412 ABSORPTION OF MINERAL SALTS 



are accompanied by anions of the same kind as they cross the plasma mem- 

 branes, or vice versa, if the interchanging ions are anions. 



Such ionic exchanges can only take place if the cells already contain dis- 

 solved electrolytes which have been obtained by primary absorption or by trans- 

 location into them from other cells. 



The intake of cations or anions by plant cells may sometimes involve an 

 exchange of ions between water and an electrolyte in the external solution 

 before inward diffusion occurs. The ions which pass into the cell may be 

 accompanied by hydrogen ions (if anions) or by hydroxyl ions (if cations) 

 of equal electrostatic value, leaving their erstwhile partners compensating hy- 

 drogen or hydroxyl ions. For example, the K+ ion of KCl might diffuse 

 into a cell unaccompanied by a companion Cl~ ion, provided it were attended 

 in its passage by an OH"~ ion. The deserted Cl~ ion would then pair off 

 with the original escort of the OH~ ion, a H+ ion originating from the 

 dissociation of a water molecule. Such a mechanism would result in an 

 increase in the hydrogen ion concentration of the solution. Similarly pene- 

 tration of anions accompanied by hydrogen ions into plant cells would result 

 in a decrease in the hj^drogen ion concentration of the external solution. Such 

 changes in the pH of the circumambient solution have sometimes been observed 

 upon the entrance of cations or anions into plant cells. Such a mechanism 

 will work whether the anions and cations are visualized as crossing the 

 cytoplasmic membranes as molecules or as closely associated ions. 



Since water almost invariably contains dissolved carbonic acid it is also 

 probable that the H+ and HCOs" ions of this compound may act in a man- 

 ner analogous to the H+ and OH~ ions of water in facilitating the entry 

 of anions and cations respectively into plant cells. 



Absorption of Mineral Salts by Aerial Organs. — In nature absorption 

 of mineral salts through the aerial organs of the plant rarely occurs. Plants 

 are sometimes "fertilized," however, by direct treatment of aerial organs, 

 which involves the absorption of the added compounds through the leaves or 

 stems. For example, such a practice is followed in treating iron-deficient 

 pineapple plants in Hawaii. The plants are sprayed with a dilute solution 

 of ferrous sulfate, enough of which is absorbed directly through the leaf 

 surfaces to remedy the chlorotic condition which has developed as a result of 

 iron deficiency. Similarly manganese deficiency has been corrected in some 

 species by spraying the plants affected with a dilute solution of a manganese 

 salt. 



