434 
Vines . — The Suction-force of 
As the branch begins to exert stress on the water in the 
apparatus, the indicator moves round the dial, marking the 
degree of tension to which the water in the tubes is being 
subjected. 
I have found, like my predecessors, that gas is evolved, 
sooner or later, from the cut end of the branch into the tubing. 
This does not occur, however, until the gauge indicates 
considerable tension : for instance, in the case of the Beech- 
branches, at tensions varying from 5-21 in . ; and in the case 
of Helianthus , at a tension of about 5 in. But in this ap- 
paratus, the evolution of bubbles of gas from the branch does 
not affect the indicator of the gauge, provided that the gas is 
not allowed to accumulate between the branch and the T-piece 
so as to interrupt the continuity of the water in the tube. 
The evolution of gas by branches freshly cut from the tree is 
frequently so rapid that I found it impossible to keep 
the tube clear : consequently I was obliged to abandon 
freshly cut branches of Beech, and to use only such as had 
been kept standing in water for twenty-four to forty-eight hours 
after having been cut. The Yew gives off gas much less 
readily, so that in this case freshly cut branches might be 
used : one such branch did not begin to give off bubbles until 
a tension of over 17} in. had been reached: another similar 
branch which had been kept standing in water for forty-eight 
hours after having been cut from the tree, did not give off 
bubbles until a tension 19 J in. had been attained: moreover 
the volume of gas evolved was small in both cases. 
If at any time during an experiment a leakage develops in 
the tubing so that air enters, the indicator goes backward on 
the dial ; and if the leakage is considerable, or if the stop-cock 
be opened, the indicator at once returns to zero, and the 
water sinks in the tube a. 
I may now pass to an account of some illustrative experi- 
ments, beginning with the Beech [Feigns sylvatica ) and the 
Yew [Tcixus baccata ), as representing woody perennial plants. 
The branches used were cut from large trees. The experi- 
ments were carried on in a well-lighted room facing south : 
