Grafts et al. — 28 — Water in Plants 



fundamental nature of the process is the same in both cases, namely, a 

 satisfaction of electrical forces of attraction between molecules. 



Some have pointed out the close connection between imbibition and 

 osmotic pressure. Heuser states (1944, p. 62), that thermodynamically 

 swelling (or imbibition) pressure appears to be identical with osmotic pres- 

 sure, and quotes Steinberger to the effect that "osmotic pressure is noth- 

 ing but swelling pressure, made evident by the external device of a semi- 

 permeable membrane." 



Brooks and Brooks (1941) similarly point out the lack of a clear 

 distinction between the two phenomena: "Imbibition is simply absorption 

 of water by a system which is in effect an exceedingly concentrated solu- 

 tion, and can often be shown to obey the laws of ideal concentrated solu- 

 tions. ..." And "Imbibition, in any fundamentally sound sense, is not 

 a property of colloidal systems alone, but may be exhibited by homogeneous 

 solutions of crystalloids, among which are such diverse substances as 

 gases, molecules of solid or liquid non-electrolytes, and ions of electrolytes." 

 This constitutes a redefinition of imbibition. We favor the use of hydra- 

 tion in the sense that Brooks and Brooks define imbibition. 



Bull (1943) uses an equation to calculate swelling pressures of a gel 

 that is identical with the vapor pressure equation for osmotic pressure 

 (equation 2, Chapter IV). This equation is: 



Ph V = RT In Ho. <^-^'') 



P 



Where Ph is the swelling pressure and V is the partial molal volume 

 of the liquid. Since he found that the swelling of a gel can be evaluated in 

 terms of vapor pressure lowering of the solvent, regardless of the fac- 

 tors responsible, its close similarity to osmotic pressure is evident. Be- 

 cause such vapor-pressure lowering creates an energy gradient, it accounts 

 for water absorption by vacuolated cells and colloidal imbibants whenever 

 water at higher diffusion pressures is available. 



At their extremes, osmotic and imbibition pressures are clearly differ- 

 ent. For instance, the retention of imbibitional water by cellulose fibers 

 involves little or no osmotic effect, and the reduction of the diffusion pres- 

 sure of water by the presence of ions or small molecules (i.e., in a solution 

 of NaCl) entails no imbibitional effect. However, when one considers 

 larger and larger molecules or molecular aggregates in solution, he en- 

 counters a marginal region where the two effects are not distinct. Just 

 as it is impossible to differentiate sharply between the crystalloidal and 

 colloidal states, so it is difificult to draw a line between osmotic and im- 

 bibitional forces. In the crystalloidal solution, all of the molecules are 

 relatively mobile and if a differentially permeable membrane is used, the 

 solute and solvent may be separated. In the colloidal solution, on the other 

 hand, some of the water of imbibition may be held so firmly that it is re- 

 movable from the colloid only by vaporization. Because of the localiza- 

 tion of imbibed water on the surface of the colloid, its solvent powers for 

 crystalloidal molecules may not be uniform throughout a given system. 

 Hence, where both crystalloids and hydrophyllic colloids are involved in 

 cells, although an equilibrium of forces exists, measurable in terms of the 

 osmotic concentration of the cell sap, some water may be held by imbibi- 

 tional forces, and such water must be accounted for in any attempt to 

 evaluate the water status of the cell. In this connection, it should be em- 

 phasized that the effects of both osmotic and imbibitional forces are simi- 



