558 University of California Publications in Agricultural Sciences [Vol. 1 



THEORETICAL CONSIDERATIONS 



A few words may not be out of place here with regard to the 

 mechanism of the action of the different salts employed in our 

 experiments, be such action in the direction of stimulation or 

 in that of toxicity. In the first place, the salts in question must 

 exercise some effect on the cell of the root itself and, through 

 it, on the whole plant. If this were not so, we should not obtain 

 the stimulating as well as the toxic effects of a given salt in 

 solution cultures, as well as in soil cultures. In the latter, we 

 do of course obtain more definite evidence of stimulation than 

 in the former, and for that reason we may claim with some 

 justice, as we have above, that stimulation effects are chiefly 

 attributable to some influence, not always the same, induced by 

 the salt on the soil, rather than on the root of the plant. This 

 does not, to be sure, deny the existence of the latter effect in soil 

 cultures and particularly in solution cultures; but when the 

 most marked stimulation occurs, it is rarely noted in the latter. 

 We therefore believe it reasonable to suppose that we are dealing 

 under such circumstances with an effect on the soil, rather than 

 with one on the plant root. What such salt effects on the soil 

 may be like are explained above. It is not easy, however, to 

 explain or even to speculate on an explanation of the effect of 

 a salt directly on the plant root in the direction of stimulation. 

 We have no unexceptionable evidence on the subject of com- 

 pounds of copper, for example, with albuminoid material of 

 living cells, and that increases the difficulty of accounting for 

 observed facts of stimulation. It is nevertheless possible that 

 stimulation of root cells by copper may be due to an effect of 

 the latter in decreasing or increasing the permeability of the cell, 

 or perhaps to the possible small content of iron in the copper 

 compounds employed, the iron acting as one of the essential 

 elements to cell development. Neither of these speculations at 

 present appears to have value other than that of inducing fur- 

 ther thought and discussion on the subject. So far as the toxic 

 effects of salts on plants in solution cultures is concerned, noth- 

 ing need be added here to the excellent discussions already given 

 bj' Czapek and Pfeffer which are cited above, and by Hober.^^^ 



125 Physikalische diemie der Zelle und der Gewebe, Leipzig, 1914. 



