'Stomata , Leaf Water- content, and Wilting of Plants. 371 
with the slight fall of temperature and rise of humidity the rate of 
evaporation from the at mometer showed a tendency to decrease. For the 
first eighty minutes the graph of transpiration rate (T.) followed a course 
very similar to that of evaporation, and the relative transpiration graph is 
therefore less irregular than that of transpiration rate itself. All observa- 
tions taken throughout this portion of the experiment show that the loss 
of water by transpiration never exceeded the amount absorbed from the 
potometer, and consequently there was no decrease in water-content. The 
stopcock on the potometer was closed after observations had been continued 
for eighty-three minutes, and further water-supply to the shoot was thus 
prevented. From this point to the end of the experiment environmental 
conditions, as indicated by readings of temperature, relative humidity, and 
evaporation rate, showed the same trend as hitherto. There was no sudden 
external change which could be held responsible for the changes exhibited 
by the plant. After a few minutes the stomatal aperture showed a sudden 
increase amounting to about 40 per cent, of its previous magnitude, the 
maximum being reached forty-one minutes after the cessation of the 
absorption of water by the stem. The opening of the stomata was 
accompanied by a simultaneous increase in the rate of transpiration, and, as 
the evaporation rate continued to exhibit a downward tendency, this 
increase of transpiration is also reflected in the relative transpiration 
graph. 
As the decrease of water-content proceeded, the preliminary opening 
of the stomata gave place to a closing movement, usually associated with 
wilting. This closure was accompanied by a decrease in the transpiration 
rate, which was again greater than could be attributed to the change in the 
evaporation rate. 
This experiment combines the features of previous experiments on 
transpiration rate with those on the changes of stomatal aperture of wilting 
plants. It shows that simultaneously with the temporary opening of the 
stomata which accompanies the initial stages of wilting, there also occurs 
a corresponding temporary increase in the rate of water loss from the leaves. 
In a series of sixteen experiments on two species, Enpatorium adenophorum 
and Peristrophe speciosa , this increase of transpiration rate has occurred on 
every occasion. The magnitude of the increase varies considerably, and 
shows a rough correlation with the magnitude of the increase in stomatal 
aperture, but undoubtedly there are other factors concerned in deter- 
mining the extent of the increase. This point will be discussed later. 
An attempt was made to correlate the variation in the magnitude of 
the increase with the rate at which the plant wilted. Unfortunately, 
different atmometers were used in different experiments, so that it was 
impossible to compare the evaporation rates from day to day. However, 
as the velocity of the air motion was constant throughout, the temperature 
