4o8 ABSORPTION OF MINERAL SALTS 



The concentration of every ion analyzed for was greater, and often many 

 times greater, in the cell sap than in the pond water. However, since the 

 cells had been growing continuously in the pond such marked differences in 

 concentration probably represent an equilibrium condition between the cells 

 and the pond water. In other species it has been shown that, under some 

 conditions at least, the NOs" ion may also accumulate within cells in greater 

 concentration than in the outside medium. The total concentration of elec- 

 trolytes in the cell sap of Nitella was about twenty-five times as great as in 

 the pond water which bathed the cells. 



Formerly it was generally assumed that the accumulation of ions within 

 plant cells is possible because they are adsorbed, precipitated, or otherwise 

 rendered kinetically inactive. Evidence from conductivity measurements 

 (Hoagland and Davis, 1923) indicates very strongly that an overwhelm- 

 ingly large proportion (probably at least 90 per cent) of the inorganic ele- 

 ments in the cell sap of Nitella are present in the ionic state and are not tied 

 up in some non-dissociated form. The total concentration of the cations in 

 the sap of this species exceeds the total concentration of anions, although 

 not greatly. Most of the excess cations are probably paired off with the 

 anions of organic acids. 



An increase in the concentration of the free ions in the cell sap to a value 

 many times greater than their concentration in the external medium can only 

 be attained as a result of the diffusion of those species of ions against a 

 concentration gradient, i.e. from a region of the lesser concentration of each 

 individual ion to a region of its greater concentration. A number of other 

 species of submerged aquatic plants such as Valonia, Elodea, Halicystis, and 

 Chara apparently possess a similar capacity of accumulating electrolytes (Os- 

 terhout, 1936). In some of these species, notably Valonia^ it also seems to 

 have been demonstrated beyond question that the ions are present in the cell 

 sap in the free state. 



Accumulation of electrolytes also occurs in some of the tissues of ter- 

 restrial plants. Steward (1932, 1933) has obtained convincing evidence of 

 a correlation between the rate of aerobic respiration (Chap. XXIX) and 

 the accumulation of electrolytes by potato tuber tissue. When thin disks 

 cut from a potato tuber were immersed in dilute solutions of potassium bro- 

 mide it was found that accumulation of K+ and Br~ ions occurred only 

 when the solution is suitably aerated, the two ions often, but not always, 

 being absorbed in approximately equivalent amounts. Within limits absorp- 

 tion of Br~ ion closely paralleled the percentage of oxygen in the air used 

 to aerate the solution (Fig. 95). The absorbed ions were found to reside 

 chiefly in the superficial layers of cells of the disks of potato tuber tissue in- 



