SYMBIOTIC NITROGEN FIXATION 179 



nitrogen-fixation, but the process is essentially a symbiotic 

 one, in that neither the plant alone nor the free-living 

 Rhizobium can fix nitrogen. The plant, as shown above, can 

 grow in sterile soil as long as it is provided with a source of 

 fixed-nitrogen, but it cannot utilise atmospheric nitrogen 

 under these conditions. Rhizobium can lead a free-living 

 existence and can grow normally in a medium containing 

 fixed-nitrogen, but it cannot fix atmospheric nitrogen when 

 growing apart from a host-plant. There is an important 

 specificity between the particular Rhizobium and the plant 

 with which it can enter into a symbiotic relation, thus Rh. 

 trifolium can form nodules only on the roots of clover, and 

 can fix nitrogen only in symbiosis with that plant, while 

 Rh. leguminosarum can form nodules only on the roots of 

 the pea. Further than this, some strains of Rh. trifolium, 

 for instance, are better nitrogen-fixers in symbiosis with clover 

 than are others, and it often pays a farmer nowadays to obtain 

 a suitable strain with which to inoculate his soil before sowing 

 this type of crop. 



The chemistry of the symbiotic nitrogen-fixation process 

 has been the subject of much work in Helsinki University and 

 also in American laboratories. Prof. Virtanen of Helsinki 

 has published a series of papers in which he claims to have 

 elucidated the chemical processes involved but, unfortunately, 

 attempts to confirm this work in other laboratories have not, 

 so far, met with success. The scheme put forward by Virtanen 

 was as follows: — 



Nitrogen is fixed by Rhizobia with the formation, after 

 unknown intermediate stages, of hydroxylamine. At the 

 same time carbohydrate is broken down within the host- 

 plant with the formation of oxalacetic acid. Hydroxylamine 

 and oxalacetic acid combine spontaneously to form the 

 corresponding oximo-succinic acid which is then reduced by 

 the organism to aspartic acid. The aspartic acid, in turn, 

 acts as a source of other amino -acids in the plant through 

 transamination. 



