Appropriation of Carbon 31 



mineral salts and water derived from the soil, and carbon 

 derived from the air, are changed into the numerous and 

 almost endless varieties of substances found in the 

 vegetable kingdom. 



The sources of carbon. — The bacteria classed among 

 plants with very few exceptions produce no chloro- 

 phyl, and cannot, therefore, invoke the aid of sunlight 

 for the decomposition of the carbon dioxid of the atmos- 

 phere. They must depend for their carbon on compounds 

 other than carbon dioxid, and they find it in numerous 

 combinations of vegetable and animal origin. We see, 

 thus, that the great majority of bacteria differ from green 

 plants in their inability to decompose the carbon dioxid 

 of the atmosphere. There are, however, a few impor- , 

 tant exceptions to this rule. 



The study of the nitrifying bacteria, the organisms 

 which produce nitrates, has demonstrated that they 

 are able to use carbon dioxid as their only source of 

 carbon. Other organisms, recently discovered, can 

 apparently utilize carbon monoxid (coal gas) in a similar 

 manner, while still others may use methane, or marsh 

 gas, for their growth. After making due allowance for 

 these exceptions, we still find it to be true that bacteria 

 differ from green plants in relation to their carbon food. 



While sugar is undoubtedly an acceptable source of 

 carbon for many species of bacteria, it is not the only 

 source. Preference is frequently given to other carbon 

 compounds. In fact, there are bacteria that are actually 

 hindered in their development, or entirely suppressed, 

 by the presence of sugar in the culture medium. For 

 instance, the germ of cholera, and that* of typhoid, are 



