Stomata , Leaf Water-content , and Wilting of Plants. 363 
present in the path of the transpiration stream would gradually increase as 
the summer advanced. It is true that the root system steadily develops, 
but not only does the total area of the transpiring surface of the plant 
steadily increase, but in addition the atmospheric evaporating power also 
gradually increases and soil moisture diminishes. All these factors would 
tend to increase the tension on the water columns. The actual wilting 
process may obviously be reg. . ded as the result of a further extension of 
this general seasonal tendency of the environment. As the plant passes 
from a state of turgor to the wilted condition, there is no reason to expect 
that at any particular stage all the water columns are suddenly ruptured. 
It is more natural to suppose that the replacement of water by air is 
a gradual process during wilting, as during the ordinary seasonal changes, 
the water columns being severed one by one with the increasing tension, 
until the number remaining unbroken cannot supply the leaves with 
sufficient water to keep the plant alive. This view is susceptible to experi- 
mental test by means of determinations of density changes during wilting, 
on the lines of Farmer’s observations. It is hoped shortly to carry out an 
investigation of this point. 
Having demonstrated the fact that during wilting the plant reaches 
a stage at which its transpiring power ceases to diminish and suddenly 
increases, Bakke (2) sought to discover possible explanations other than his 
theory of the rupture of the water columns. He attempted to establish 
a correlation between the increase of transpiring power at the ‘ permanent 
wilting’ stage and the opening of the stomata which Darwin (7, 8) and 
Darwin and Pertz (9) showed to occur on wilting. Darwin’s original 
methods estimate water loss and not stomatal aperture, but the porometer 
method of Darwin and Pertz was more direct and confirmed the previous 
results. Lloyd, using his alcohol method, failed to demonstrate the stomatal 
opening on wilting, but Laidlaw and Knight (16), with the porometer, 
obtained results 1 in agreement with those of Darwin and Pertz. Bakke (2) 
failed to find any appreciable stomatal opening during wilting, but rightly 
insists that it is important that the question of stomatal influence on trans- 
piration during wilting should be settled. 
It may be considered significant that investigators who have demon- 
strated the occurrence of stomatal opening during wilting have all used the 
porometer method, indicating that the result obtained might be due to some 
inherent quality of the method. It has been shown, however (16), that 
there is no reason to suppose that the method is at fault in this particular 
case. A study of the limitations of the method has already been made (14), 
but it is perhaps desirable to mention some remarks of Gray and Peirce 
(12) upon the method. They state that ‘ Obviously, stomata within 
a porometer are shut off from both light and air’. It is true that the 
1 For a consideration of the exph !; tion of this phenomenon, see the paper referred to. 
