456 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 64 



oxidized state, and for an unidentified iron component. This iron 

 factor is distinct from ferredoxin, mentioned earlier; in fact, ferre- 

 doxin does not produce EPE signals and is not present in Azotohacter. 

 It was possible to reduce this iron and molybdenum by hydrogen and 

 culture media factors and they were reoxidized by nitrogen (fig. 4). 

 Comparisons with similar effects in the oxygen-fixing respiratory 

 mechanisms suggest that nitrogen and oxygen are alternative targets 

 for chemical reducing power that is transferred successively from 

 hydrogen to iron to flavin and then via cytochrome to capture oxygen 

 or molybdenum to capture nitrogen. Support for the view that mo- 

 lybdenum is a key link in the chain comes from experiments in which 

 particles from cells that do not fix nitrogen proved to contain very 

 little molybdenum. 



Much work has been done on the mechanism of root nodule forma- 

 tion in clover, and similar studies have also been made in other types 

 of flowering plants. It is clear that the host plant derives the advan- 

 tage of the readymade products of nitrogen fixation and the micro- 

 organisms in turn get a supply of food that the plant makes during 

 photosynthesis, as well as a favorable environment for nitrogen 

 fixation. 



Bergersen and his associates in Canberra have shown that the 

 rhizobia bacteria in the nodules of clover are enclosed in a double 

 layered membrane envelope. The bacteria are devoid of cell walls 

 and are termed bacteroids (fig. 5). They are bathed in a solution of 

 hemoglobin, which has a high affinity for oxygen, and this may provide 



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Figure 5. — Rods and bacteroids from root nodules. A, rods from a white ineffective 

 nodule. B, bacteroids from a red ineffective nodule. C, rods from a green ineffective 

 nodule. D, various forms of bacteroids. (After Virtanen.) 



