288 



Sir F. Darwin. 



The following experiments were made in an improvised laboratory having 

 a bright north light ; but the dark room was not perfect and the plants 

 ■could not be exposed to absolute darkness. The potometer used was of the 

 Kohl type, having a horizontal tube - 95 mm. internal diameter. The march 

 of the meniscus was generally timed over half a centimetre. The meniscus 

 is brought back to zero by turning a tap and allowing water to enter, as in 

 the instrument designed by Prof. Ganong. 



The rate of transpiration is corrected for (relative humidity). 



Experiment 16. — May 7, 1913. P. laurocerasus. 



May 6. — Branch, with seven last year's leaves, gathered at night. 



May 7, 10 A.M. — Finished vaselining and making incisions in the leaves. 

 Fresh surface made to branch ; apparatus at an east window ; dull morning. 

 yjr between 65 and 82 per cent. T 127-16-8 C. 



For each period, light or dark, the average rate of transpiration is given. 



Time. 



Rate. 



Time. 



Rate. 





Light. 



12.18 p.m. 



Dark. 



10.20 a.m. 1 

 10.40 „ I 



302 



1.47 „ 1 

 1-51 „ J 



187 



10.47 „ 



Dark. 



1.52 „ 



Light. 



n.io „ 1 



11.18 „ J 



229 



2.16 „ I 

 2.22 „ J 



255 



11.19 „ 

 11.50 „ \ 

 12.9 p.m. J 



Light. 



277 



6.53 „ 1 

 6.55 ., J 



195 

 dull light. 



Omitting the last reading (as giving the effect of dull light), we have the 

 average rates : Light = 278, Dark 208, or L/D = 134/100. 



Experiment 17. — May 12, 1913. P. laurocerasus. 



9.45 a.m. — Cut fresh surface to a branch which had been in water since 

 May 10. Dull sky. f 80-88 per cent. T 12-6-13-8 C. The average rate 

 of transpiration for the light and dark periods is given as before : — 



Time. 



Rate. 



10.41 A.M. 



10.56 „ 



10.57 „ 

 11.45 „ 

 11.54 „ 

 11.56 „ 



1.1 P.M. 



2.34 „ 



Light. 







179 



Dark. 







129 



Light. 







172 



Time. 



Rate. 



2.36 p.m. 

 3.52 „ 



3.55 „ 



3.56 „ 

 6.9 „ 

 6.13 „ 



Dark. 



114 



Light. 



129 



