ENTRY INTO THE BIOSPHERE 



A. Fixation in the Root Nodules of Legumes 



363 



It is to Boussingault that we must give credit for having in 1838 estab- 

 lished that Legumes fix the nitrogen of the air. After a period of great 

 confusion, in 1885 Boussingault's views were confirmed by Atwater in 

 America. Towards the middle of the nineteenth century a relationship was 

 noticed between this phenomenon and the presence in legumes of the root 

 nodules which had first been described by Malpighi in 1679. The presence 

 in the nodules of rod-shaped bacteria associated into filaments had already 

 been described in 1858, but their presence was incorrectly interpreted by 

 the morphologists for over thirty years. It was not until 1888 that 

 Beijerinck isolated these bacteria in a pure culture. The parasitic bacteria 

 on the roots of legumes are very similar and they are collected together in 



Fig. 98 (Guilliermond and Mangenot) — Contamination of cells in the region of a root 

 nodule, by branches of the bacterial filament (c.b.). 



Notice the bacteria contained in the mucilaginous mass of the filament whose branches 

 insure the infestation of each cell; N and n, nucleus and nucleolus of the infected cells. 



he genus Rhizohium. These bacteria do not contain cellulase and they 

 apparently penetrate the plant at the extreme point of the root hairs which 

 seem to be devoid of cellulose. 



Neither the bacteria nor the plant, considered alone, fix nitrogen. In 

 association, not only do they fix nitrogen but they excrete nitrogen com- 

 pounds into the soil: aspartic acid, glutamic acid, oximino-succinic acid, etc. 



These lead us to postulate the following mechanism: 



Na <- NH2OH 



+ 

 CO— COOH 



I 



CH,— COOH 



H2O 

 C = NOH— COOH CHNH2— COOH absorption 

 I "^ I "*■ "ito 



CH2COOH 4(H)CH2— COOH the plant 



Oxaloacetic acid oximino-succinic acid aspartic acid 



There is no accumulation of hydroxylamine, so that it is probably 

 rapidly reduced to ammonia when reacting with the ketonic acid. Alterna- 

 tively, as believed by some workers, ammonia may be the first compound 



