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 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 

 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 



