96 AIR-PUMP. [CH. IV 



nothing corresponding to the increased negative pressure 

 due to continued transpiration in experiment 109. 



(112) Strasburgers air-pump experiment 1 . 



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 opposi- 

 tion to negative pressure. 



Cut a sound branch of yew (Taocus), peel the lower 8 

 or 10 cm. 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 

 well below the eosin solution, which should not more than 

 half fill the bottle. When the air-pump is set in action, 

 it is obvious that its tendency is to suck out the contents 

 of the tracheids at the cut end, and as a fact air-bubbles 

 are seen to issue at that point. Nevertheless, in spite of 

 this the eosin rises in the branch. Leave the pump running 

 for 6 or 7 hours, when the branch should be sawn off 

 above the cork, without stopping the pump, so as to avoid 

 injection of the wood with eosin. Readings of the baro- 

 meter should be taken in the course of the experiment 

 and compared with the readings of the pump-manometer 2 . 



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

 Pringsheim's Jahrb. 1887. 



2 We have only used a negative pressure of 50 cm., but Strasburger used 

 72 cm. 



