Stomata , Leaf Water-content, and Wilting of Plants . 365 
l 
physiological correlation is inadmissible. The duration of the time interval 
must be determined by such factors as the rate of transpiration and the 
amount of water in the plant. It is to be expected that the time interval 
will be shorter for a single leaf severed at the petiole than for a leaf 
attached to a severed shoot, the stem of which could supply water to the 
leaves longer than the petiole alone. No real comparison is permissible 
which does not take into consideration the physiological state of the plants 
in the two cases, but in the present case the disparity between the two 
intervals is significantly large. 
Again, the duration of the period of increased transpiring power appears 
excessively long when compared with the short time occupied by the 
opening movement of the stomata. 
In view of these discrepancies and of the importance of the question 
whether stomatal opening could be held to account for the increase of 
transpiring power accompanying permanent wilting, the writer felt that 
further experiments were desirable in order to obtain more complete 
information with regard to the significance of the wilting process. The 
trend of the work has already been indicated in the writer’s summary of 
transpiration work (15). 
The first investigation undertaken was the determination of the change 
of transpiration rate accompanying the stomatal opening caused by wilting. 
Darwin (7,8) in effect determined transpiration rate changes during wilting, 
and Darwin and Pertz determined stomatal changes, but there has been 
until now no attempt made to determine the two simultaneously for the 
same plant. In the present investigations plants were caused to wilt under 
controlled conditions, and observations were made at short intervals of 
stomatal aperture, transpiration and evaporation rates, and of environmental 
conditions such as temperature, relative humidity, and wind velocity. Since 
the whole wilting period during which observations were made was usually 
of less than two hours’ duration, and since very frequent observations were 
necessary, the technical details of the experiment were not simple, and 
therefore merit some description. 
Detached shoots, set up in potometers, were used in this work, the 
usual precautions being observed in severing the shoots from the plants. 
Potometers were used in preference to potted plants in order that observa- 
tions could be made on the rate of water absorption by the stem as well as 
on the rate of water loss. In this way it was. possible to detect any changes 
in the water-content of the plant during the preliminary period before the 
water-supply to the stem was intentionally cut off. The potometers were 
placed in the air flue previously described (3), by means of which the move- 
ment of the air passing over the plant could be maintained at a constant 
velocity. Throughout the experiments the velocity of the air stream has 
been seven metres per minute. Owing to the short duration of the experi- 
D d 
