulphur and iron bacteria). The 

 chemical changes are not well understood, 

 but they indicate that these organisms, without 

 chlorophyll and sunlight, can appropriate the 

 carbon dioxide from the atmosphere, as well 

 as assimilate and store up free nitrogen. These 

 bacteria thus give some indication of how the 

 first forms of life to appear on the earth 

 obtained their nourishment, and they may 

 possibly form an exception to the general 

 statement that green chlorophyll-bearing 



