330 LIFE MOVEMENTS IN PLANTS 



pulvinus is shown by the fact that the response of the 

 lower half of the pulvinus to ten seconds' exposure is 



Fig. 118. Fig. 119. 



Fig. 118. — Series of up-responses of Aiinwsa leaf to light applied on upper 

 half of pulvinus. 



Fig. 119. — Down-responses given by the same plant on application of light 

 from below. 



even larger than that given by the upper half under the 

 prolonged exposure of 240 seconds. 



TRANSFORMATION OF POSITIVE TO NEGATIVE PHOTO- 

 TROPIC CURVATURE. 



J^xperiment 126. — A beam of light from a small arc 

 lamp was thrown on the upper half of the pulvinus. 

 After a latent period of 5 seconds, a positive curvature 

 was' initiated, by the contraction of the upper and expan- 

 sion of the lower side of the organ. But under continued 

 action of light, the excitatory impulse reached the lower 

 half of the organ, causing a rapid fall of the leaf, 

 and a negative curvatare. The arrival of transmitted excita- 

 tion at the more excitable distal half of the organ is 

 clearly demonstrated by the very rapid down-movement, 

 seen as the up-curve in the record (Fig. 120). In sensitive 

 specimens this movement is so abrupt and rapid, that the 

 writing lever is jerked off above the recording plate 

 before making a dot on it. The thickness of the 



