A CRITICAL EVALUATION OF THE TERMINOLOGY OF DIFFUSION PHENOMENA.

Diffusion phenomena are involved, directly or indirectly, in practically all fundamental plant processes, playing particularly important r?les 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 diffusion 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, ani 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 mechanism 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. The fact that the terminology of diffusion phenomena is far from being stabilized is the principal justification for the writing of this paper. Physi1 A report of the Physical Methods Committee of the American Society of Plant Physiologists. The membership of the committee is as follows: S. ?. Dexter, K. C. Hamner, B. 8. Meyer, Gordon Marsh, and R. B. Withrow, Chairman. This manuscript has been critically read by all members of this Committee and also by Dr. O. F. Curtis of Cornell University, Dr. C. A. Shull of the University of Chicago, Dr. H. C. Sampson of the Ohio State University, Dr. D. B. Anderson of North Carolina State College, Dr. A. S. Crafts of the University of California, Dr. P. J. Kramer of Duke University, Dr. R. O. Freeland of Northwestern University, Dr. ?. ?. Scofield of the Ohio State University, and Dr. C. A. Swanson of the Ohio State University. All of these persons have made valuable suggestions and criticisms which are hereby gratefully acknowledged. The author alone, however, is responsible for the final form in which all statements are made.