oui 
102 VEGETABLE PHYSIOLOGY 
from the cells into the intercellular spaces is not at first 
affected. 
The influence of the hygrometric condition of the air, 
apart from changes of temperature, can be seen when a 
plant which has been exposed to a dry atmosphere till its 
leaves have become flaccid is transferred to one saturated 
with moisture. After a short time the drooping leaves 
again become turgid. ‘This is not due to an absorption of 
water in the form of vapour by the leaves, but to a diminished 
loss by the checking of transpiration. The return of turgidity 
is caused by the accumulation of the store drawn from the 
earth by the roots. This can be shown by comparing the 
behaviour of two plants treated in the way described, one 
of which is allowed to remain rooted in goil, while the other 
is taken up from the earth and exposed in that condition to 
the saturated air. There is in the latter case no recovery of 
turgescence. 
The temperature of the soil in which the roots of a plant 
are embedded has also an influence upon the exhalation 
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 
