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 240 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 leguminous 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