1882.] 



on the Excitability of Plants. 



161 



at all, or are so in a mnch less degree. In this way only can we 

 account for the bending inwards of the lobe. In the nnexcited state 

 both lavers are eqnaUy turgid; as the effec-t of excitation the 

 internal layers become limp, the external remaining tense and dis- 

 tended. 



I will now endeavour to illustrate the moHons of the leaf by 

 projecting them on the screen. Here are several leaves which have 

 been prepared by attaching one of the lobes to a cork support ; the 

 other IS free, but a very small concave mirror has been attached to 



Fig. 6. 



Transverse section of lobe ©f leaf ot Dionsa comprising the root of a 



sensitive hair. 



its external surface near the margin. The image of the licrht which 

 falls on the mirror is reflected on the wall behind me. In^this wav 

 the sUghtest movement of the lobe is displaved. Bv this contrivance 

 I wish to show you two things— first, that a verv' appreciable time 

 elapses between the excitation and the mechanical effect- and 

 secondly, that when the leaf is subjected to a series of verv Untie 

 excitations, the effects accumulate until the leaf closer TMs we 

 hope to show by bringing down a camel-hair pencil several times in 

 succession on a sensitive hair, doing it so deftly that at the first touch 

 the lobe will scarcely move at all. At each successive touch it will 

 bend more than at the preceding one, until you see the lever suddenly 

 rise, indicating that the leaf has closed. The purpose which I have 

 m view is to demonstrate the contrast l^tween the motion of the 

 leaf and muscular contraction. A muscle in contracting acts as one 

 Vol. X. (Xo. 75.) 



