354 



Mr F. Darwin and Mr R. W. Phillips, [Nov. 28, 



Exp. 26. Aug. 14, 1884. A similar experiment was made 

 with a branch of Yew 55 cm. in length. The branch was con- 

 nected to the potometer and to the Sprengel pump at 10.7 a.m. 

 The sucking force being kept at about 6 to 6'5 cm. of mercury* 

 Time Rate 



In the above experiments, the cut end of the branch which 

 was not fixed to the potometer was exposed to a rarefied at- 

 mosphere in the air-pump tube, and therefore the evaporation of 

 this end would cause a slight transpiration current in the wood. 

 It was therefore determined to immerse both ends of the branch 

 in water ; thus the negative pressure from the pump was trans- 

 mitted to the wood through a small column of water. 



In some cases a siphon was used instead of a Sprengel pump ; 

 this was the case in the following experiment. 



Exp. 27. Aug. 19, 1885. A branch of Portugal laurel 68 cm. 

 in length was fixed by its basal end to a potometer, at the other 

 end was applied a siphon with a fall of 118 cm. of water. 



Time Rate 



(i) 



("> 



12.5 

 15 

 20 

 21 



22 

 32 

 25 

 37 



42 



74-1 



72-5 

 75-2 

 Sawed 



through 



T ?g- of diameter, at 

 21 cm. from lower end 



46-5 



46-5 



46-9 



Sawed through T 7 g of diameter oppo- 

 site to and 2 cm. from (i) 



4-5 



Exp. 28. Aug. 20,1885. A Portugal laurel branch 62 cm. 

 in length was fixed into the potometer, the upper end being 

 attached to a siphon having a fall of 135 cm. of water. 



Time Rate 



5.1 

 10 



46-5 

 47-6 



* We purposely employed far lower pressures than Dufour used, as they afforded 

 readings more comparable with those observed in transpiring branches. 



