CH. IV] LIGHT. 85 



(96) Light and darkness. 



Kohl 1 has succeeded in demonstrating the effect of 

 light on transpiration by a method which in his hands 

 gives excellent results. Variations in transpiration are 

 estimated by changes in the amount of water absorbed 

 as in experiment 93. To avoid the difficulty of keeping 

 the hygrometric condition of the air constant during the 

 alternation of light and darkness, Kohl encloses the plant 

 in a large bell-jar resting on a ground-glass plate. The 

 leaves and stem of the plant are within the bell while the 

 apparatus for recording the rate of absorption is below the 

 glass plate and free to be manipulated. By means of an 

 aspirator a current of dry air is drawn through the bell-jar, 

 and the plant can be darkened by a cardboard cylinder 

 slipped over the bell-jar without affecting the hygrometric 

 condition of the air within. Kohl's curves show that the 

 effect follows a change from darkness to light very 

 quickly, and that the depression in the rate of absorption 

 produced by darkness occurs still more rapidly. Plants 

 of Nicotiana tabacum growing with their roots in water 

 seem to give the best results. 



We have noticed a source of error in this method 

 which seems to be of importance. If the aspirator runs 

 too quickly it is possible to produce diminished air- 

 pressure within the bell-jar and thus produce an increase 

 in the rate of absorption. We found it necessary to 

 attach a mercury manometer to the apparatus in order 

 that this source of error may be avoided. We have not 



1 Die Transpiration der Pjianzen, 1886, p. 61. 



