266 The Physiology of Plants BOOK in 



changes in the guard cells, causing a considerable increase 

 in the quantity of vapour exhaled. The active rays were 

 determined to be those which correspond to the absorption 

 bands of chlorophyll. Wiesner made his observations on 

 etiolated and normal plants of maize. He arranged such 

 conditions that in his plants the stomata were nearly closed 

 while they were exposed to light, and found that when 

 illuminated the transpiration of the etiolated plants was 

 doubled and that of the normal ones was increased seven- 

 fold. Observations made upon Hartwegia showed that the 

 stomata remained open in the dark, yet the access of light 

 caused the amount of exhaled vapour to be one and a half 

 times as great as before. 



Wiesner suggested that the result might be due to the 

 conversion of the light rays into heat rays by the chloro- 

 phyll. A similar suggestion as to the influence of the 

 pigment was made by Van Tieghem in 1886, and it received 

 considerable support in France. The view propounded was 

 that one of the normal functions of chlorophyll is to increase 

 transpiration, a function which was spoken of as chloro- 

 vaporization. Dehe"rain, in 1876, had stated that he observed 

 that when carbon dioxide was not admitted to the leaf, 

 the incidence of light increased transpiration. The state- 

 ment was made again by Jumelle as the result of observa- 

 tions carried out in 1889. 



The view has not obtained acceptance in England, and 

 in Germany Pfeffer appears doubtful about it. The results 

 of Deherain and Jumelle do not receive confirmation from 

 the later work of Kohl and of the Verschaffelts. The 

 increase of transpiration under the influence of light cannot 

 be dependent on the action of chlorophyll, for Bonnier and 

 Mangin determined in 1884 that it is not confined to green 

 plants, but is exhibited also by certain fungi. 



The value of the process of transpiration to the plant 

 as a whole has been but gradually recognized. During the 



