Biological Nitrogen Fixation 119 



In leaving this example, once again note that a new 

 method suggested by the advances made in our knowledge 

 of bacterial physiology sooner or later would be applied 

 to the Clostridia. Once this was done, the conditions inevi- 

 tably would be discovered that enabled us to use nature's 

 own method — excretion — for isolating the sought-for in- 

 termediate. The extension to Azotobacter represents only 

 the introduction of a somewhat more subtle technique to 

 exploit the finding further. 



I could cite other case histories from our experience that 

 would illustrate my argument, but I suspect that by this 

 time you are probably recalling some from your own in- 

 vestigations, either in support of or against my thesis. What 

 remains is to draw some suitable conclusion from this sur- 

 vey of the pathways we have used in the past quarter cen- 

 tury. Although we might not agree that these roads have 

 led anywhere in particular, I am sure that none would dis- 

 pute that, at least in the way of complicated apparatus, we 

 have traveled far since Lipman first inserted an unglazed 

 pot inside a glazed one. 



Refe 



fences 



1. Adelberg, E. A. The use of metabolically blocked organisms for 

 the analysis of biosynthetic pathways. Bad. Rev., 17:253-267, 

 1953. 



2. Burk, D., and Burris, R. H. Biochemical nitrogen fixation. Ann. 

 Rev. Biochem., 10:587-618, 1941. 



3. Green, Margaret, and Wilson, P. W. Hydrogenase and nitro- 

 genase in azotobacter. /. Bad., 65:511-517, 1953. 



4. Hamilton, P. B., Magee, W. E., and Mortenson, L. E. Nitrogen 

 fixation by Aerohader aerogenes and cell-free extracts of the 

 Azotobacter vinelandii. Bad. Proc, 82, 1953. 



5. Hyndman, L. A., Burris, R. H., and Wilson, P. W. Properties of 

 hydrogenase from Azotobacter vinelandii. J. Bact., 65:522-531, 

 1953. 



6. Kamen, M. D., and Gest, H. Evidence for a nitrogenase system 

 in the photosynthetic bacterium Rhodospirillum rubrum. Science, 

 109:560, 1949. 



