CH. IV] AIR-PUMP. 87 



that used in experiment 109, and select one having a part 

 of about 12 inches in length bare of side branches ; leave 

 it in water for some hours, then cut off the 12 inches 

 and attach one end of it to a potometer, and the other 

 to a water air-pump. When the air-pump is in action 

 water will be sucked through the branch out of the poto- 

 meter and readings can be taken with a stop-watch. Adjust 

 the suction of the pump so that the readings of the poto- 

 meter are roughly the same as those obtained in experi- 

 ment ] 09. Now make the two overlapping saw-cuts as 

 explained under experiment 109 and note the result. The 

 point of interest is that here there is no recovery after 

 the depression in rate of absorption, because there is 

 nothing corresponding to the increased negative pressure 

 due to continued transpiration in experiment 109. 



(112) Strasburger's air-pump experiment^. 



The last experiment depends on a current of water 

 being drawn through wood by diminished air pressure. 

 In the following experiment the current moves in spite of 

 negative pressure. 



Cut a sound branch of yew, peel the lower 4! or 5 

 inches and place it in water for about 12 hours; cut a 

 clean surface and fix it tightly in a perforated rubber 

 cork fitted into a bottle with a ground mouth. The cork 

 is also perforated for a tube connected with an air-pump. 

 The tube must only just project below the cork, while the 

 yew branch must be thrust through far enough to dip 



' Leitungsbahnen, p. 795. A similar experiment is given by Janse, 

 Pringsheim's Jahrb. 1887. 



