THE ACCUMULATION OF IONS BY PLANT CELLS 407 



that electrolytes pass into cells in the form of ions. Because of electro- 

 static attraction between oppositely charged ions passage of a cation into a 

 plant cell must be accompanied by passage of an anion or anions of equal 

 electrostatic charge, and vice versa, unless the unbalanced electrical forces 

 which would develop as the result of such a situation are compensated for 

 in some other manner. Actually it maii.es very little real difference in our 

 picture of the kinetics of penetration of electrolytes whether we visualize 

 them entering cells as molecules or as closely associated ions. 



The Accumulation of Ions by Plant Cells. — In most elementary dis- 

 cussions of the absorption of mineral salts by plants it has usually been stated 

 or implied that they enter the cells of submerged aquatics or the epidermal 

 cells and root hairs in the absorbing region of roots by a process of simple 

 diffusion from the external inedium. The principal implication of such a 

 statement is that the diffusion of an electrolyte into cells takes place as a re- 

 sult of its greater concentration in the external solution than in the cells. 

 The results of a number of investigations indicate, however, that the absorp- 

 tion of electrolytes by plant cells is, under many conditions at least, a much 

 more complicated process than simple diffusion. 



That factors other than simple diffusion are involved in the absorption 

 of ions is illustrated by the data in Table 40. These data are based on ac- 

 tual chemical analyses of sap expressed from the cells of the alga, Nitella 

 clavata, and of the pond water in which it was growing. As much as a 

 drop of sap can be obtained from a single cell of this species. The samples of 

 sap actually used for analysis generally ranged in volume from 15 to 40 cc. 



TABLE 40 ANALYSIS OF THE SAP OF isUella clavata AND OF THE POND WATER IN WHICH IT WAS 



GROWING (dAT.'V of HOAGLAND AND DAVIS, I929) 



1 A milliequivalent of an ion is one-thousandth its gram ionic weight divided by its valence. 



