The Effect of Light on the Transpiration of Leaves. 285 



Experiment. 



Date. 





Effect of light or of 

 dark. 



1 



JJec. y, lauy ... 



L — * 



D 



123 — ► 



100 1 









D — 



L 



100^ 



127 }> 



19 per cent. 







L-^ 



D 



108 — 



100 J 



2 



Apr. 11, 1911 ... 



L > 



j) 



105 : 



100 



5 



3 



19 1911 



L — 



D 



100 : 



100 





4 



" 20^ 1911 ... 



L-> 



D 



183 : 



1001 









D — 



L 



100 : 



113 J 



48 „ 



5 



„ 22, 1911 ... 



L — 



D 



136 : 



1001 









T) 



L 



100 : 



110 J 



23 „ 



6 



Not. 15, 1911 ... 



D-> 



L 



100 : 



125 



25 „ 



7 



„ 17, 1911 ... 



L — * 



D 



114 : 



100 



14 „ 





Average 



L/D 



= 11 



9/100. 







In some cases transpiration is but slightly affected by darkness, as in the 

 ollowing experiments. 



The material was supplied by small branches of laurel (P. laurocerasus), 

 having, as a rule, four leaves, vaselined and cut (four incisions per leaf) in 

 the usual way. A branch was fitted to a simple form of potometer consisting 

 of a pipette graduated to 0"01 c.c. The pipette was fixed vertically and the 

 branch attached to the lower end by rubber tube ; as the plant absorbs 

 water the descent of the meniscus is read with a lens, by which means 

 errors of parallax are fairly well avoided. 



The experiments were made alternately in a dark room and at the north or 

 east window of the laboratory.* Headings were generally continued for an 

 hour before the change from light to darkness, or vice versa, was made. The 

 results, i.e. the amounts of water absorbed per hour in light and darkness 

 were corrected for psychometric differences. The dates of the experiments 

 summarised below were April 21, 22, 23, 28, 29, May 1, 2, 3, 1913. 



The results were somewhat irregular and are therefore given in the form of 

 an average. A single experiment is, however, given in detail. 



* In a few cases in a dark room which could be illuminated by opening the shutter. 

 The room was to the south and care was taken to avoid sunshine. 



