238 Knight . — The Interrelations of Stomatal Aperture, 
means necessarily the same as transpiration. The differences between the 
two can be large enough to influence the transpiration rate without causing 
any obvious change in the turgor of the leaf, as has been demonstrated by 
many of the above experiments, so that the fact that a plant is not flaccid 
is no assurance that its rate of absorption is equal to its rate of transpiration. 
Darwin (9) is of the opinion that stomata play a primary role in the regula- 
tion of transpiration, and considers (p. 418) that the water-content does not 
sensibly affect the rate of transpiration. Lloyd (19, p. 137) concluded that 
stomatal regulation of transpiration does not occur, except when the 
apertures are almost or completely closed. In his later work (20, p. 14) he 
again states that stomata f are not closely regulatory of the loss of water 
from the leaf and are ineffectual in maintaining a constant supply of leaf 
water’, basing this on the fact that in many of his experiments the stomata 
continued to open when the water-content of the plant was decreasing. The 
experiments in the present paper have confirmed Lloyd’s results. Lloyd 
considers, however, that stomata may limit transpiration ‘ in a purely passive 
manner ’. This conception is no doubt the same as that expressed on p. 138 
of his earlier work (19), namely, that there is no stomatal closure in anticipa- 
tion of wilting, to conserve the water-supply. It is clear, however, that if 
the difference between the partial pressures of water vapour on the two sides 
of the epidermis is constant, a decrease in the size of the stomatal apertures 
is bound to decrease the rate at which moisture diffuses through to the 
outer air, so that a change in the size of the stomatal apertures must result 
in a change in the transpiration rate. Limitation of transpiration by stomata 
in this manner is presumably what Lloyd conceives as the ‘passive’ regula- 
tion. It is unlikely that any one would maintain that stomata are capable of 
adaptive closure in anticipation of wilting, if by ‘ wilting ’ is meant a decrease 
in the water-content of the leaf. By ‘closure in anticipation of wilting’, 
Lloyd doubtless means closure in response to a reduction of water-content, 
thereby preventing the further loss which would otherwise occur. The 
response of stomata to loss of water from the leaf has been indicated by the 
experiments of Darwin and Pertz (11), of Lloyd (20), and of Laidlaw and 
Knight (16), and also in the present work. A small decrease in the water- 
content of the leaf does not cause the stomata to close, and as the loss of 
water proceeds the first noticeable effect of wilting is to cause the stomata 
to open. 1 Closure finally takes place only at a comparatively late stage of 
wilting. 
Iljin (13) concluded that an excessive transpiration rate caused stomatal 
closure, but his statements that open stomata are often found in wilted plants, 
and that the rate of stomatal closure is not dependent upon the rate at which 
the water-content of the plant is reduced, confirm the work mentioned 
above. 
1 Lloyd failed to find any evidence of preliminary stomatal opening due to wilting. 
