FIXATION OF NITROGEN BY BACTERIA 197 



surface of the liquid into the air are they of service to 

 their symbiotic partner. 



The greatest amount of nitrogen assimilation takes 

 place at the time when the formation of bacteroids has 

 reached a maximum, at which stage the plants are just 

 beginning to flower ; during the ripening of the seed the 

 organic nitrogenous compounds are removed from the 

 nodules, and their nitrogen content becomes reduced to 

 near that of the rest of the plant. Stoklasa found that 

 lupin nodules in which the bacteroidal tissue was fully 

 developed contained 5.2 per cent, of nitrogen, most of 

 which was present in the form of proteins : the plants 

 were then in flower. Later, when fruit and seeds 

 were ripe, the nodules contained only 1.7 per cent, an 

 amount about equal to that normally present in the 

 unaltered parts of the roots. Analyses of the nodules of 

 peas and dwarf beans by Frank showed a nitrogen content 

 of about 7 to 7 1 per cent. 



Maze and Golding have shown that the nodule 

 bacteria are able to assimilate the free nitrogen of the 

 air to a limited extent when grown in pure cultures apart 

 from the leguminous plant, and it is possible that under 

 certain favourable conditions, where a suitable supply of 

 carbohydrates or other organic carbon compounds is 

 available, they may be able to do the same during their 

 existence as separate isolated agents in the soil. But 

 very little is known of their life and nutrition in the soil. 

 Nodule-forming bacteria are ubiquitous, and it is ex- 

 ceptional to find soil in which nodules are not formed on 

 one or another species of leguminous plant, even in 

 cases where the land has carried no crop of this class for 

 forty or fifty years. Certain poor soils, however, are met 

 with in which the specialized organisms adapted to lupins, 



