On the Sources of the Nitrogen of Vegetation. 



113 



interest as confirming the supposition that the insoluble nitrogenous 

 compounds in soils are amide bodies, and as indicating the changes to' 

 which they are subject when acted upon by acids. Supposing the 

 acid root-sap so to act on the insoluble organic nitrogen of the soil, 

 and especially of the subsoil, as already said, the question still remains, 

 whether the amide rendered soluble is taken up as such, or undergoes 

 further change before serving as food for the plant? It is seen that 

 ammonia is an essential result of the reaction ; and as, so far as our 

 experiments go, nitric acid seems to be a more prominent constituent 

 of the root-sap than ammonia, the question arises whether the liberated 

 ammonia is not oxidated into nitric acid before being taken up ? 

 Then, again, is the soluble amide subjected to further change — perhaps 

 first yielding ammonia, and this again nitric acid ? On this supposi- 

 tion we are again met with the difficulty as to the sufficient aeration 

 of the subsoil. 



Supposing any considerable amount of the amide rendered soluble 

 may be taken up by the plant as such, it is obviously of interest to 

 consider what is the evidence bearing on the question whether plants 

 can take up such bodies and assimilate their nitrogen ? The condi- 

 tions of experiment and the results obtained by various experimenters, 

 have therefore been considered. The substances which have been 

 experimented upon are — urea, uric acid, hippuric acid, guanine, phos- 

 phate of ammonia, glycocoll, creatine, and tyrosin. In some cases the 

 experiments have been made in soil, but in most by the water-culture 

 method. 



In the majority of cases there could be little doubt that the complex 

 nitrogenous body contributed nitrogen to the plant, either directly or 

 indirectly. In the case of the experiments with soil as a matrix, there 

 was no direct evidence that the plant took up the complex organic 

 body, as such; and the probability is that it suffered change before 

 becoming available. In some of the water-culture experiments, 

 especially when urea was used, that substance was found within the 

 plant, and it was concluded that it contributed directly as a source of 

 nitrogen to it. Hampe also concluded that glycocoll was as available 

 as nitric acid as nitrogenous food to plants. 



Upon the whole it seems probable, that green-leaved plants can take 

 up soluble complex nitrogeuous organic bodies, when these are pre- 

 sented to them under such conditions as in water-culture experiments, 

 and that they can transform them, and appropriate their nitrogen. 

 If this be the case, it would seem not improbable that they could take 

 up directly, and utilise, amide bodies rendered soluble within the soil 

 by the action of their acid root-sap. 



In connexion with the subject of the conditions under which the 

 insoluble organic nitrogen of soils and subsoils may become available 

 to chlorophyllous plants, some results of Frank may be briefly con- 



