180 THE NITROGEN CYCLE 



N, > ? > NHoOH 



Rliizohium \ 



\ — ^ HOOC.CHo.C (NOH).COOH 



/ I Ehiznhiii.m 



c^Hj^Oe — > H00C.CH2.C0.C00H ; 



^""'* HOOC.CHo.CHNHs.COOH 



Rhizolnuyn ^--^^ Plant 



HOOC.CH2.CH2NH2 ^^ R.CHNH2.COOH 



The evidence put forward by Virtanen in support of this 

 scheme includes the following points: — 



1. The demonstration that aspartic acid and its decarboxyla- 

 tion product, jS-alanine, are excreted into the soil around the 

 roots of young pea plants, and that this excretion occurs 

 only from the nodules. 



2. The demonstration of oxalacetic acid in the sap of the 

 host-plant. * 



3. The isolation of oximo-succinic acid from the soil around 

 the nodules. 



4. The demonstration that Rhizobia can grow on a nitrogen- 

 free medium provided that oxalacetate is added to the medium 

 — when growth occurs with nitrogen-fixation. 



5. That transamination between aspartic acid and pyruvic 

 acid will take place in the presence of extract of pea with 

 the formation of oxalacetic acid and alanine. 



COOH CCOH 



CH2 ^113 01x2 ^^z 



I + I > I + I 



CHNH2 CO CO CHNH2 



II II 



COOH COOH COOH COOH 



This evidence is quite formidable, and would seem to provide 

 satisfactory confirmation of the fixation hypothesis, but 



