Chapter X — 193 — Loss and Retention 



pressure in value, all walls are highly hydrated and it has been assumed that the outer 

 surfaces constitute virtually free water surfaces. Little energy is required to move 

 water up to and through cell walls under these conditions for movement is simply a 

 process of exchange. When the water balance shifts far to the negative side some 

 water is withdrawn from the walls, the constituent molecules come closer together, and 

 the capillary structure becomes more dense. The walls perform two essential functions 

 in relation to transpiration by leaves in addition to their usual structural service ; by 

 virtue of their colloidal and microcapillary nature they act as conductors; because of 

 their imbibitional properties they store water during periods of abundance against 

 deficits that develop during rapid water loss. 



Bangham and Lewis (1937), using cut strips of Ficus elastica leaves, found that 

 water did not readily enter the intercellular passages by capillarity. Less polar com- 

 pounds (paraffin oil, benzene, chloroform, ether, and essential oil) did enter. The 

 writers concluded that transpiration does not take place from a liquid film of water on 

 the cell walls of the mesophyll. 



Lewis (1945) has performed experiments indicating that the mesophyll walls in 

 leaves are hydrophobic in nature : that is, they are not readily wet, the interfacial ten- 

 sion between walls and water being low. The effect of such a condition would be that 

 water would remain deep within the walls and that the surfaces would remain rela- 

 tively dry. Lewis' experiments indicate this to be true. Earlier Hausermann (1944) 

 had demonstrated that such a lyophilic liquid as butyl alcohol which has some hydro- 

 philic properties best wets the inter-cellular surfaces of Dianthus barbatus leaves. 

 From this observation he proposed that such surfaces are composed of cutin. 



Transpiration from such surfaces could not approach that from free water sur- 

 faces of equal area. This may explain the low ratio of transpiration to evaporation 

 from free water surfaces found by Turrell in experiments to be described. Release 

 of fatty substances by living cells has been postulated to explain deposition of cutin 

 and suberin in the endodermis and cuticle of plants. Possibly this same type of ma- 

 terial may escape from mesophyll cells in minute quantities and impregnate the outer 

 exposed wall surfaces rendering them hydrophobic. 



Another characteristic of the cell wall which may greatly influence the water hold- 

 ing capacity of the cell is the degree of elasticity which it possesses. The cell walls of 

 some xerophytes exhibit little or no elasticity. In this connection, it has been pointed 

 out (Pringsheim, 1931; Ernest, 1934c) that there is essentially no change in volume 

 from the water-saturated state to limiting plasmolysis. Hence, the DPD can increase 

 rapidly and considerably with the loss of a very small amount of water. This is be- 

 lieved to enable plants with inelastic walls to adapt themselves to drought conditions. 

 These cells may rapidly change from a condition of complete turgor to zero turgor and 

 hence from zero DPD to a DPD equal to the OP of the cell sap with little correspond- 

 ing change in the volume of water lost. Such a condition would greatly enhance the 

 plant's resistance to water loss. 



Physical Resistance to Water Movement: — As mentioned above the 

 volume of the cell walls varies with water balance and as the walls become 

 thin during periods of high deficits resistance to longitudinal movement 

 parallel to the cell surface must be appreciably increased. Covalent forces 

 that space the carbohydrate molecules along the chains, however, are little 

 affected by water content and so lateral movements across walls as occur in 

 movement from cell to cell are not greatly changed. Hence dehydration of 

 cell walls probably does not become a major factor of resistance to trans- 

 piration until a point is reached where air begins to replace water in the 

 capillary structure. This does not occur until other disturbances have 

 severely affected the cells. Study of the walls of living mesophyll by means 

 of the microscope shows that they remain plastic, elastic, and normal in 

 appearance throughout the normal stages of wilting and in fact until the 

 cells are almost completely collapsed. Probably cell wall structure has 

 little direct effect upon water movement and loss in leaves, at least until 

 they reach a stage of permanent wilting. 



Nevertheless, the resistance offered by even the minor veins of the 

 leaves is extremely small compared to that offered by the cell (Wylie, 



