Crafts et al. — 194— Water in Plants 



1938; Mer, 1940). Root pruning and stem incisions have shown that over 

 50 per cent of the cross section of the root as well as of the stem may be 

 prevented from functioning without severe injury to young citrus trees, and 

 ready lateral transfer of water may be observed (Elazari-Volcani, 1936). 

 The protoplast particularly exhibits a high resistance to rapid water move- 

 ment which may be increased or decreased by the presence of specific ions, 

 by changes in temperature, or by light intensity, as will be discussed in a 

 subsequent section. 



Osmotic Properties: — Just as imbibitional forces enable cell walls to 

 maintain their form and water content, so do osmotic forces through the 

 development of turgor make possible the normal unwilted stature of plants. 

 And in proportion to the efifect of such form upon evaporation from leaves 

 does osmosis affect transpiration. The direct effects of solute concentra- 

 tion upon vapor pressure of water are shown by Table 18 to be almost 

 negligible at concentrations that occur in plants. Furthermore, as explained 

 in Chapter V, turgor increases the vapor pressure of water in the cell so 

 that at full turgor the water-vapor pressure of the cell sap is equal to that 

 of free water of the same temperature and pressure. 



Shreve (1931) found that free cell sap with osmotic pressures of 4.9, 

 11.7, and 20.0 atmospheres respectively evaporated in the ratios 1.011:- 

 1.005 : 1.000. This reflects the relatively minor influence of small additions 

 of osmotically active solutes upon vapor pressure. When sugar solutions 

 were allowed to evaporate through semi-permeable membranes the reduc- 

 tion in rate was much greater, but in this case the solutes were probably 

 able to accumulate in the solution immediately in contact with the membrane 

 or actually within the pores of the membrane, a phenomenon that does not 

 occur in leaves. 



Boon-Long (1941) similarly found that the presence of a collodion 

 membrane reduced by 10 per cent the rate of evaporation of a sucrose solu- 

 tion over that brought about by osmotic concentration alone. Increase in 

 osmotic pressure of leaf cell sap by exposure to light, scalding of petioles, 

 or direct introduction of glucose caused a decrease in the permeability of the 

 tissues to water as determined by the plasmolytic method. According to 

 Boon-Long this lowering of permeability is chiefly responsible for reduc- 

 tion in the transpiration rates which he found in plant organs subjected to 

 these treatments. However, the hardening of cabbage tissues during ex- 

 posure for seven days to 5° C. caused an increase in permeability which 

 more than counterbalanced a simultaneous increase in osmotic pressure. 

 Consequently the transpiration was greater for the hardened than for the 

 non-hardened plants. 



Internal Surface: — Because evaporation is a function of surface it 

 follows that, when transpiration is high, the exposed intercellular surface 

 of the leaf might become a limiting factor. Most investigators have fol- 

 lowed the development of air spaces in leaves by determining their volume 

 by infiltration methods. In the leaves of vascular plants the volume of 

 space differs not only among plants but also as a response to environmental 

 conditions. 



SiFTON (1945) in a review of the subject reports that Unger (1854) 

 found in 41 species a minimum air space of 77 parts per 1000 in leaves of 

 Camphora officinalis and a maximum of 713 parts per 1000 in Pistia 

 texeris. Shade leaves with more spongy parenchyma have a greater vol- 



