100 VEGETABLE PHYSIOLOGY 
The modification of the turgescence of the guard-cells 
is caused by the transference of water between them and 
the other cells of the epidermis, from which they are separated 
by thin walls. The vapour which is in the intercellular 
space below them does not penetrate them, the walls 
abutting on the space being thick and cuticularised. The 
changes in the turgor of the guard-cells are effected by 
their own protoplasm, which modifies its permeability to 
water in consequence of their receiving a stimulus caused 
by the variations in the quantity of water in the plant and 
especially in the epidermis. The commencement of wilting, 
for example, causes the protoplasm of the guard-cells to 
allow water to pass from them with increased facility, and 
they consequently diminish the size of the opening, checking 
the loss of water from the general intercellular space system, 
and hence tending to restore the general turgidity. 
It was held till recently that the guard-cells regulated 
their turgor by producing osmotic substances in their 
interior in greater or smaller amounts, so setting up osmotic 
currents between the guard-cells and the general epidermis. 
This was supported by the presence of chloroplasts in the 
guard-cells. Quantitative considerations do not support 
this view of the mechanism. A short period of darkness 
is sufficient to close the stomata, and it is unlikely that so 
short a time can reduce to a sufficient extent to explain the 
closure the amount of the osmotic substances which have 
been drawing water into the guard-cells till the failure of 
the light. Nor is there sufficient evidence that darkness 
causes a diminution of the quantity of osmotic substance 
at all. 
Transpiration is markedly increased by sunshine, rising 
to many times its original amount when a plant is trans- 
ported into it from a dim light. No doubt this is due in a 
very large measure to the heat rays which then fall upon 
the plant, and which would raise its temperature very 
dangerously were they not applied to the evaporation of 
the water. But it is not due entirely to them, nor to the 
