Grafts et al. — 64 — Water in Plants 



material filling the interstices, and by the binding of water by such OH 

 groups as are exposed on the micellar surface. But in the amorphous 

 regions of the micelle, where the chains are somewhat curled and arranged 

 in a random manner, water can enter to cause swelling. This has recently 

 been demonstrated by Hermans and Weidinger (1946). 



According to Stamm (1936), there are three principal ways in which 

 water is held by cell walls: (a) water of constitution, (b) surface ad- 

 sorbed water, and (c) capillary condensed water. No sharp line separates 

 (a) from (b), nor (b) from (c). The graphical relation between water 

 content of the cellulosic material and relative water vapor pressure of the 

 surrounding atmosphere is represented by a smooth sigmoid curve (Figure 

 10). According to Babbitt (1942), the initial portion (first water taken 

 up) of the adsorption isotherm of cotton fibers is best explained by a 

 monomolecular adsorption, the middle portion by a polymolecular surface 

 orientation. Finally, capillary condensation predominates to produce the 

 steep upper portion of the curve. 



Often it is not possible to distinguish between adsorptive and chemical 

 binding of water. Bull (1943, p. 214) has clearly stated the viewpoint 

 that there are varying affinities between adsorbent and adsorbate particles 

 due to the degree to which the latter are exposed on the surface, and the 

 resulting degree to which their force fields have been satisfied. The forces 

 are the same for both types of binding. 



From the above description, it is obvious that cell wall material is highly 

 colloidal and characterized by immense internal surface. Stamm (1936) 

 states that the internal surface of one gram of cellulose is between one and 

 ten million square centimeters. On much of this surface must be exposed 

 the polar OH groups of which there are three per glucose unit. These 

 hydroxyls are hydrophilic and experiment has shown that potentially each 

 may coordinate up to three water molecules. 



Studies on sorption of water vapor by cellulose have indicated only one 

 half to two thirds of the theoretical water uptake, based on that expected 

 by the attraction of one water molecule by each hydroxyl (Stamm, 1944). 

 Since only an estimated half of the OH groups are on the micellar surface, 

 and some are used in bonding the cellulose together, Stamm estimates that 

 only about one out of four hydroxyls is effective in holding a water mole- 

 cule, an amount of surface bound water which he states is equal to a mois- 

 ture content of about 8 per cent. From this it is apparent that not a great 

 amount of water is held on the surface of the micelles. 



The intimate relation between the cellulose fibrils and the infiltrating 

 substances would suggest a partial satisfaction of OH bonds by attraction 

 between the two materials. The subsidiary substances of the nature of 

 pectic compounds, hemicelluloses, and gums are more hydrophilic than 

 cellulose, and like cellulose, are polymeric substances composed of chains 

 of sugar or uronic residues. Hence in the primary wall particularly they 

 would account for considerable uptake of water. 



Water condensed in the smallest capillary spaces is held relatively firmly, 

 but as the water content increases the successive increments of water fill- 

 ing the increasingly larger capillaries may be considered as approaching 

 the nature of water in bulk, mechanically held. 



In studies on hydration of cellulose a strong hysteresis is noted, more 

 water being held by cellulose that has not been subject to drying. Urqu- 

 HART (1929) attributes this to the condition of the secondary valence bonds 



