138 C. J. T. DORYLAND 



oxidation of ammonia or nitrite nitrogen, those bacteria which 

 obtain their energy by the oxidation of sulphur and those which 

 obtain their energy from the oxidation of iron carbonate. We 

 are now in a position to summarize the food requirements of 

 microorganisms. They need in general the common nutrient 

 salts. Their nitrogen requirements differ, some being able to 

 utilize ammonia, some nitrate, some cyanide and probably 

 some nitrite nitrogen. They differ too in the compounds which 

 they use as sources of energy. Among the prototrophic forms 

 we find, respectively, those groups which can utilize iron, sul- 

 phur, ammonia, nitrite or a non-nitrogenous organic compound 

 as a source of energy, while among the saprophytic and para- 

 sitic forms we find those which can utilize the non-nitrogenous 

 organic compounds as sources of energy, as well as those which 

 can utilize nitrogenous organic compounds. If we prepare 

 nutrient synthetic solutions containing one of the simplest ni- 

 trogen and one of the simplest carbon compounds there will 

 grow in that medium only those organisms which can utilize 

 both the nitrogen and carbon compounds present. For example, 

 a medium which contains the necessary nutrient mineral salts 

 and ammonia as a source of nitrogen . with a formate as a source 

 of energy will allow the development of only those organisms 

 which can assimilate ammonia nitrogen and utilize a formate 

 as a source of energy, and for the carbon of its cell construc- 

 tion. If an organism is present which can assimilate ammonia 

 nitrogen but cannot utilize a formate as a source of energy and 

 cell construction it cannot develop. Conversely, if an organism 

 is present which can utilize the formate but not the ammonia 

 nitrogen there will be no growth. Therefore, starting with 

 the simplest nitrogen and carbon compounds we can arrange 

 a series of media having a common source of nitrogen and a 

 changing source of energy, or vice versa, having a common 

 source of energy but a changing source of nitrogen. We have, 

 therefore, the possibiHty of developing as many media as the 

 product of the number of nitrogen sources multiplied by the 

 number of nitrogen free energy compounds. To these may 

 be added a large number of media which may be made from 



