TRANSPIRATION 429 



The protoplasm under the stimulus allows more vapour to pass 

 through it, as that of the cortex of the root permitted water to 

 pass out of its cells under the stimulus of over-turgescence. 



The ultimate exudation of watery vapour is thus seen to be 

 chiefly carried out through the stomata of the green parts, at 

 any rate in those plants which possess them. The mechanism of 

 the stomata has already been alluded to. The guard-cells which 

 surroTind the aperture are capable of varying turgescence, the 

 water entering and leaving them from the other cells of the 

 epidermis, which abut upon them. Those walls of the guard- 

 cells, which abut upon the air space underlying them, are, as a 

 rule, thickened and cuticularised (fig. 1185), so that vapour can- 

 not enter them. The wall between them and the next epidermal 

 cell is thin and readily allows of osmosis. The guard-cells 

 themselves differ from the rest of the 

 epidermis in the higher plants by con- "' '-^°°- 



taining ohloroplasts, which suggests the 

 presence in them of more osmotic sub- 

 stances than the other epidermal cells 

 contain. The guard-ceUs are so attached 

 to each other that turgescence causes them 

 to separate, opening the aperture. Loss of 

 water induces in them a flaccid condition, 

 and their edges fall together and partly or 

 wholly close the sHt. Hence, when the '''Snth'Lf rEpMeS^; 

 cells of the epidermis are turgid, the sto- <=ells. i. Opening of the 

 1 . 1 , J 1 • J stoma, p. Parencliyma 



mata open ; when they lose their turges- of tlie leaf. 



cence these organs are closed. Thus the 



escape of watery vapour is accelerated or retarded by their 

 'action. 



Transpiration is markedly increased by light, rising to many 

 times its original amount when a plant is transported from 

 darkness into sunshine. This is not due to the rise of temperature 

 accompanying the illumination, though the greater warmth may 

 have some influence upon it. The effect of light upon the stomata 

 is to cause them to open ; hence they are open during the day and 

 closed during the night. It is probable, however, that tViis is 

 not directly due to the light. If coloured light be employed, it 

 is found that the rays which influence the opening or closing are 

 those which are absorbed by chlorophyll — a fact which has some 

 significance when it is remembered that the guard-cells contain 

 that pigment. 



Other external conditions which have an influence upon this 



