A Critical Evaluation of the Terminology of Diffusion Phenomena

Abstract

Diffusion phenomena are involved, directly or indirectly; in practically all fundamental plant processes, playing particularly important roles in the movement of substances into and out of plants and in the translocation of substances within the plant. In recent years the results of a number of different lines of investigation, which need not be cited here in detail, have increasingly indicated that energetically much more complex mechanisms than dliffusion are involved in the transfer of at least certain kinds of solute particles, and perhaps also water molecules, from the environment into the plant, from the plant into its environment, and from cell to cell within the plant. In spite of such more complex mechanisms which are superimposed on the kinetically and energetically simpler process of diffusion, this latter process does account principally or entirely for certain of the movements of materials which occur within a plant or between a plant and its environment. Furthermore, diffusion almost invariably accompanies, as a subsidiary or underlying mechanism, those processes in which the predominant niiechanism is thermodynamically more complex.The now generally recognized fact that diffusion is only the simplest of the mechanisms which play a part in the movement of substances into, out of, and within plants should not disincline us from attaining as clear an insight as possible into the dynamics of this process. A lucid picture of the kinetics of diffusion phenomena as they occur in relation to plants is necessary as a foundation for an understanding of other more complicated processes involved in the movement of materials in plants. In this paper little attempt is made to explain or conceptualize even the relatively simple phenomena of diffusion, but only to evaluate the terminology used in descriptions of such phenomena, and to make certain recommendations regarding preferences in such terminology. Any clarification or simplification in terminology which can be accomplished should, however, make possible a more effectual presentation of the concepts of diffusion.

On the Theory of Osmotic Water Movement

“…. is the amount by which its diffusion pressure is less than that of pure water at the same temperature and under atmospheric pressure" (34).…”

The Osmotic Cell, Solute Diffusibility, and the Plant Water Economy.

“…Now Pd = 0 at t = 0, so Pd is given by the particular integral of equation 13, Pd=Pl-[ e P(t)o (14) Usingf equation 14 -(e + PO)v] (15) which is subject to the initial condition t = 0,v = P,/(E+PO) (16) Numerical solutions of equation 15 Let us interrupt the previous cycle of operations on the cell at the point where it has reached equilibrium with free water (C of fig 3). We now suppose that a diffusible solute is introduced continuously into the water, so that, at any time, t, the osmotic pressure of the external solution is at.…”

Further Theoretical Considerations of Modes of Expression and Factors Possibly Concerned in the Movement of Materials Through a Two-Phased Solution System

“…Discussion of modes of expression In earlier articles, various terminologies have been used as means of expressing the factors concerned in solute and water movement (2,3,4,5,6,10,14,16,20,24). References 2, 3, 4, and 5 should be consulted in particular.…”

“…As with specific free energies, there may appear to be certain advantages or disadvantages to the use of the terms diffusion pressure (5,26) or diffusion pressure deficit (20) in the sense of FICK (12). Flux of materials in all natural systems, involves diffusion in part, this tendency being a func-tion of the relative concentrations of the constituent whose migration is in question.…”

“…Where based on the usual definition of OP, the terms are useful for discussion of the tendency for movement of the solvent component only. The (20). One circumstance which might be considered in favor of certain older usages, e.g., suction force or pressure or tension, waterabsorbing capacity; or osmotic pressure, osmotic value, et cetera, is historic precedence.…”