IONIC EXCHANGES BETWEEN CELLS 411 



otherwise entirely favorable for the process. For example the parenchyma 

 tissue of apple and pear fruits is fully mature and shows no accumulation, 

 while disks cut from potato tubers and many other similar storage organs are 

 capable of renewed growth and exhibit rapid accumulation under favorable 

 conditions. 71ie most rapid rates of accumulation yet discovered have been 

 found in the apical region of j^oung roots. It is possible that all or most 

 plant cells possess this capacity when in a meristematic condition, but that 

 it is lost by many as they grow older. 



Certain non-electrolytes will also accumulate in plant cells. It has long 

 been known that plant cells will continue to absorb certain dyes (methylene 

 blue, neutral red, etc.) until their vacuolar concentration far exceeds that 

 in the outside medium. It is doubtful, however, whether the mechanism of 

 dye accumulation is the same as that operating in the case of electrolytes. 

 Evidence is also gradually accumulating that sugars may also move into cer- 

 tain plant cells against a concentration gradient and ultimately it may be 

 found that plant cells can accumulate other kinds of organic compounds. 



Ionic Exchanges between Cells and the External Solution. — Brief 

 reference should be made to another type of phenomenon which often has an 

 influence on the mineral salt composition of the cell sap of plant cells. The 

 accumulation of salts, as previous discussion has shown, seems to occur only 

 in actively metabolizing cells. In this process both anions and cations are 

 absorbed, usually in approximately equivalent quantities. Steward (1935) 

 has suggested that this process be called "primary salt absorption" in order 

 to distinguish it from other processes which also influence the ionic com- 

 position of the cell sap. 



Exchange of ions between the cell sap and a circumambient solution has 

 been observed in a number of different plant tissues. This process appears 

 to be more characteristic of mature cells which have ceased to accumulate 

 salts, although it also occurs in younger, metabolically active cells. The 

 simplest mechanism which can be postulated to account for such ionic ex- 

 changes is that the ion which diffuses into the cell sap is compensated for 

 by diffusion out of the cell by one or more ions of another kind, but of equal 

 electrostatic value. An example of such ionic interchange between cells and 

 an external medium is the previously cited exchange of H+ ions from root 

 cells for other cations in the soil. Such an exchange mechanism will explain 

 the apparent capacity of cells under certain conditions to absorb cations or 

 anions without their satellite ions of opposite charge. Even if it is assumed 

 that the electrolytes cross the plasma membranes in the form of molecules 

 rather than as ions, essentially the same picture of the process is adequate. 

 It is only necessary to assume that both the outgoing and incoming cations 



