TRANSFORMATIONS OF CARBON AND NITROGEN. IOI 



trast can hardly be conceived. Not only bacteria, but all other 

 colorless plants, are obliged to depend upon organic food as a 

 source of energy, in this respect resembling animals.- But 

 here is a group of organisms that not only does not need, but 

 cannot grow in the presence of, organic matter. They do not, 

 therefore, need any other living organisms to interpose between 

 them and the mineral world, but may develop under condi- 

 tions in which they are supplied with mineral substances alone. 

 It is more surprising perhaps to find that they do not need 

 light, but can utilize the mineral substances while growing in 

 perfect darkness. This fact was at first conceived as quite 

 contrary to our general ideas of the relation of life to physical 

 energy. We have supposed that the only source of energy 

 for living things is sunlight, and that this energy is stored up 

 by green plants in the form of chemical compounds of high 

 complexity. The animals and colorless plants use these stores 

 as food, breaking them up and using the energy liberated for 

 their own use. But here we have organisms which do not 

 require organic material as a source of energy and are not able 

 to utilize sunlight itself directly. How are these facts to be rec- 

 onciled with the general views of physical energy ? Evidently 

 these nitrobacteria must obtain their energy from some other 

 source than that which is commonly utilized by animals and 

 plants. 



Of course, these surprising and almost revolutionary facts 

 demanded thorough verification and they soon received it from 

 various sources. Some additions and some corrections were 

 made to Winogradsky's first conceptions, but the general facts 

 remained as he gave them, and it is necessary to believe that 

 these bacteria obtain the energy for their growth from some 

 source not available to ordinary' animals or plants. That they 

 do have a supply of energy at command is clear enough from 

 the fact that they assimilate CO., and build it into carbonaceous 

 compounds. Winogradsky thought that they obtained their 



