Chapter VIII — 145 — Active Relations 



gives a measure of the electrokinetic component. As an example, for red 

 beetroot the DPD in sugar solution was 7.90 atm. ; in CaCl2 it was 6.75 

 atm. Possibly one should inquire if at least some of the effect of the salt 

 is not due to its penetration into the interior protoplasm, with a resulting 

 change in capacity to hold water. 



The suggestion that electroosmosis and related phenomena are operative 

 in the control of water by plant cells is very plausible; their relative im- 

 portance, however, is difficult to evaluate. The requirements for electro- 

 osmotic flow include: a permeable membrane with a heterogeneous pore 

 system ; electrolytes of suitable concentration on each side ; and continuous 

 diffusion of electrolyte demanding some device for maintaining concentra- 

 tion differences. It is this last that designates the process in living systems as 

 an active one. If a mechanism suited to the maintenance of such concen- 

 tration differences can be demonstrated, the above requirements can be met 

 by living cells. Hober (1945), who has very adequately discussed mechan- 

 isms of electroosmosis, suggests, with reference to frog skin, that a con- 

 tinuous production and removal of hydrogen and bicarbonate ions would 

 establish a permanent concentration gradient across the protoplasm and so 

 produce the potential difference required for such flow. 



Up to now our discussion of mechanisms has dealt with active transport 

 of water across membranes. At the beginning of the chapter, we included 

 in a definition of active water that water held within or outside the proto- 

 plasm due to the expenditure of energy. 



The water retaining mechanisms in protoplasm are almost unknown. 

 The views, however, of Sponsler and Frey-Wyssling, outlined in Qiap- 

 ter VI with respect to binding of water by protoplasm, aided by modern 

 concepts of the structure of water and of proteins, are very suggestive. 

 The ability of the protoplasm to vary its volume is known from many 

 observations — stimulative plasmolysis, vacuolar contraction, frost harden- 

 ing, etc. It is true that some of these responses could be the result of 

 change in solute concentration but it is just as likely that water is actively 

 moved, as revealed for example, by vacuolar contraction studies (see page 

 125, Chapter VIII). Perhaps a distinction is unnecessary in the light of 

 recent suggestions of Steward and of Lundegardh that water and solute 

 uptake may be intimately related. Furthermore, salts can markedly affect 

 the imbibitional properties of colloids. 



Northern (1942) found that stimulation decreased the structural 

 viscosity of protoplasm, and concluded that this result was conditioned by 

 dissociations of cellular proteins, at least in part. It was suggested that 

 such dissociations produced a greater imbibition pressure in the protoplasm. 



In their studies on foliar hydration, Phillis and Mason (1945) enter 

 into this matter, with the thesis that the hydration of cotton leaves is largely 

 controlled by protoplasmic imbibition. The latter is in turn controlled by 

 the amount and kind of salts present. Sugars are ineffective in causing 

 absorption of water. An imbibitional mechanism is favored in which micro- 

 scopic vacuoles in the protoplasm are the proposed loci of accumulation of 

 both water and salts. Since water in some instances was absorbed without 

 an increase in the concentration of salt, an enhanced OP as the cause of 

 water intake was ruled out. Following a suggestion of Lloyd and Pleass 

 (1927) the effect of salt was considered as weakening the protoplasmic 

 framework. 



Reinders subsequently (1942) verified her earlier results and by ex- 



