54 Laidlaw and Knight.— A Description of a Recording 
aperture shown in Fig. 3, the opening reached a maximum in both cases 
about five minutes after the severance from the plant, whilst in the case 
of the thick leaf of Prunus Laurocerasus , even when the temperature was 
20 0 C., tending to produce rapid wilting, nearly twenty minutes elapsed 
before the maximum opening was reached. 
Similarly, with any particular plant, a higher temperature, tending 
to produce more rapid wilting, causes the maximum to be reached more 
quickly, and a roughly graded series may be obtained, as in the following 
experiments upon Maranta bicolor : 
9 
Experiment No. 
Mean greenhouse 
temperature (° C.). 
Twie elapsing before 
max. opening is reached. 
100 
16* 3 
35 minutes. 
102 A 
18*0 
13 
102 c 
18-0 
T 4 ?> 
103 
18-8 
20 „ 
106 
20*5 
9 
The extent to which the stomata open on severing the petiole is 
variable, depending probably upon the plant used, and possibly also upon 
the rate at which the leaf wilts. The greatest opening observed was with 
a leaf of Phaseolus vulgaris , in which the stomatal aperture, as deduced from 
the speed of the air-stream, being represented by the value 168 before 
cutting the leaf from the plant, increased to 475 within five minutes after 
cutting. 
The results of these experiments are thus in close agreement with those 
obtained by Darwin and Pertz. 
In his first reference to the phenomenon, Darwin suggested ( 4 , p. 617) 
that the temporary stomatal opening is a direct result of wilting, and is due 
to the guard-cells retaining their water, and therefore remaining turgid 
longer than the other epidermal cells. Thus the pressure of the rest of the 
epidermis upon the guard-cells is reduced early in the process of wilting, 
with the result that the size of the stomatal pore increases. 
The explanation offered by Darwin is no doubt the most probable one, 
but no experimental evidence was adduced in support of it, and alternatively 
there seem to be two other possible explanations which would fit the 
observed facts. The stomatal opening might be due, not to any effect 
of wilting, but to the shock sustained by the leaf as a result of severing 
it from the plant. In another connexion it has been demonstrated that the 
mere handling of some leaves may produce stomatal closure, but opening has 
never been observed to occur as the result of shock. 
Another possibility was that the change in the porometer readings 
on detaching a leaf from the plant might be due to some cause other than 
stomatal opening. 
In the unpublished work above referred to, it was observed that in 
