The Effect of Light on the Transpiration of Leaves. 299 



1913, when the transpiration in light was double that in darkness. The 

 average ratio for transpiration in light and darkness is: ivy, 136/100; 

 laurel, 132/100. But between May 14 and June 16 the laurel gives an 

 average 150/100, and, speaking generally, it cannot be doubted that the 

 laurel reacts to illumination more in early summer than in spring. The 

 winter experiments are not sufficiently numerous to justify any comparison 

 with those obtained in summer. 



It is at present impossible to form any conclusion as to the cause of the 

 increased reaction in June. I have no evidence as to whether the 

 increased permeability to water is a periodic effect, or connected with the age 

 of the leaf, or with the brightness of the summer sky, as compared with 

 illumination earlier in the year. 



§ 4. With regard to the main fact that transpiration is increased by light 

 or diminished by darkness, we may either accept the view of Wiesner,* viz., 

 that in light the chloroplasts are warmed by the absorption of radiant 

 energy, or we may believe that light produces an increased permeability of 

 the plasmic membrane to water, a point of view to which the interesting 

 work of Lepeschkin and Trondlef on the increased permeability to dissolved 

 substances produced by illumination may possibly give some support. Or we 

 may combine Wiesner's theory with those of the other writers. 



It is a pleasure to express my thanks to Miss D. F. M. Pertz for the 

 valuable aid she has given me throughout the research. 



* Wiesner, ' Sitzb. d. k. Akad. Wiss.,' 1877, vol. 74, p. 477. 



t Lepeschkin, ' Ber. d. Bot. Ges.,' xxvi, a ; Trondle, ibid, xxvii. 



