Transpiring Branches . 433 
conditions that the cut surface is maintained in contact with 
water : in some cases I removed the cortex from the portion 
of the branch introduced into the india-rubber tube, in others 
1 simply attached the branch as it was, without any per- 
ceptible difference in behaviour. When all the connexions are 
made, the tubing is filled with water by raising and inverting 
the water-reservoir A ; then the reservoir itself is completely 
filled by raising the tube a so that its lower end does not 
project beyond the lower surface of the india-rubber stopper, 
and then pouring water down it from a wash-bottle, when all 
the air escapes up the tube : the tube a is then lowered so that 
its lower end is about on a level with that of the other tube 
in the reservoir ; and, finally, the india-rubber tube attached 
to its free end is closed at a by two stop-cocks whilst still full 
of water. The apparatus, if properly set up, is now entirely 
free from gas ; it is air-tight, and the indicator of the vacuum- 
gauge stands at zero. It should be mentioned that the 
india-rubber tubing must be provided with an internal spiral 
wire to give it strength to withstand the atmospheric pressure 
from without, so as not to become occluded ; or, better still, 
it should be thick-walled pressure-tubing ; and the water 
used must have been boiled, so as to render it as nearly as 
possible air-free. The dial of the gauge is divided into 
thirty parts, representing inches of mercury, so that a complete 
revolution of the indicator would mark a perfect vacuum. 
