126 Introduction to Botany. 



tion at B, after being touched or shaken. If one of the 

 leaflets of the upper pair be lightly touched, it will rise, 

 and its mate will quickly rise with it ; the next pair of leaf- 

 lets soon fold together in a similar manner, and so on with 

 succeeding pairs, until all of the leaflets of a secondary 

 petiole have responded to the stimulus received by the 

 upper leaflet ; indeed, all of the leaflets on all of the sec- 

 ondary petioles may respond to a stimulus received by a 



single leaflet. 



There is, therefore, a 

 transmission of a stimulus 

 from one part of the plant 

 body to another, such as 

 takes place in animals by 



means of the nerves ; but 

 FIG. 62. -1,1 



in plants there are no struc- 



A, mature leaf of the Sensitive Plant fully tures Corresponding to the 

 expanded; B, the same after stimula- 



tion. After DUCHARTRE. nerves of animals, and the 



exact manner of the trans- 

 mission of a stimulus is not positively known. It is known, 

 however, that in the case of the sensitive plant the trans- 

 mission can take place through portions of the petiole 

 which have been killed. Whatever the method of trans- 

 mission may be, the reception of the stimulus is by the 

 living protoplast, and the reactions which follow are 

 caused by it. The benefits to be derived from actions of 

 this kind are obvious : in rain or hail storms, or in strong 

 winds, the leaflets fold together, and become much less 

 liable to injury. Mimosa pudica affords an example of 

 extreme sensitiveness. There are many other sensitive 

 plants belonging to the same family which are of common 

 occurrence; Cassia chamcecrista and nictitans and Schrankia 

 uncinata are good examples. 



