HISTORICAL AND INTRODUCTORY 25 



1*242 and '789 grm. per pot respectively. They drew two 

 conclusions: (i) the peas took their nitrogen from the air; 

 (2) the process of nitrogen assimilation was conditioned by 

 some factor that did not come into their experiment except 

 by chance. In trying to frame an explanation they connected 

 two facts that were already known. Berthelot (26) had made 

 experiments to show that certain micro-organisms in the soil 

 can assimilate gaseous nitrogen. It was known to botanists 

 that the nodules on the roots of leguminosae contained bacteria. 1 

 Hellriegel and Wilfarth, therefore, supposed that the bacteria 

 in the nodules assimilated gaseous nitrogen, and then handed 

 on some of the resulting nitrogenous compounds to the plant. 

 This hypothesis was shown to be well founded by the follow- 

 ing facts : 



1. In absence of nitrates peas made only small growth 

 and developed 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 II. 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 organ- 

 isms. 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 1886. Gilbert was present at the meeting, and on 

 returning to Rothamsted repeated and confirmed the experi- 

 ments (165). At a later date Schloesing fils and Laurent 



J This had been demonstrated by Lachmann in 1858 (158) (p. 21) and by 

 Woronin in 1866 (322). Eriksson in 1874 (Doctor's dissertation, abs. in Botan. 

 Ztg., 1874, 32, 381-384) carried on the investigation, while Brunchorst in 1885 

 Ber. d. Deutsch. Bot. Ges. iii., 241-257, gave the name " bacteroids". 



