Excretion of Nitrogen by Legumes 



225 



legumes can excrete nitrogen from the nodule is substantiated by 

 the following evidence: 



1. The quantitN' of nitrogen excreted is too large to be explained 

 by the sloughed-off nodules and portions of the roots. Frequently 

 50 per cent or more of the total nitrogen fixed is excreted, more 

 nitrogen than is usually present in tlu^ entire root system. 



Fig. 92. Influence ot a legume (peas) on growth of a nonlegume (oats). Oats 

 in inner pot, peas in outer. Porous inner pot on left, glazed inner pot on right 



( from Lipman ) . 



2. Virtanen grew the plants under bacteriologically controlled 

 conditions. Excretion took place, thus proving that the origin of 

 the nitrogen is the legume bacteria and not other soil microorganisms. 



3. Nitrogen compounds were excreted even in the absence of non- 

 legumes. They were identified as comprising largely aspartic acid I ^ 

 and beta-alanine. The formation of the beta-alanine was ascribed l 



to decarboxylation of the excreted aspartic acid by the root-nodule 

 organisms. If the nitrogen originated from sloughed-off portions of 

 the plant, the presence of more than one amino acid would be 

 expected. 



