1904] DARWIN: MOVEMENTS OF STOMATA 89 
is, should affect the recorder bulbs in the same way. On the 
whole, we may conclude that the effect of variations in the 
humidity of the air on the temperature of a leaf fitted to the 
recorder is comparatively small, especially when counteracted by 
the use of a layer of damp linen on the control bulb of the instru- 
ment. And this must be some advantage in experiments like 
the present, in which it is sought to estimate the opening and 
shutting of the stomata by temperature changes. 
A possible source of error, in long-continued experiments, is 
that a leaf will transpire less in darkness than in light, quite 
apart from the closure of the stomata. This is believed to be 
due to the fact that the radiant energy absorbed by the chloro- 
plasts is in part made evident as heat. It follows that a relative 
rise of temperature occurring at night might be put down to 
closure of the stomata, when it might in reality be due to the 
checking of ‘chloro-vaporization” by darkness. The following 
experiment, however, shows that this is not a serious source of 
error. 
EXPERIMENT 102. february 22, 1901. 
A tulip leaf (attached to the plant) was placed on bulb R, an “ artificial 
leaf” made of wet linen on bulb B. Readings with the horn hygroscope 
showed that the stomata were open from 4 P. M., when the experiment began, 
until 10:32 P. M.; a slight closure had occurred, but the nocturnal reading 
corresponds to a fair amount of transpiration. The temperature of the air fell 
during the same period from 18,2° to 16.6°, but the relative moisture of the 
air remained the same, z.¢., 60 percent. The reading of the resistance ther- 
mometer remained almost constant. 
As far as one experiment may be trusted, it would seem there- 
fore that darkness, apart from its effect on the stomata, need 
not produce any serious check to transpiration. 
THE EFFECT OF WITHERING. 
One of the points investigated in my work on stomata (doc. 
cit., p. 548) was the phenomena which occur when certain leaves 
are severed from the plant. The first effect is that the stomata 
open more widely, and this ‘‘preliminary opening” is followed 
by a gradual closure of the stomata. This seemed a good sub- 
ject for the recording method, since the effects occur rapidly, 
