200 Mineral Nutrition of Plants 



early by Donnan that under certain circumstances a modified equili- 

 brium of diffusion may obtain. This phenomenon occurs where one of 

 the ion pairs of a compound in solution does not readily penetrate a 

 differentially permeable membrane interposed between two aqueous 

 solutions. The equilibrium conditions imposed by this phenomenon are 

 such that, for monovalent ions for example, the product of the molar 

 concentrations of the cation (C+) and anion (A~ ) pairs in the one 

 medium equal the same product in the solution on the opposing side 

 of the limiting surface, i.e., (C+) i X (A~) i = (C + )e X (^ _ )e- Ex- 

 pressed otherwise, at equilibrium the ratio of the concentrations of the 

 diffusible cations in the two media should equal the inverse ratio of the 

 diffusible anions, i.e., (C+)j/(C+) e = (A~) v /(A~) i . Studies in vitro 

 have shown this phenomenon to be qualitatively valid; however, in 

 nature the ratios required at equilibrium are not usually observable. 

 The latter observations indicate that although generally applicable, ionic 

 equilibria of this sort are not universally attained in systems with living 

 cells. Compare the relevant inverse ratios of cation to cation and anion 

 to anion, particularly for K+, Na+, and Cl~, for Halicystis, Nitella, 

 and Chara in Table I. One must infer, therefore, that the Donnan 

 equilibrium as originally proposed is not to be regarded as of primary 

 importance between at least some living cells and their natural environ- 

 ment (//, 54). 



Exchange adsorption 



With living tissues, we are dealing with colloidal systems, and the 

 phenomenon of exchange adsorption can play a role in cellular physi- 

 ology of inorganic solute absorption. Ions are adsorbed on the surfaces 

 of charged colloids. These may be exchanged for ions with the same sign 

 from the dispersion medium. This mode of ionic migration has been 

 extended recently to include an exchange of ions between colloidal 

 micelles where interpenetrating spheres of ion oscillation exist. Thus, 

 also, the movement of ions along surfaces, without intermediate passage 

 into the dispersion medium, may be effected. These exchange phe- 

 nomena are schematically represented in Figure 7, from Jenny and 

 Overstreet (j6, ^7). Such movements are discussed in detail elsewhere 

 in this book. 



