226 Knight . — The Interrelations of Stomatal Aperture , 
tion of severed shoots in a potometer, and the following is a typical 
example : 
A shoot was severed under water at 2.0 p.m. on October 4th, and 
mounted in the usual manner in a potometer. Porometer and transpiration 
measurements were made up to 6.0 p.m., and continued from 11.0 a.m. till 
3.0 p.m. on October 5th. During that day the stomata responded in the 
usual manner to the illumination changes and the rate of transpiration also 
was comparable with that of a rooted plant. Comparison of the potometer 
readings and weighings showed that the plant was not suffering from lack 
of water, the quantity absorbed being equal to the quantity transpired. 
During October 6th transpiration and stomatal movement went on as usual, 
but it was found at 2.0 p.m. that the rate of absorption of water was slightly 
below the rate of transpiration and therefore the water-content of the plant 
was diminishing. There was no apparent wilting, however, until an hour 
later, when the leaves were distinctly flaccid. At this stage an inch of the 
end of the stem was removed, to provide a fresh absorbing surface, and the 
plant immediately began to absorb at a greater rate, so that by 5.0 p.m. the 
leaves had almost recovered their former turgidity. By 10 a.m. on October 
17th the plant, having absorbed during the night more water than it had 
transpired, was quite turgid, and the observations were continued. By noon, 
however, wilting had commenced again and the experiment was stopped, 
although, as the plant was apparently quite healthy, there appeared to be 
no reason why a freshly cut absorbing surface should not again have enabled 
it to recover. 
It appeared then from these experiments that as far as transpiration 
and stomata are concerned, a shoot in a potometer is capable in ordinary 
environment of carrying on its functions normally for some two or three days 
after its removal from the plant. It was found, as was to be expected, 
that plants raised out of doors remained normally active for longer periods 
after mounting in the potometers than similar ones grown in a greenhouse. 
Since, therefore, most of the experiments in this work lasted less than 
twenty-four hours, it was considered legitimate to draw conclusions from the 
behaviour of plants mounted in potometers. 
Consideration was given to the possibility that, as the plants were 
placed in the air-flue during the experiments, the continuous stream of air 
passing over the leaves might cause some change of stomatal aperture, e.g. 
the closure due to shaking already referred to. Although the speed of the 
air-current was never sufficiently high to cause any visible movements of 
the leaves, nevertheless experiments were carried out to test the possible 
effects of the wind on the stomatal aperture. Rooted plants and shoots in 
potometers have been placed in the flue and subjected for short and long 
periods to air-currents of velocities up to twenty metres per minute. 
Periods with the air moving have been alternated with periods of fairly still 
