1885.] On the transpiration-stream in cut branches. 353 



so that it is evident that when it was acting as a cork it was not 

 adding materially to the water absorbed, and therefore did not 

 interfere with the reading given by the Portugal laurel. 



Conclusions as to Dufour's Experiments. 



We may safely conclude from the result of the above experiments 

 that the effect of double sawing on the transpiration-stream of 

 angiosperms is great. And we can by no means agree with 

 Dufour's statement, that the difference in absorbing power between 

 normal and doubly-sawn branches is "at first only slight*". 

 Dufour only gives three experiments in which the amounts ab- 

 sorbed by sawn and not sawn branches are compared. In two of 

 these the difference is certainly slight (126 : 124, and 52 : 41) 

 at first ; but even in these the difference mounts up so that on 

 the last day of the experiment the intact branch absorbed 97 cc. 

 while the sawn one absorbed 2 cc. 



§ 5. Estimation of the effect of " double sawing " on branches 

 through which a current of water is drawn by means of a 

 pump, &c. 



The method employed was as follows : 



A branch is fixed by its upper end to a thick india-rubber pipe 

 (a fig. 2) connecting it to a water air-pump, the other end of the 

 branch being fixed to the potometer b. A slight suction of 

 the pump causes a rapid current of water through the branch, 

 the rate being read by the potometer. The branch is then sawed 

 at c and d and the effect on the reading of the potometer 

 noted. 



Exp. 25. Aug. 13, 1884. A branch of Portugal laurel 658 mm. 

 in length was cut between 9 and 10 in the morning, was kept 

 damp until it could be immersed in water in the laboratory, 

 i.e. about half an hour after it had been cut from the tree. It 

 was fitted to the Sprengel pump (at 11.13 a.m.) in such a way 

 that the current of water was drawn through in a natural 

 direction, i.e. from the basal to the apical end. The sucking power 

 was about 10 cm. of mercury. 



Time Rate 



12.0i 125-0 

 Sawed half through at 27 cm. from the base. 

 83-3 

 78-1 

 37 Sawed half through on the opposite side at 



25 cm. from the base. 

 42 | 34-7 



* Sach's Arbeiten, p. 50. 



2 

 33 



