236 Plant Physiology 



131. Nitrogen fixation. - - Since by nitrification (in- 

 cluding ammonification) nitrogenous bodies are merely 

 transformed into inorganic nitrogen, this does not increase 

 the total nitrogen of the soil. Moreover, the nitrogen 

 brought to the soil as a result of electrical discharges is a 

 small amount. It is then apparent that the loss of com- 

 bined nitrogen over the surface of the earth through the 

 washing away of sewage, the leaching of soils, and the 

 liberation of free nitrogen in denitrification would mean 

 in time a nitrogen famine. There is, however, a more than 

 compensating process of nitrogen fixation. 



132. Organisms which fix free nitrogen. - - Since the 

 classical researches of Hellriegel and Wilfarth, there has 

 accumulated a vast array of facts and observations with 

 respect to the fixation of free nitrogen by micro-organisms. 1 

 The role played by the bacteria of the leguminous tuber- 

 cles was the first to be clearly demonstrated, but the im- 

 portance of certain saprophytic soil bacteria in the process 

 of nitrogen accumulation was fully recognized a short 

 time later. Strikingly little has been said in agricultural 

 publications regarding the role which may be played by 

 fungi in this process. Nevertheless, as a result of a series 

 of observations and experiments, it is now commonly 

 held that certain fungi are likewise important in fixation, 

 and this view is regarded in the succeeding discussion, 

 although there is some doubt respecting the real impor- 

 tance of the fungi in this connection. 



133. Bacteria of leguminous tubercles. - - In recent 



1 The evidence now commonly accepted is to the effect that certain 

 bacteria and fungi alone among all organisms possess the capacity for 

 nitrogen fixation. 



