VOLUME XXXVII NUMBER 2 
BOTANICAL Gazer 
| FEBRUARY, 1904 
ON A SELF-RECORDING METHOD APPLIED TO THE 
MOVEMENTS OF STOMATA. 
FRANCIS DARWIN. 
(WITH FIFTEEN FIGURES) 
IN my paper on stomata’ it was incidentally shown that the 
temperature of a leaf varies, other things being equal, with the 
condition of the stomata. Thus if there are two leaves, in one of 
which (O) the stomata are open, while in the other (C) they 
are closed, it is found that O is cooler than C. It is evident that 
this must be so because the evaporation from O is greater than 
that from C; in fact, one corresponds to the wet bulb, and the 
other to the dry bulb of a psychrometer. Inthe experiments above 
referred to the leaves employed were those of Tropaeolum, C 
being a leaf in which the stomata were closed by separating it 
from the plant, and thus allowing it to wither; while O was a leaf 
still attached to the plant, with normally open stomata. As the 
stomata of O closed in the evening, its temperature approached 
more and more to that of C, and to that of the dry bulb ther- 
mometer, while the temperature of an aquatic leaf (of a species 
in which the stomata are open at night) remained cooler than 
the dry bulb thermometer. 
These experiments suggested that the changes in the tem- 
perature of a leaf might be used, with certain precautions, as an 
index of the condition of the stomata. It was hoped that by 
the use of this method it would be possible to check and control 
* DARWIN, FRANCIS, Observations on stomata. Phil. Trans. Roy. Soc. London 
B. 190: 582. 1898. 
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