225 
Leaf Water-content , and Transpiration Rate . 
pipette. These improvised clamps sometimes interfered with the readings 
of the potometer, but were very convenient to adjust and were relatively 
light. 
Transpiration rates were determined by weighings ; readings of the 
potometer were made simultaneously in order to indicate the amount of 
water absorbed by the shoot and consequently any changes in the water- 
content of the plant. 
Experiments on the Potometer Method. 
Curtis (6) and Livingston (17) have taken exception to methods which 
involve mutilation of the plant, such as detaching a shoot and using it in 
a potometer, since such treatment is liable to cause the plant to behave in 
an irregular manner. Lloyd (19, 20) appears to have found the potometer 
satisfactory, and as the method is convenient some experiments were carried 
out to compare the behaviour of plants in pots with that of cut shoots set 
up in a potometer. 
Leaf chambers were fixed to three leaves of each of two shoots on 
a plant of Eupatorium adenophorum and porometer readings taken simul- 
taneously from each shoot by means of the automatic recorders. After 
a few hours one shoot was severed from the plant, a fresh surface was cut, 
under water, four inches above the first cut, to exclude the air as far as 
possible from the water-conducting system, and the cut shoot was mounted 
in a potometer, the porometer records being continued for some hours with 
the shoot in the potometer. The graphs of stomatal aperture ran closely 
parallel until one shoot was severed from the plant ; but when this was done 
the stomata of the severed shoot immediately began to close, the move- 
ment lasting about ten minutes. From this point onwards they opened 
again until, about fifty minutes after the shoot was cut, the two graphs 
bore the same relation to each other as before cutting. There seems to be 
no doubt that the temporary tendency to close was due to the handling 
involved in cutting the shoot and mounting it in the potometer. Other 
examples of this phenomenon have been described elsewhere (Knight, 15, 
P- 59)- 
Records of the stomatal behaviour of shoots in potometers have been 
continued for as long as three days after setting up, and it has been found 
that the graphs of stomatal aperture correspond closely for the whole 
period with those obtained at the same time from potted plants under the 
same conditions. The differences are no greater than are normally to be 
found between different potted plants in these circumstances. In the case 
of the cut shoots, about half an inch of the end of the stem was cut off every 
day in order to provide a fresh absorbing surface. 
Similar experiments have been carried out with regard to the transpira- 
