TEANSPIEATION 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 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 

 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 



