The Effect of Light on the Transpiration of Leaves. 287" 



Another series of similar experiments was made by Miss Pertz on 

 P. laurocerasus, using a Ganong potometer.* 



The following example shows a definite light and dark effect, in spite of a 

 good deal of irregularity. The figures are corrected for relative humidity, 

 which varies between 56 and 60 per cent., while the temperature lay between 

 15-5° and 16-1° C. 



Experiment 9.— May 3, 1913. 



Time. 



Transpiration. 



. Average. 



Time. 



Transpiration. 



Average. 





Light. 











10.49 a.m. 



150 





12.0 NOON 



Light. 





11.1 „ 



163 





12.5 p.m. 



157 





11.11 „ 



167 





12.10 „ 



144 





11.26 „ 



164 \ 



170 



12.15 „ 



133 





11.35 „ 



176 J 



12.20 „ 



142 T 



149 





Darkness. 





12.35 „ 



156 J 



11.43 „ 



172 











11.50 „ 



139 1 



138 









11.59 „ 



137 J 









In the third column is given the average of the last two readings in the 

 three periods Light, Dark, Light ; thus the result of the experiment of 

 May 3 is L 170, D 138, L 149. 



The following table gives the results of the series, May 3 to May 12, 1913,. 

 Experiments 9 to 15 : — 



Date. 



Expt. 



L. 



D. 



L. 



1913. 

















9 



170 



133 



149 



5 





10 



110 



100 



104 



6 





11 



134 



107 



134 



7 





12 



144 



121 



132 



8 





13 



113 



109 



103 



„ 10 





14 



70 



65 



67 



„ 12 





15 



77 



60 



67 





Sum 



818 



700 



756 







117 : 



100 



108 



Or taking the 



average of the two light readings, we 



have — 





L/D = 



= 113/100 







* ' Plant Physiology,' by W. F. Ganong, 1908. The instrument is supplied by 

 Messrs. Bausch and Lomb. 



VOL. LXXXVII. — B. Y 



