152 



Dr. J. Burdon-Sanderson 



[June 9, 



acquainted with the mechanism by which the excitable motions of 

 i:ilants are brought about, I will confine myself to a very few 

 examples, selecting, of course, those which have been most carefully 

 investigated. 



Every one is acquainted with the general aspect of the sensitive 

 plant. Probably, also, most persons have observed the way in which 

 the leaves behave when one of them is touched, namely, that the leaf, 

 instead of being directed upwards, suddenly falls, as if it had lost its 



Fig. 1. 



Leaf of Mimosa : a, in the unexcited state ; 6, after excitation (after Pfeffer). 



power of supporting itself, and that the little leaflets which spring 

 from the side-stalks fold together upwards (Fig. 1). But perhaps 

 every one has not observed exactly how this motion is accomplished, 

 namely, that by means of little cylindrical organs the leaflets are 



