458 CHARACTER OF VEGETABLE VITALITY. CHAP. XI. 



SECTION II. 



Irritability. 



PLANTS are not only susceptible to the action of 

 the natural stimuli of light and heat, exciting them 

 gradually to the exercise of the functions of their 

 different organs in the regular progress of vegeta- 

 tion ; they are susceptible also to the action of a 

 variety of accidental or artificial stimuli, from the 

 application of which they are found to give indica- 

 tions of being endowed also with a property similar to 

 what we call irritability in the animal system. 

 This property is well exemplified in the genus Mi* 

 mosa ; but particularly in that species known by the 

 name of the Sensitive Plant. 

 Exempli- If a leafit of this plant is but touched, however 



fiedinthe 



Sensitive slightly, by any extraneous body, it immediately 

 ant> shrinks into itself, and communicates the impulse, 

 if strong, perhaps to the whole wing, each leafit 

 shrinking, or each pair of leafits collapsing in suc- 

 cession, and the leaf-stalk itself sinking downwards 

 as if by a joint, at its point of union with the stem. 

 The following experiments were made by Duhamel 

 with a view to ascertain the extent of its susceptibi- 

 lity :* At eight o'clock in the morning of a day in 

 September a leaf-stalk of a Sensitive Plant formed 

 with the lower part of the stem an angle of 135, 

 which upon being touched fell to an angle of 80; 

 * Phys. des Arb. liv, iv. chap, vi, 



