240 JOSEPH HENRY GILBERT 



in practice leguminous crops assimilate from some source 

 so very much more nitrogen than graminaceous ones under 

 ostensibly equal circumstances of supply of combined nitrogen, 

 it is desirable that the evidence of further experiments with 

 these plants under conditions of more healthy growth should be 

 obtained." 



As long as Gilbert's investigations were confined to non- 

 leguminous plants and to leguminous plants grown in calcined 

 soil the "nitrogen theory" was triumphant. When, however, 

 leguminous plants were grown in uncalcined soil or in the open 

 the results were uncertain, and in many cases the manures 

 supplying ash constituents alone proved the most effective. The 

 elucidation of these uncertain results has been a tedious problem, 

 and has taken many years of patient investigation, but gradually 

 the evidence accumulated which led to its solution. 



Field and pot experiments in Germany, France, England 

 and the United States in the late seventies and early eighties 

 furnished abundant proof that under certain conditions legu- 

 minous plants do obtain nitrogen from the atmosphere, and 

 gradually, from the work of Rautenberg, Frank and others, the 

 idea was evolving that fungi or micro-organisms play some 

 important part in the process. 



Gilbert, however, would not listen to any such heresy, as 

 he considered that the question of the assimilation of the free 

 nitrogen of the air by plants had been finally settled by the 

 experiments of 1857-60. It was therefore a most happy chance 

 that Gilbert was present at the scientific congress in Berlin in 

 1886 when Hellriegel described his experiments on leguminous 

 plants, showing that the formation of nodules on these plants 

 was associated with the fixation of atmospheric nitrogen. In 

 commenting subsequently on these experiments, Gilbert said, 

 "It must be admitted that Hellriegel's results, taken together 

 with those of Berthelot and others, do suggest the possibility 

 that, although the higher plants may not possess the power of 

 directly fixing the free nitrogen of the air, lower organisms, 

 which abound within the soil, may have that power, and may 

 thus bring free nitrogen into a state of combination within the 

 soil in which it is available to the higher plants at any rate to 



