374 Knight. — Further Observations on the Transpiration, 
This is no doubt due to the fall in leaf water-content and indicates the onset 
of wilting. This constitutes a special case of the well-known generaliza- 
tion which has now been frequently demonstrated, that in its daily course j 
the transpiration rate reaches a maximum before the atmospheric evaporat- 
ing power, owing to the check exerted by the decreasing water-content of 
the leaf. 
Having demonstrated by this series of experiments the occurrence of 
a rise of transpiration rate accompanying the temporary opening of the 
stomata which occurs during wilting, it is necessary to consider these 
phenomena in relation to the rise of transpiration rate which Bakke has 
recorded in association with permanent wilting. It has been pointed out 
earlier that the interval elapsing before the appearance of the transpiration 
increase accompanying permanent wilting is very much longer than the 
interval in the present experiments, but in such a connexion time intervals 
are useless for comparison unless some criterion of physiological condition 
is available. It may be urg^d that in Bakke’s experiments the plants were 
wilting slowly as the result of the gradual failure of the soil water-supply, 
whereas in the present investigation the velocity of wilting was much greater 
owing to the sudden complete cessation of the water-supply. This is true, 
but it is equally true that in the present case, even with the more rapid 
wilting, the plants never approached the stage of permanent wilting. In 
order to be sure of this, at the end of each experiment, when transpiration 
rate was falling and the stomata were closing, the stopcock of the potometer 
was reopened, allowing water to enter the cut stem again. In every case 
the shoot recovered its turgidity in the ordinary greenhouse atmosphere 
without recourse to increased humidity, thus demonstrating that the 
permanent wilting stage had not been reached. It is evident, then, that 
the stomatal opening and rise of transpiration rate which have been shown 
to accompany the initial stages of wilting cannot be associated with the 
rise of transpiration found by Bakke at the permanent wilting stage. 
This statement is not intended to rule out the possibility of the recurrence of 
stomatal opening at a later stage of wilting. In the course of a large number 
of experiments on the behaviour of stomata during wilting, the writer has 
never seen any indication of such recurrence, but, although leaves have 
frequently been allowed to wilt to extreme flaccidity, in none of the 
experiments has either Bakke’s or Shive and Livingston’s test for permanent 
wilting been applied. 
An attempt was next made to discover whether the attainment of the 
maxima of stomatal aperture and transpiration rate represented a stage of 
the wilting process which could be associated with a definite water-content 
of the leaf. The relation between the time occupied in reaching these 
maxima and the air temperature during wilting appeared to indicate that 
these maxima might be found to occur when a definite quantity of water 
