Crafts et al. — 144 — Water in Plants 



ideal semipermeable protoplast." To organic acids such as malic is assigned 

 an important role in this process. 



Figure 42 from Bennet-Clark and Bexon (1943) will aid in follow- 

 ing the suggested series of events. In Figure 42A, malic acid diflfuses 

 from the vacuole into the protoplasm where it is enzymatically converted 

 into carbohydrate. The latter moves into the vacuole, whereupon it is 

 broken down to malic acid again. This completes a continuous diffusion 

 cycle of non-electrolyte inward and electrolyte outward. In this way a 

 potential difference might be maintained which in turn would account for 

 an electroosmotic flow into the vacuole. Figure 42B represents a simple 

 model used by Bennet-Clark for illustrative purposes. A container, the 

 bottom of which comprises a permeable ("oxidized collodion") membrane, 

 contains a saturated salicylic acid solution with excess of solid. This is 

 immersed in another vessel containing pure water. A hydrostatic pressure 

 is developed as indicated by the manometer, which is greater than the 

 osmotic pressure of the solution. This excess pressure is due to an electro- 

 osmotic flow through the pores of the membrane. One such pore is indi- 

 cated as Figure 42C. 



In a fourth paper on the water relations of plant cells, Bennet-Clark 

 and Bexon (1946) give further evidence that bioelectric forces exert an 

 influence on water movement. Inner epidermis of onion bulb scale was 

 mounted in a perfusion apparatus designed for rapid replacement of the 

 bathing solution while the tissue was under microscopic observation. When 

 tissue, which had been treated with KCl solution (27.9 atm.) long enough 

 to cause complete plasmolysis (40 minutes), was exposed to a sucrose 

 solution of the same osmotic pressure, a temporary swelling of the vacuole 

 was observed. After 15 to 30 minutes the initial volume was reestablished. 

 Transfer back to KQ produced the reverse effect — a shrinkage of the 

 vacuole, followed by a return to the initial volume. That this result was 

 not simply a diffusion effect on the OP of the solution between the wall 

 and the protoplast was shown by the fact that isolated protoplasts behaved 

 in the same way. 



Bennet-Clark and Bexon give the following explanation. Assuming 

 a negative charge on the membrane, the permeability to K"" is greater than 

 to CI". On transfer from sucrose to KCl, the greater penetration of K"" 

 produces a positive charge on the inner membrane surface. This potential 

 difference effects an electroosmotic flow outward, and cell volume is reduced. 

 When KCl is replaced by sucrose, preferential diffusion of KCl outward 

 causes the inner surface to become negatively charged, and the electro- 

 osmotic flow is directed inwardly. Equilibrium obtains in both instances as 

 the membrane potential drops to zero. 



One point made insufficiently clear is whether the volume of protoplasm 

 remains approximately constant during swelling and shrinkage of the 

 vacuole. Vacuolar contraction, and its reversal, frequently reported in 

 onion epidermis, could conceivably account for at least some of the volume 

 changes observed. 



Brauner and Hasman, in a series of papers {see 1947) have defended 

 the view that an electroosmotic flow of water is a significant component of 

 DPD of plant cells. In their theory, bivalent cations should act to neu- 

 tralize the electric potential in the pores of the double membrane (cell 

 wall-cytoplasm). The difference between gravimetric determinations of 

 DPD, one in sucrose solution, another in CaCl2 orMgCl2, in their view 



