On the application of the Donnan equilibrium to the ionic relations of plant tissues

Abstract

The conception of a simple Donnan membrane equilibrium operating between 2 homogeneous phases is inadequate to explain the phenomena of ionic intake by plants. Experimental evidence (carrot tissue at various temp.) indicates that the product of the apparent internal concentrations of cation and anion, estimated on basis of total volume of tissue, may be greater than that of the concentration of the 2 ions in the external medium, instead of equal to it, as the above conception would require. The interior of the tissue comprises a number of phases, each of which may be in Donnan equilibrium with the external solution. The product of the apparent internal ionic concentrations resulting from the total effect of all these internal phases under such circumstances is greater than the external product. Even in such a system as this, however, if the ra''tio of the apparent internal to the external concentration of a cation is greater than unity, the same ratio of the corresponding anion should be less than unity; whereas the experimental evidence indicates that both may at times be greater than unity. This may be accounted for if we suppose that one internal phase contains indiffusible cations and that another contains indiffusible anions. The effect of temp. on the relative uptake of the cation and anion of a salt can be interpreted in terms of its possible effect on dissociation of the tissue colloids.