HISTORICAL AND INTRODUCTORY 



17 



roots of leguijiinosae contained bacteria.^ Hellriegel and Wilfarth, 

 therefore, supposed that the bacteria in the nodules assimilated gas- 

 eous nitrogen, and then handed on some of the resulting nitrogenous 

 compounds to the plant. This hypothesis was shown to be well 

 founded by the following facts : — 



1. In absence of nitrates peas made only small growth and de- 

 veloped no nodules in sterilised sand ; when calcium nitrate was 

 added they behaved like oats and barley, giving regular increases in 

 crop for each increment of nitrates (the discordant results of Table 2 

 were obtained on unsterilised sand). 



2. They grew well and developed nodules in sterilised sand watered 

 with an extract of arable soil. 



3. They sometimes did well and sometimes failed when grown 

 without soil extract and without nitrate in unsterilised sand, which 

 might or might not contain the necessary organisms. An extract that 

 worked well for peas might be without effect on lupins or serradella. 

 In other words, the organism is specific. 



Hellriegel and Wilfarth read their paper and exhibited some of 

 their plants at the Naturforscher-Versammlung at Berlin in i886. 

 Gilbert was present at the meeting, and on returning to Rothamsted 

 repeated and confirmed the experiments (165). At a later date 

 Schloesing fils and Laurent (247) showed that the weight of nitrogen 

 absorbed from the air was approximately equal to the gain by the 

 plant and the soil, and thus finally clinched the evidence : — 



The organism was isolated by Beijerinck (p. 95) and called Bacterium 

 radicicola. 



Thus another great controversy came to an end, and the discrep- 

 ancy between the field trials and the laboratory experiments of Lawes, 

 Gilbert and Pugh was cleared up. The laboratory experiments gave 

 the correct conclusion that leguminous plants, like non-leguminous 

 plants, have themselves no power of assimilating gaseous nitrogen ; this 

 power belongs to the bacteria associated with them. But so carefully 

 was all organic matter removed from the soil, the apparatus, and the 

 air in endeavouring to exclude all trace of ammonia, that there was no 



1 This had been demonstrated by Woronin in 1866 (322). Eriksson in 1874 (95) made 

 an admirable investigation, while Brunchorst in 1885 (64) gave the name " bacteroid ". 

 Swellings are sometimes formed containing no bacteroids : these do not bring about 

 nitrogen fixation. 



