284 



Sir F. Darwin. 



Transpiration was approximately steady before darkening and fell from 213 

 to 175, or from 121 to 100, during actual darkness; or, if we include the 

 reading taken at 11.54, it fell from 213 to 157, or from 136 to 100. 



There is the same doubt about the effect of subsequent illumination. If 

 we compare the end of the dark period with the last reading taken in the 

 light the effect is nil. If we compare reading at the beginning of the light 

 (157) with that at the end (174) we get a rise of 100 to 111. 



On the whole it is fairest to take the darkening effect as 136 : 100, the light 

 as 100 : 111. The average of the light and dark effects is L/D = 124/100. 



Experiment 6. — November 15-16, 1911. P. laurocerasus. Potometer. 



Time. 



Rate. 



Time. 



Kate. 



Nov. 15. 

 10.25 a.m. 

 11.18 „ 



Nov. 16. 

 10.17 a.m. 

 10.5-1 „ 

 11.16 „ 

 11.27 „ 



Leaves slit. 

 In dark room. 



Cut fresh surface to branch. 

 119 

 120 

 126 



11.30 A.M. 



11.40 „ 

 11.54 „ 

 12.1 P.M. 

 12.25 „ 

 12.40 „ 

 12.50 „ 

 12.57 „ 



Light — at east window. 

 146 

 134 

 137 

 143 

 148 

 162 

 153 



The effect of light may be taken as increasing the rate from 126 to 158 

 (the average of last two readings), or L/D = 125/100. 



Experiment 7. — November 17, 1911. P. laurocerasus. Potometer. 



Time. 



Bate. 



Time. 



Rate. 



10.15 A.M. 



Leaves cut. 



11.16 A.M. 



In dark room. 



10.18 „ 



192 



11.28 „ 



317 



10.34 „ 



316 



11.40 „ 



292 



10.39 „ 



325 



12.0 NOON 



294 



10.50 „ 



300 



12.37 p.m. 



278 



11.12 „ 



330 



12.38 „ 



286 



11.15 ., 



308 



12.56 „ 



278 



Shortly before the period of darkness the rate may be taken as = 320 

 (average of last two readings), at the end of the dark period it is 280 

 (average as above) ; this gives a diminution in transpiration equal 114 to 100, 

 or L/D = 114/100. 



The result of the above series is given in the following table ; L— *D means 

 that darkness followed light, D— *L indicating the opposite. The last column 

 gives the effect as a percentage. Where, as on April 20 and April 22, there 

 is a L— *D as well as a D— effect, the average is given : — 



