Crafts et al. 



24 — 



Water in Plants 



are those influences that determine the behavior of molecules in the liquid 

 state. 



Molecular Interaction and Water Binding: — From the foregoing 

 discussion it seems evident that the nature of aqueous solutions is definitely- 

 conditioned by the forces exerted by water molecules among themselves 

 and upon molecules of the solute. In simple one salt solutions the force 

 relations may be analysed without too much difficulty but as the solutions 

 become complex a satisfactory analysis is impossible. The problem is 

 made difficult not alone by the complexity of the relations between solvent 

 molecules and between solvent and solute molecules but by interactions be- 

 tween solute and solute both as these relate to the solvent and to the state 

 of aggregation of the solute. Although a number of types of forces are 

 exerted between the molecules of a solution as illustrated by Table 3, 

 these forces are all electrical in nature and are so integrated that within a 

 single system they form a smooth series. This is reflected in the form of 

 the vapor pressure : water content curves for soils and cellulose as illus- 

 trated by Figures 9 and 10. 



/oo 



t 



20 



.04- .06 . IZ ./6 ,10 .24 .26 iZ 



Moisture content 



Fig. 10. — Curves showing the relation between moisture content 

 and relative humidity of tlie atmosphere surrounding cotton (solid 

 line), and spruce wood (broken line). Redrawn from Babbitt 

 (1942), figure 3. 



In biological systems molecular structures of great size and complexity 

 are often encountered. Much research has concerned the forces binding 

 water to molecularly dissolved solutes and to colloids. Studies on colloidal 

 hydration have given rise to a concept of water adsorption termed for con- 

 venience, "bound water" about which much controversy and confusion has 

 centered. The many definitions given including "water — in living tissues 

 in a state different from that of water as we know it in bulk" (Gortner, 

 1938), "adsorbed water," "oriented water molecules," "non-solvent water," 

 "non-freezable water," and the like, all indicate that water in the bound 

 state deviates in physical or chemical properties from water in an ideal solu- 

 tion. Gortner states, "Heats of hydration, dielectric constant measure- 

 ments, and the contraction of the system colloid-water — all indicate that 

 the H2O molecules in bound water are more closely packed and are prob- 

 ably specifically oriented in relationship to each other possibly in a more 

 or less true crystal lattice which is more densely packed than the crystal 



