1915-16.] BOTANICAL SOCIETY OF EDINBURGH 61 



In eveiy case records continuing over several days were 

 obtained, and as far as possible they were uninterrupted. 

 The temperature was kept as constant as possible, the 

 variation being from 2° to 4°. The percentage humidity 

 was on an average between 60 and 70. 



A. Normal Conditions of Light and Dark. 



1. Piniis ■sylvcstris. 



(1) nth June to 23rd June 1914. 



The natural conditions at that time were approximately 

 sixteen hours' light to eight hours' darkness. Transpiration 

 Avas found to be more active during the light than during 

 the darkness period, the ratio being as 1 : "32, while the 

 ratio of light to dark is as 1 : '5 



An analysis of the hourly graph shows that at this 

 season there is on an average a maximum of transpiration 

 at 4 o'clock in the afternoon, a minimum at 3 o'clock in the 

 morning. 



(2) 25th November to 18th December 1914. 



The conditions as regards the illumination of the plant 

 at this time were eight hours' light to sixteen hours' dark- 

 ness — ^just the exact reverse of the state of affairs in June. 

 In this case the average mean ratio of transpiration in 

 light to transpiration in the dark is 1 : 2"9, while the ratio 

 of light to dark is as 1 : 2. 



The hourly graphs show as an average a maximum at 

 4 o'clock in the morning, a minimum at 5 p.m. 



(3) 19tli January to 19th February 1915. 



In this case the ratio of light to dark is for January 

 as 1:2, and for February as l:r4. Here again the 

 figures show transpiration during the period of darkness 

 to be greater than that of light, although the difference is 

 not so marked as in the November-December records ; the 

 ratio of transpiration in light to that in dark is as 1 : 1'5. 



The maximal value of transpiration at this time as seen 

 from the average graph occurs at 8 o'clock in the morning. 

 The minimal values are very variable, but approximately 

 there is a minimum at 1 p. in. 



