TRANSPIRATION 95 
tiguous to them. When, therefore, the epidermal cells are 
charged with water, this is osmotically drawn into the 
guard-cells, which become turgid, and consequently separate, 
opening the aperture. When the contiguous epidermal cells 
lose their water, the osmotic constituents of their contents 
become more concentrated, as these do not leave the cells 
with the water. The direction of the osmotic stream is 
consequently reversed, the guard-cells lose some of their 
turgidity, so that their edges fall together and partially or 
wholly close the slit. Thus the escape of watery vapour 
is accelerated or retarded by their action. 
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 piant, 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 
higher temperature of the air accompanying their passage. 
The light has, indeed, an influence apart from the heat. 
No doubt, so far as the visible rays of the spectrum are 
converted into heat vibrations after absorption, they must 
influence transpiration indirectly in this way. Besides 
acting thus indirectly, light has a direct effect upon the 
process, for it influences the size of the stomatal apertures. 
These have been observed to be open during the day and 
more or less completely closed during the night. The 
gaseous interchanges which light induces, in causing the 
decomposition of carbon dioxide and the evolution of 
oxygen, on the whole favour the exhalation of watery 
vapour. When green plants are exposed to light of various 
colours the most marked increase of transpiration is caused 
by the light of which the plants absorb most. This can 
be observed not only in the green parts of plants, but in 
those which are not green, as in the petals of the flowers. 
The fact that the rays which are absorbed by chloro- 
phyll are the most active in promoting the process has 
