SPASMODIC FALL OF LEAF 27I 



which give rise to the characteristic responses — the pre- 

 maximum, the maximum, and the post-maximum. 



Fig. 157. Photometric record showing the variation of 

 intensity of Hght from morning to evening. 



Successive dots are at intervals of 30 minutes. 



The Spasmodic Fall of the Leaf 



Pfeffer regarded the fall of the leaf which occurs abruptly 

 late in the afternoon as due to the increased mechanical 

 moment of the secondary petioles moving forward on with- 

 drawal of light. I proceed to show that this characteristic 

 movement occurs even after complete removal of the sub- 

 petioles, so that an increased mechanical moment cannot be 

 the true explanation of the fall. 



Experiment 148. Diurnal movement of the petiole after 

 removal of sub-petioles.— In this experiment the possibihty 

 of any variation of mechanical moment was obviated by 

 cutting off the end of the petiole, which carried the sub- 

 petioles. The cut end was coated with collodion flexile 

 to prevent evaporation. The intense stimulation caused 

 by the amputation induced excitatory fall of the leaf, but 



