1904] DARWIN: MOVEMENTS OF STOMATA 87 
interpreted to mean that the stomata had opened more widely. 
This is not of great practical importance, since the method is 
used chiefly to demonstrate sudden changes in the condition of 
the stomata. But in taking readings of the closure of the 
stomata occurring naturally at night, it would be necessary to 
take account of the effect of the dampness of the night air. 
One way of counteracting the error is to place on the control 
bulb a leaf of a species whose stomata do not close at night. 
Another method, which is more easily managed, is to use fora 
control a wet strip of linen.6 Any change in the moisture of the 
air will affect both leaf and linen in roughly the same way, and 
thus an increase in the leaf temperature may be put down as 
practically all due to closure of the stomata. It should be noted 
that the effect of moist air is not nearly so great in a resistance 
thermometer as in an ordinary wet and dry bulb thermometer, 
no doubt because the temperature of the bulbs is maintained ~ 
above that of the air. It follows that the error against which 
the use of a wet bulb is meant to guard is not nearly so great as 
might be expected. 
EXPERIMENT 106. March 14,1901. 
Bulb B was covered with a strip of wet linen, while bulb R 
was exposed to the air. By watering the floor and the hot pipes, 
the air was made damp, as shown by the following readings : 
ene fe “ Per cent. 
tas eee . were oor a 479° 68 
Watered floor 
A:5 - - 15.8 eo 
AG = a ee 14.0 83 
4517 . 16.0 a oe 89 
4:19% - - 16.0 15.2 9! 
Brit =e 14.9 12.8 77 
Between 4:0 p. M. and 4:19% P. M. the difference between the 
wet and dry bulb mercury thermometers decreased from 3.0° C. 
to 0.8", #4, by 22. 1 oe resistance thermometers behaved 
like an ordinary wet and dry bulb psychrometer, the difference 
It must be separated by a layer of oil-silk from the surface of the bulb, lest the 
thermometer wire should be wetted through a crack in the varnish by which it is 
covered. 
