Chapter VI — 65 — Intracellular Distribution 



of the hydroxyl groups of the cellulose molecules. In the original water 

 soaked condition these bonds are saturated ; when the cellulose is dried these 

 groups are freed of water and, as a result of shrinkage, pairs are drawn 

 together so that the individual groups of adjacent cellulose molecules mu- 

 tually satisfy each other. Upon remoistening, some of the bonds originally 

 binding water are not readily freed for water absorption. This results 

 in reduced uptake. Other factors affecting hydration of cell walls are the 

 presence of the infiltrating materials listed above (page 63) the osmotic 

 and imbibitional properties of the protoplast, the presence of ions and 

 molecules passing through or along the walls, and the hydrostatic status 

 of water in the tissue. The latter is considered important for cells with 

 thick walls, as in certain marine algae (de Zeeuw, 1939). 



The ability of cell walls to hold water in competition with the evaporat- 

 ing power of the air in the stomatal chambers of leaves and the colloids of 

 the soil is of great importance in the economy of the plant. Apparently 

 the imbibition process in living plants is completely reversible for even 

 after severe wilting, if the plant is supplied with water the wall shows no 

 appreciable damage except where collapse and death of cells has destroyed 

 the organized structure. Though considerable water may be lost from the 

 walls, they apparently can maintain enough to retain their normal properties. 

 No hysteresis effects comparable with those noted for dried cotton fibers 

 have been observed in connection with wilting of plants. 



The amount of water held by cell walls may be considerable. Crafts 

 (1931) found that phloem walls of potato stolon were relatively thick in 

 their natural condition, and that their loss of volume on drying indicated 

 a high water content. While considerable shrinkage resulted when they 

 were dried in alcohol, drying in air at 80° C. produced a 50 per cent re- 

 duction in volume (Table 20). This indicates a minimum water content 

 of 50 per cent in these walls. 



The above is cited to illustrate the condition in one type of tissue. 



Water held by hydrogen linkages along the cellulose chains of cell walls 

 is probably relatively immobile. On the other hand, considerable water 

 must exist in microcapillaries between the fibrils and this water should be 

 fairly free. Such water plays an important role in water movement along 

 cell walls. This movement occurs during absorption by roots, from root 

 hairs across the cortex and into the xylem. It also takes place laterally 

 through the wood, across the cambium, and in the bark. It undoubtedly 

 accounts for flow from the last tracheids at the bundle ends to the meso- 

 phyll of leaves. And it probably occurs in a large measure during develop- 

 ment of fruits, tubers, and other storage organs. The microstructure of 

 many cell walls seems highly adapted to such water movement. 



Protoplasmic Water: — Considering the properties of protoplasm, it 

 is remarkable that its water content often attains values above 90 per cent ; 

 and that an average value for many plant cells may be as high as 85 per cent. 



