TRANSPIRATION 101 
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 legs 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 
some significance when it is remembered that the guard- 
cells of the stomata contain this pigment. The nature of 
the action of chlorophyll in this direction is not, however, 
fully understood. 
Apart from direct radiation, the temperature of the 
air and its hygrometric condition are important factors 
in causing an increase or a diminution of the watery vapour 
exhaled. They act principally by exerting an influence 
directly upon the evaporation from the cells, but several 
indirect effects can also be noticed. The general movements 
of water in the plant, as well as its absorption, are influenced 
particularly by variations of temperature, and the latter has 
also an effect upon the width of the stomatal orifices. A 
rise of the external temperature causes the saturated air in 
the intercellular passages to expand, as the air acquires 
the new temperature more rapidly than do the tissues of the 
plant. The escape of vapour is consequently accelerated 
as the temperature rises, even though the rate of evaporation 
