234 ^' ^' Hoagland 



derived from the assumption that hydrogen ion concentration is 

 mainly operative through some relatively simple physicochem- 

 ical system which may be explained by reference to Donnan equi- 

 libria, protein isoelectric points, hydrogen ion gradients, and 

 other factors of the kind. 



Transpiration and Absorption of Mineral Solutes 



The relation of the absorption and transpiration of water by 

 plants to the absorption of mineral elements, or to their move- 

 ment within the plant, has been discussed by plant physiologists 

 decade after decade. Yet the most recendy published articles re- 

 flect a striking diversity of opinion with respect to the function 

 of transpiration in the economy of the plant — whether it is a 

 necessary evil or is beneficial. There remains great uncertainty 

 concerning many features of the movement of water and solutes 

 within the plant's conducting systems, but several aspects of the 

 absorption of mineral elements in relation to absorption and 

 transpiration of water can be clarified on the basis of simple 

 and convincing experiments. 



Metabolically active root cells, when in a low salt condition, 

 have the capacity to accumulate mineral solutes with great ra- 

 pidity from a dilute solution, even when the root is severed from 

 the shoot, as has already been demonstrated. Under suitable con- 

 ditions and over a brief interval of time, almost as great a quan- 

 tity of certain mineral solutes can be absorbed from a culture 

 solution by the excised roots alone as by the whole plant. Trans- 

 piration is therefore not involved in the first step of the process 

 by which plants remove solutes from the soil. 



The plant as a whole being considered, root cells, when in a 

 state of high metabolic activity and not already saturated with 

 mineral solutes, may absorb some ions from the culture solution 



