SECT. I. EXCITABILITY. 443 



change is not effected by the stimulus of the light 

 acting on the vital principle, but rather on the 

 fibres of the leaf. But the reply is that the leaf is 

 not yet entirely deprived of the vital principle ; as 

 it is not to be supposed that the experiment would 

 succeed upon a leaf that is withered and decayed. 



Such are the effects produced. Is light the sole Light the 

 agent? It had been conjectured that the effect is" 

 partly attributable to the agency of heat ; and to 

 try the value of the conjecture Bonnet placed some 

 plants of the Atriplex in a stove heated to 25 of 

 Reaumur. Yet the stems were not inclined to the 

 side from which the greatest degree of heat came ; 

 but to a small opening in the stove. Heat then 

 does not seem to exert any perceptible influence in 

 the production of the above effects. Does mois- 

 ture? Bonnet found that the leaves of the Vine 

 exhibited the same phenomenon when immersed in 

 water as when left in the open air. Whence it 

 seems probable that light is the sole agent in the 

 production of the effects in question. 



But as light produces such effects upon the leaves. Counter 

 so darkness or the absence of light produces an effect 

 quite the contrary ; for it is known that the leaves 

 of many plants assume a very different position in 

 the night from what they have in the day. This is 

 particularly the case with winged leaves, which, 

 though fully expanded during the day, begin to 

 droop and bend down about sun-set and during the 

 fall of the evening dew, till they meet together on the 



