Hoagland — 68 — Plant Nutrition 



Contemplation of the results of the investigations 

 as a whole does not enable us to propose any general 

 solution of the mechanism of salt accumulation by 

 living cells ; nevertheless we do perceive more clearly 

 than before that ideas concerning permeability do not 

 in themselves suflEice. In fact, in metabolizing cells, 

 permeability and active transport of salt are so inter- 

 related that a separate treatment of the two concepts 

 is exceedingly difficult. Tentatively, a general guiding 

 view might be proposed that seems to be consistent 

 with the data I have cited. This is that vacuolar 

 membranes are relatively impermeable so long as the 

 protoplasmic organization is not too greatly impaired. 

 Salts can, nevertheless, be pumped into the vacuole 

 with the use of metabolic energy, in the utilization 

 of which an aerobic respiratory system is involved. 

 The mechanism of the pump is not revealed, but there 

 occurs apparently some preliminary combination of 

 protoplasmic constituents with the solute and it is 

 almost impossible to avoid some concept of ion ex- 

 change as part of the general process of salt accumula- 

 tion. As regards the application of energy to the 

 salt accumulation process this remains an unsolved 

 problem. All explanations become more difficult when 

 we consider the upward movement of the salt absorbed 

 by the roots. We shall find that ions can be accumu- 

 lated by root cells and subsequently they can leave 

 these cells to move upward and be accumulated by 

 cells in the upper part of the plant. A polarized move- 

 ment of salt is implied. 



General Remarks : — My discussion so far has been 

 rather narrowly limited to the accumulation of salt. 

 It should not be assumed that living cells cannot also 

 move organic substances inward or outward against 

 gradients with the utilization of metabolic energy, for 

 there is reason to believe that this does at times occur. 

 Indeed the accumulation of salt by plant cells seems 

 to be but one aspect of a general attribute of living 



