LECT. II.] VITAL FUNCTIONS OP PLANTS. 67 



agents which are considered as natural stimulants 

 to the animal body, as heat, light, air, and food, 

 act also on the irritability of the vegetable ; and 

 their moderate application is absolutely necessary 

 for the preservation of the life and health of a 

 plant. When they are withdrawn the irritability 

 accumulates, and the plant languishes ; when they 

 are applied in excess the vital powers are exhausted, 

 and the plant also languishes ; and if they be 

 withdrawn, and then again applied suddenly, and 

 in too great quantity, the plant dies, exactly in 

 the same manner as an animal who is killed by 

 indulging in a full meal after having been nearly 

 starved. 



To bring forward examples in proof of the ex- 

 istence of this principle in plants, in this stage of 

 our inquiries, would be anticipating, our future 

 observations, nor could they be fully understood 

 without a knowledge of the structure of the dif- 

 ferent organs in which it is more peculiarly dis- 

 played; we shall, therefore, only observe generally, 

 that to it must be referred all the motions of plants, 

 both general and partial ; except such as depend 

 upon the agitation of the air, or winds, and other 

 mechanical causes, and which, like exercise to an 

 animal, are absolutely necessary for the preserva- 

 tion of the health and vigour of the vegetable. I 

 shall only now observe, that among the motions 

 depending on irritability are those observed in some 



