The Effect of Light on the Transpiration of Leaves. 289 



The average of the three light readings is 160, of the two dark ones 122 

 or L/D = 160/122 = 131/100. 



If the L reading for 6.9-6.13 p.m. is omitted, which is fairer owing to the 

 fading of the light, we have L/D = 176/122 = 144/100. 



Experiment 18. — May 14, 1913. P. laurocerasus. 



10.20 a.m. — Branch, bearing last year's leaves, vaselined and fitted to 

 potometer. 



11.2 A.M.— Incisions made in leaves. Xorth window, fair light, yfr 71-78 per 

 ■cent. T 15-2-15-9 C. 



Time. 



Kate. 



Average. 



Corrected for 





Light. 







11.45 A.M. 



223 1 







11.46 „ 

 11.48 „ 



228 1 

 231 f 



228 



228 



11.52 „ 



231 J 







11.53 „ 



Dark. 







12.19 P.M. 

 12.25 „ 



184 \ 

 180 J 



182 



208 



12.28 „ 



Light, fairly 



bright. 





4.44 „ 



202 i 







4.46 „ 



191 y 



196 



190 



4.51 „ 



194 J 







The average of the two L periods is 209, which is practically equal to the 

 transpiration in the D period. This result is not explicable, as the light 

 was good at 4.44. ISTor was there any evidence of the wood-vessels being 

 blocked, as sometimes occurs in potometer experiments. 



Average: L/D = 209/208 = lOQ-5/100-0. 



Experiment 19. — May 17, 1913. P. laurocerasus. 



Eresh surfaces cut to the branch on May 15, May 16, and May 17. 

 9.30 A.M.— between 76 and 82 per cent. T 137-14-4 C. 



Time. 





Time. 





Time. 





10.39 a.m. "| 

 10.42 „ f 



Light. 

 168 



Circa 10.45 a.m. 

 11.19 „ \ 

 11.30 „ J 



Dark. 

 105 



| 



11.34 A.M. 



2.0 P.M. \ 

 2.20 „ J 



Light. 

 221 



The average of the two light periods is 194. L/D = 194/105 = 185/100. 



The following experiments were made by Miss Pertz on P. laurocerasus 

 growing at the Botany School, Cambridge : — Shoots bearing five to seven 



Y 2 



