CH. IV] NEGATIVE PRESSURE. 79 



changes from light to darkness, and idee versa, namely, 

 that the hygrometric state of the air does not remain the 

 same when the change in illumination takes place. For 

 some reason we have not found it easy to keep this 

 condition uniform even with Kohl's apparatus, but it is 

 clear from his excellent results that it is to be done. 

 Kohl performed the experiment with rooted plants, which 

 is in every way preferable. His method will be under- 

 stood from his illustrations, fig. 2 : and his results from the 

 curves on page 63'. 



(97) Negative pressure. 



The object of this experiment is to prove the need of 

 the precaution mentioned under experiment 94, viz. 

 leaving the cut end of the branch under water for some 

 hours before using it. Cut a branch from a Portugal 

 laurel, fit it at once to Kohl's apparatus, and take a series 

 of readings. The absorption will be found to be very 

 quick at first, and then to become slower. 



(98) Negative pressure. 



Fit up a laurel branch in a potometer and allow it to 

 remain for a day or so. Having taken a series of readings 

 take out the branch and shave a few millimeters off the 

 cut end, which will have become dirty: the partial 

 stoppage of the vessels, which gives the dark tint, 

 produces a slowing of the current, and when it is removed 

 the readings of the stop-watch show an immediate 

 increase in rate. When the readings are fairly steady 



1 Kohl, Die Transpiration der PJlansen, 1886. 



