APPENDIX II. 
The Potometer.—This apparatus enables us to calculate the 
rate of transpiration by observing the rate of absorption by a 
cut shoot. The special glass tube required can be obtained 
from any maker of botanical apparatus. It consists of a bent 
tube communicating with a straight tube, open at both ends. 
A branch with green leaves is cut under water, then attached 
to the bent tube by rubber tubing. The branch must be of 
the same diameter as the tube, and the connection must be 
made under water. Still keeping the apparatus under water, 
the straight tube is closed at the upper end by a cork, and 
at the lower end by another cork, through which a length 
of thermometer tubing has been passed. The length of the 
thermometer tubing should be about 18 inches. As all this 
has been done under water, all the tubes will be full of water, 
and if the apparatus is airtight they will remain full when it 
is lifted out. If any water runs out the fastenings are not 
airtight and the experiment must be begun again. When the 
apparatus is taken out-of the water, the potometer tube with 
the branch is clamped to a retort stand, so that the branch 
stands upright. The thermometer tube should be made to 
dip into a beaker of water. 
The rate of absorption will be measured by the rate at 
which water ascends the tube. To see this the tube is lifted 
out of the beaker of water, and a piece of blotting-paper is 
placed at the open end. This draws out a little water and 
therefore causes a bubble of air to goin. The tube is then 
put back into the beaker, and the rate at which this air 
bubble ascends the tube is measured. 
If the apparatus is placed first in the sunshine and then in 
the shade, the effect of sunlight on the rate of transpiration 
can be observed. 
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