TRANSPIRATION 97 
tion of watery vapour, which increases as the soil is 
warmed and diminishes as it becomes cooler. 
If the protoplasts of the cells of the turgid leaves of a 
branch are stimulated by violently shaking it, the leaves 
become flaccid. The protoplasm under the stimulus allows 
more water to pass through it to the cell-walls, and hence 
evaporation is promoted. The effect may be compared 
with that which has already been mentioned as set up in 
the cells of the cortex of the root by their over-distension 
by the water which accumulates in them in consequence of 
the continuous osmotic activity of the root-hairs. The 
stimulus of this distension is responded to by the proto- 
plasm by its becoming more permeable by the water of the 
vacuoles of the cells. The response made by the protoplasts 
of the leaves to the stimulus of shaking may help to explain 
the flaccid condition observable in the foliage of certain 
trees after the prevalence of a high wind. Besides this 
effect upon the protoplasm, the continuous removal of the 
air around the transpiring organs has, no doubt, a consider- 
able influence upon the removal of the watery vapour from 
their intercellular passages. 
The effect of alteration of the external conditions upon 
transpiration may be investigated by means of Darwin’s 
potometer, which enables approximately accurate determina- 
tions of its amount to be made from time to time. This 
instrument is shown in fig. 68. It consists of a glass tube 
with a side arm which is bent upwards so as to be parallel 
with the tube itself. A capillary tube of about -2 mm. bore 
is fastened by an indiarubber cork into the lower opening 
of the tube so as just to project beyond the cork. A con- 
venient length of the capillary tube is about 20 cm. Its 
lower end dips into a small vessel of water, arranged so as 
to be easily withdrawn from the tube. The upper orifice 
of the potometer is closed by a tightly fitting cork, and the 
plant whose transpiration is to be observed is fitted into 
the side arm by means of an indiarubber band or tube 
which embraces the glass arm and the end of the cut 
7 
