592 LTPE MOVEMENTS IN PLANTS 



The reason of. this similarity between the records of 

 Cassia and Mimosa was found in the fact : 



(1) That the main pulvinua of the leaf of Cassia is, 

 like the pulvinus of Mimosa, differentially excitable, the 

 lower half being more excitable than the upper. This is 

 demonstrated by sending a diffuse electric shock through 

 the leaf, the response being by a fall of the leaf due to the 

 greater contraction of the lower half of the pulvinus. 

 The leaf recovered after an interval of 20 minutes, the curve 

 of response being similar to that of Mimosa. The only 

 difference between the two organs is in the lesser excitabi- 

 lity of the pulvinus of Cassia, on account of which a 

 greater intensity of shock is necessary for producing the 

 responsive fall. 



(2) The responses to light are the same in both as 

 will be seen in the following experiment. 



Experiment 225. — In Cassia, as in Mimosa, light acting 



Via. 216. — Post-maximum after-effect of light on response of leaf of Casaia. 

 There is aa over- shooting on cessation of light at arrow witliin a circle. 



from above induces at first an erectile movement which 

 reaches a maximum ; after this there is a neutralisation 



