[Chap. XXIV TRANSPIRATION 



other soluble compounds that dilute the water. 

 As water molecules leave the walls and diffuse 

 into the intercellular spaces, other water mole- 

 cules are pulled into their places by the forces 

 of cohesion and surface tension. This cohesion 

 of the molecules of water in closed tubes is so 

 great that the pull extends all the way down 

 the veins and vessels of the leaves and stems. 

 For example, when the cut ends of stems in a 

 bouquet are placed in a vase of water, tran- 

 spiration from the leaves results in a movement 

 of water from the vase and up the veins of the 

 stems to the leaves and flowers exposed to the 

 air. 



The removal of water from the cells of the 

 leaf increases the concentration of the sugar 

 and other solutes and decreases the concentra- 

 tion of the water. The water becomes less con- 

 centrated in the mesophyll cells than it is in 

 the xylem of the veins. Water molecules, there- 

 fore, diffuse osmoticallv from the vessels into 

 the adjoining mesophvll cells, and finally into 

 the epidei-mal cells, in both of which the con- 

 centration of water has been lowered by tran- 

 spiration. 



The lifting power of evaporation may be 

 demonstrated by evaporation from a porous 

 porcelain cup suitably mounted on a long glass 

 tube, both of which have been completely filled 

 with freshlv boiled water ( Fig. 79 ) . The lower 

 end of the tube dips into a vessel of water and 

 mercury,^ When the porous cup is exposed to 

 the air, evaporation occurs at the surface of the 

 cup. This develops a tension which is trans- 



^ Full details for setting up these experiments may be 

 obtained from papers by H. Thut, Ohio Jour. Science, 

 28:292, 298, 1928; Amei'. Jour. Bot., 19:358-364, 1932. 



with a stopcock when the apparatus is not in use to pr 

 siphoned from the beaker. 



227 



X 



T 



\= 



ill 



*is=i> 



A 



Water 



Mercury 



Fig. 79. Diagram of ap- 

 paratus used to demon- 

 strate the lifting power of 

 evaporation. Tul^e C is a 

 meter or more in length. 

 This tube and the porous 

 cup, which is immersed in 

 water in beaker A, are 

 filled with air-free water. 

 The beaker is removed 

 onlv when the apparatus 

 is being used. The short 

 tube in bottle B is closed 

 event the water from being 



