270 Mineral Nutrition of Plants 



tively high concentrations of Fe 55 may build up in the veins of the 

 leaves. We are unable to determine whether the iron accumulates be- 

 cause of failure of the mesophyll cells to accept it as rapidly as the root 

 cells accepted it or whether a preponderance of anions of one or more 

 species resulted in precipitation. We suspect the latter and offer as 

 partial proof the pattern of distribution of P 3 " in the leaf. It also is 

 present in greatest concentrations along the veins (Figure 6). The 

 presence in the veins of a precipitate such as ferric phosphate might be 

 explained on the basis of more rapid withdrawal of water than of 

 minerals, resulting in a concentration beyond the solubility limits of the 

 least soluble salt (ferric phosphate at low pH's or a similar complex 

 with calcium at pH 6.0 or above). The role of hydrogen ions in this 

 phenomenon seems important, but, in considering them, we must re- 

 member the results of Arnon and Johnson (/) who have shown that 

 the pH of "expressed sap" of leaves is not altered by the pH of the 

 nutrient solution in which the plants grew. From these results it 

 would seem, then, that the plasma membranes of the root cells are at 

 least a partially effective screen against the ready entrance of hydrogen 

 and hydroxyl ions. In the nutrient solution these ions play an important 

 role in determining solubilities. They would play a similar role in the 

 vein extremities if they reached these areas. We find evidence of preci- 

 pitates on root surfaces and in vein extremities under particular growth 

 conditions. These precipitates are essentially absent in both areas at pH 

 4.0 and occur in both areas at pH 7.0. It would appear that some of the 

 hydroxyl ions are penetrating the root membranes and influencing the 

 vein extremities even though they are insufficient to overcome the buffer 

 effect of the cell sap. This is our present view. We see no great dis- 

 similarities between the cluttering up of the root surfaces with insoluble 

 materials and the similar apparent accumulation of insoluble materials 

 in vein extremities. 



RE-EXPORT OF MINERALS FROM LEAVES 



Leaves differ in their basic metabolism according to age. In young 

 leaves the predominant effect is the synthesis of new protoplasm which 

 results in growth toward maturity. In older mature leaves, photosynthe- 

 sis is the dominant function and little growth results. Carbohydrate ex- 



