86 NEGATIVE PRESSURE. [CH. IV 



however had much experience of the apparatus and have 

 never thoroughly tested it. 



If the suction tube from an air-pump is fitted to the 

 bell-jar and a good sized tube^ for the admission of air is 

 provided, it is easy to keep the air in the bell-jar dry 

 enough to keep up a good transpiration current. This is 

 an easier apparatus to fit up, because no air-tight joints 

 are needed, but it is obviously faulty because the air in the 

 bell-jar varies in dryness with the air of the laboratory. 

 Still, from some few trials, Ave are inclined to think that 

 the effect of light and darkness may be demonstrated in 

 this way. 



(97) Negative j^^^essure. 



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 (Primus lusitanica), fit it at once to Kohl's ap- 

 paratus, and take a series of readings. The absorption 

 will be found to be quick at first, and then to become 

 slower. 



(98) Negative jy^^essure". 



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 



1 A curved rubber tube which excludes light. 



2 See von Hohnel, Botan. Zeitung, 1879, p. 318. 



