Permeability of Certain Plant Membranes to Water

Abstract

1. Quantitative measurements were made of the permeability to water of certain non-living semipermeable plant membranes under experimentally controlled conditions. 2. The apparatus and method employed had the following advantages over osmometers ordinarily employed: (1) the passage of as small a quantity of water as 0.000337 gm. could be detected; (2) the exact area of the membrane used could be calculated; (3) the concentration of the solution exerting the osmotic pressure could be kept constant. 3. The effect of temperature upon the permeability to water of the seed coat of Arachis hypogaea was measured and the temperature coefficients for 10⚬ rise in temperature were obtained. An average coefficient was not calculated. Since the temperature coefficients are not constant, but vary with the temperature, an average coefficient is without significance. 4. The temperature coefficient is lower than that according to the van't Hoff law, and is higher than the diffusion coefficient. There is no evidence that either chemical or physical processes are exclusively involved in the passage of water through the membrane. 5. The temperature coefficients showed higher values at lower temperatures and lower values at higher temperatures, and this is in agreement with the behavior of temperature coefficients in other processes. 6. A comparison is made with the temperature coefficients obtained in the permeability experiments of (1) Krabbe with living membranes, (2) Rysselberghe with living membranes, (3) Brown and Worley with non-living seed coat membranes, (4) Pfeffer with copper ferrocyanide membrane. 7. No hysteresis or after effect of a previous temperature was observed. 8. It was found that the seed coats of peanut and almond showed a difference in permeability to water in opposite directions through the membrane, the faster rate being from the external toward the internal portion of the seed. 9. When distilled water was placed on one side of the membrane, the rate of water movement was proportional to the osmotic pressure applied upon the other side, when sodium chloride solutions were used; but this proportionality did not exist when cane sugar solutions were used. 10. When solutions of varying concentrations were placed on opposite sides of the membrane, it was found that the relation between rate and concentration difference was complex, and that in general equal osmotic differences do not necessarily produce equal rates; the rate is greatly affected by changes in the concentration of the internal solution; no mathematical relation was noted between the concentration on opposite sides and the rate of water movement through the membrane. The bearing of these facts upon plasmolytic experiments based on rate of water movement through membranes was pointed out. 11. A comparison of the permeability of several plant membranes under similar conditions was made, large differences appearing.