32 FERTILISERS CONTAINING NITROGEN [chap. 



probably been operative since the beginning of life on 

 the earth, and this power is the property of certain 

 groups of bacteria only. The history of nitrogen-fixing 

 bacteria began some thirty years ago with the resolu- 

 tion by Hellriegel and Wilfarth of the great outstanding 

 difficulty in the theory that plants only make use of 

 combined nitrogen. Though the demonstration in the 

 laboratory of this opinion seemed perfect, and though 

 in the main it was corroborated by field experiments, 

 there was one group of plants — peas, beans, clover, and 

 their allies — which seemed to derive little or no benefit 

 from nitrogenous fertilisers, and yet actually left the 

 land richer in nitrogen after their growth, although in 

 the crop removed there was an exceptional amount of 

 nitrogen. That beans or vetches or lupins were the 

 best preparation for a wheat crop was a commonplace 

 of Roman agriculture, and the same observation became 

 afterwards enshrined in that most fundamental of 

 rotations, the Norfolk four-course system, in which 

 wheat follows clover or beans. Hellriegel and Wilfarth 

 found that leguminous plants did gather nitrogen from 

 the atmosphere, and could, therefore, become wholly 

 independent of nitrogenous manures ; but this only 

 took place when, by infection from the soil, certain 

 characteristic nodules were formed upon the roots. 

 These nodules were found to be colonies of a particular 

 bacterium which seems to live symbiotically on the host 

 plant, furnishing it with nitrogenous matter and deriving 

 from it the carbohydate required for the fixation of 

 nitrogen. As the fixation of nitrogen is a chemical 

 process analogous to going uphill, it requires a supply 

 of energy from outside, which external source of energy 

 the bacteria obtain by the oxidation of carbohydrate 

 in some form or other. The particular bacterium living 

 in symbiosis with the leguminous plants is highly 



