240 D. R. Hoagland 



differentiated from accumulation of nitrate ions in the cell sap) 

 may be less affected than the removal of other ions, the reason ap- 

 parently being that the absorption of nitrate accompanied by re- 

 duction does not depend upon an oxygen supply in the same way 

 that the accumulation of nitrate and other ions does. Accord- 

 ingly, the extent of the selective removal of nitrate may be a 

 function, not merely of the character of this ion, and of the asso- 

 ciated basic ions, but also of the kind of metabolism occurring 

 in root cells.* Furthermore, the experiments of Lundegardh and 

 Burstrom,' and of investigators in California, show that respira- 

 tion may be accelerated in the process of absorption of certain 

 ions; particularly the nitrate ion, as results obtained in this labo- 

 ratory make plain. This again illustrates the complexity of the 

 reciprocal relations existing between the soil and the plant. 



In the accumulation of mineral elements by root cells, inter- 

 ionic relations play an important role. It has been more than 

 sufficiently shown that these relations are not limited to strong 

 solutions, nor to interrelations of cations. The explanation of 

 the results cannot be given satisfactorily in terms of a simple 

 physicochemical system, for the reason that we are dealing with 

 a dynamic system in which the nature of many of the metabolic 

 reactions is still unknown. Interionic effects, nutrient deficien- 

 cies, and the relative rates of absorption of ions of varied char- 

 acter should be reinvestigated with reference to oxygen and 

 available carbohydrate supply to the root system, temperature of 

 soil, and related factors. Such an investigation must form a part 

 of any comprehensive plan of attack on the nature of soil-plant 

 interrelations. 



* The activities of microorganisms living on root tissues require additional 

 study, but at present the metabolism of the root cells seems to be primarily 

 concerned. 



