T ranspi ri ng Branches . 437 
pressure) in the branch, but is a measure of a tensile stress 
exerted by the branch. 
The escape of bubbles of gas from the cut surface of the shoot 
into the apparatus does not, at the moment, affect the position 
of the indicator on the dial, because the gas is at the same 
tension as the water in the apparatus : but as the gas evolved 
at lower tensions is removed from the tubes and accumulates 
in the reservoir A, it will gradually expand as the tension 
increases, and thus lowers the absolute measurements of the 
tension obtained. 
A point of interest is the consideration of the different 
tensions at which gas-bubbles begin to escape from the 
cut surfaces of the shoots. In the case of the Beech I have 
the following tensions recorded : — 13, i6f, 21J, 16, 5, 9 J, 13I 
inches ; and in the case of Helianthus 5-7 \ in. The tension at 
which bubbles begin to escape would seem to be determined 
by the degree of tension of the fluids in the shoot at the be- 
ginning of the experiment ; for I found that the tension was 
relatively high when the shoot had been previously well 
supplied with water, and relatively low when this was not the 
case. For instance (June 18), a branch of Beech which had 
been standing in darkness with its cut end in water for 
twenty-seven hours began to evolve bubbles at a tension 
of 5 in. : but after remaining all night connected with a tube 
by which water was injected into its cut surface by a slight 
siphon-action, it did not evolve bubbles next morning (June 19) 
until the tension reached 9I in. In most cases no gas was 
evolved by shoots from which all the leaves had been 
removed, in consequence, probably, of the low tensions 
attained. 
The highest tensions which I have observed are 23 inches 
of mercury with the Beech, and 23I in. with the Yew, 
tensions which would raise water to a height of about 24-2 5 
feet, in the case of relatively small shoots. This important 
fact being established, it becomes necessary to distinguish 
the factors which co-operate in developing so considerable 
a force. 
