Introduction 5 



structurally and physiologically must be regarded 

 as the simplest organisms of which we have any 

 knowledge. Especially are we indebted to the re- 

 searches of the Russian investigator, Winogradsky, 

 for information regarding these important little or- 

 ganisms. These bacteria possess the remarkable 

 power of assimilating the free nitrogen of the at- 

 mosphere, and some of them, like the higher green 

 plants, can decompose CO 2 ,* and they are thus able 

 to live quite independently of any organic food, a 

 condition of things hitherto supposed to be confined 

 to plants possessing the characteristic green pigment 

 chlorophyll. Unlike the green plants the assimila- 

 tions of CO 2 in these bacteria is not dependent upon 

 light; i.e., it is not a process of "photosynthesis." 

 We have then, in these nitrogen bacteria, organ- 

 isms of the simplest structure so far as we can 

 judge, although their excessively minute size may 

 account for the failure to demonstrate any definite 

 cell structure. In their ability to assimilate such 

 simple and common substances as nitrogen and 

 CO 2 they may very well be assumed to approximate 

 the earliest forms of life that appeared upon the 

 earth. It is reasonable to suppose that these primi- 

 tive organisms, like the nitrogen bacteria, were able 

 to assimilate free nitrogen, CO 2 , and water, which 

 yield the most important elements of the proto- 



* Carbonic acid gas. The reader is urgently invited to 

 fall in with the convention which now accepts this abbrevia- 

 tion quite generally in non-technical writing. 



