ASSIMILATION OF NITROGEN BY PLANTS 



231 



yearly, which is not sufficient to restore to the soil the quantity 

 of nitrogen, about 50 kg. per hectare, that is removed in the 

 harvesting of crops. In order to enrich the soil with nitrogen, 

 it is necessary, therefore, either to introduce nitrogenous fer- 

 tihzers or to resort to the cultivation of leguminous plants. 



53. Assimilation of Molecular Nitrogen by Leguminous 

 Plants. — Agriculturists have noted for a long time that the 

 growing of legumes, particularly of clover, makes the soil more 

 fertile and subsequently increases the yield of cereals. It has 



Fig. 68. — Nodules containing nitrogen-fixing bacteria on the roots of leguminous 

 plants: A, red clover; B, sweet pea; C, soybean {after Transeau). 



been estabhshed also that this augmented crop production results 

 from an increased nitrogen content of the soil. Naturally, then, 

 leguminous plants acquired the name of nitrogen fixers. These 

 observations, however, proved ta> be contradictory to Bous- 

 singault's careful experiments, which showed that under con- 

 ditions of artificial culture no higher plants, legumes included, 

 were able to assimilate free nitrogen of the atmosphere and that 

 their yield was wholly determined by the amount of combined 

 nitrogen present in the nutritive solution. Hellriegel (1886j 

 succeeded in solving this apparent contradiction. His attention 

 was drawn to the fact that in soil certain nodules, which were 

 absent in water and sand cultures, were formed on the roots of 

 leguminous plants (Fig. 68). And as these nodules were found 



