PLANT PHYSIOLOGY 301 



periment demonstrating this ascent of sap and its route 

 through the xylem will be found described in Plant Rela- 

 tions, p. 151. How it is that the transpiration current 

 moves through the xylem is not certainly known. 



163. Transpiration. — When the water carrying dissolved 

 salts reaches the mesophyll cells, some of the water and all 

 of the salts are retained for food manufacture. However, 

 much more water enters the leaves than is needed for food, 

 this excess having been used for carrying soil salts. When 

 the soil salts have reached their destination the excess of 

 water is evaporated from the leaf surface, the process being 

 called transpiration. For an experiment demonstrating 

 transpiration see Plant Relations, § 26. 



This transpiration is regulated according to the needs 

 of the plant. If the *water is abundant, transpiration is 

 encouraged ; if the water supply is low, transpiration is 

 checked. One of the chief ways of regulating is by means 

 of the very small but exceedingly numerous stomata (see § 

 79 [1]), whose guard cells become turgid or collapse and so 

 determine the size of the opening between them. It has 

 been estimated that a leaf of an ordinary sunflower contains 

 about thirteen million stomata, but the number varies widely 

 in different plants. In ordinary dorsiventral leaves the sto- 

 mata are much more abundant upon the lower surface thaD 

 upon the upper, from which they may be lacking entirely. 

 In erect leaves they are distributed equally upon both sur- 

 faces ; in floating leaves they occur only upon the upper 

 surface ; in submerged leaves they are lacking entirely. 



The amount of water thus evaporated from active 



leaves is very great. It is estimated that the leaves of a 



sunflower as high as a man evaporate about one quart of 



water in a warm day ; and that an average oak tree in its 



five active months evaporates about twenty-eight thousand 



gallons. If these figures be applied to a meadow or a 



forest the result may indicate the large importance of this 



process. 



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