IRRITABILITY. 115 



where pleasant odours operate so detrimentally, the 

 nervous system is either considerably depressed, or in a 

 state of much excitement. 



159. It is quite otherwise, however, in regard to the 

 odours of poisonous plants, especially the narcotic ones, 

 the exhalation from which will produce the same effect 

 as the internal use of the plant itself. This, it is true, 

 has been ascribed to the odours themselves ; but some 

 of the plants acting most deleteriously in this manner 

 are almost odourless. The noxious principle must be 

 looked for in an evolution from the plant of its charac- 

 teristic property which thus influences the system. 



IRRITABILITY. 



160. By the term Irritability, is most properly under- 

 stood the proximate cause of certain phenomena seen to 

 take place in plants, such as motion, both spontaneous 

 and otherwise, and what is termed their sleep ; the 

 excitement of which is followed by such results as, did 

 they occur in connection with an animal organization, 

 would often be considered as evidences of a power 

 of volition in the body in which they were seen to 

 happen, or at least of the existence of a nervous 

 system. 



161. The evidences generally received as indicative 

 of the presence of irritability, may be divided into 

 two classes : 1st, Certain actions resulting from the 

 excitement of external causes. 2nd, actions seeming to 

 have their origin and excitement within the plant in 

 which they occur. 



162. Under the first division may be included what 

 is termed the sleep of plants, though we are perhaps far 

 from right, if we consider this phenomenon to depend 

 solely upon external agents as influencing the plant ; 

 though what may be the other conditions we are com- 

 pletely ignorant of. The negation or presence of light in 



