TRACE ELEMENTS IN PLANTS 



109 



clearly the effect of calcium in reducing both the intake of man- 

 ganese and its translocation to the leaves, while no such effect 

 is suggested as regards potassium. 



Table X. Effect of varying concentrations of calcium and 

 potassium on the absorption of manganese by tobacco 

 plants in water culture. (From Swanback) 



Zinc. It has already been mentioned (p. 99) that Thatcher 

 thought that zinc and copper were a pair of catalysts concerned 

 together in oxidation-reduction reactions. That zinc, at any 

 rate, is concerned in such reactions is the conclusion reached by 

 Reed and Dufrenoy (1935) mainly as a result of their micro- 

 scopical examination of mottled leaves of Citrus. They conclude 

 that zinc is concerned with the functioning of sulphydryl com- 

 pounds such as cysteine in their regulation of the oxidation- 

 reduction potential within the cells. We have already noted that 

 they found that the stromata of the chloroplasts in the palisade 

 cells of such leaves are often rich in fat, while the vacuoles con- 

 tain phenolic material and phytosterol or lecithin, which are 

 absent from normal leaves. Reed and Dufrenoy interpret these 

 substances as suboxidized products of carbohydrates and 

 proteins, and their presence suggests a disturbance in the 

 oxidation-reduction potential within the leaf cells. 



For the view that the maintenance of the oxidation-reduction 

 potential at its normal level depends on sulphydryl compounds 



