436 Vines . — The Suction-force of 
suction-force of the branch under experiment. But this is no 
real answer, for the further question at once arises, what 
is the nature of this force ? — can it be definitely shown to be 
of the nature of a tensile stress ; or is it merely a measure 
of the tension of the gases in the wood, a measure, that is, of 
the negative pressure developed in the branch ? 
The first step towards a solution of this somewhat difficult 
problem is to obtain a clear idea of what is going on in the 
branch whilst the experiment is in progress. It must be 
understood that, so long as no gas-bubbles are evolved from 
its cut surface, the shoot is not absorbing any water from the 
apparatus. If any direct proof of this obvious physical fact 
is necessary, it is afforded by the rapid flagging of the leaves 
and younger parts of the shoots of Helianthus when under 
experiment : their cells very soon lose their turgidity, in 
consequence, no doubt, of transpiration uncompensated by 
absorption. When bubbles are evolved, however, each bubble 
displaces a corresponding volume of water from the apparatus, 
so that water is then absorbed by the shoot ; in an experiment 
with an actively transpiring branch, a considerable proportion 
(3-2) of the water in the reservoir may eventually be replaced 
by gases evolved from the cut end of the stem. 
These are the actual facts ; but how are they to be ex- 
plained ? What is the active cause of the escape of bubbles 
of gas from the cut end of the stem ? This gas cannot be air 
leaking through the shoot, for were it so there would necessarily 
be a fall of the tension indicated on the dial : it is gas derived 
from the tissue of the stem, some of it, possibly, escaping 
out of solution in the water of the tissues. Various explana- 
tions of the escape of this gas might be suggested : but 
as the matter is a purely physical one, I will not attempt to 
discuss it in detail. For my own part, I incline to the view 
that the escape of bubbles is due to a stress exerted upon the 
water in the tube by the suction-force of the branch, with the 
result that water is absorbed and gas expelled in corresponding 
volume ; and that, consequently, the tension indicated on the 
dial is not a measure of the tension of the gases (negative 
