DIURNAL MOVEMENT OF .tfl.UOSA 



589 



withdrawal of light. But the following experiment shows 

 that the increased mechanical moment cannot be the true 

 explanation of the fall. 



Diurnal movement of the amputated petiole : Experiment 

 223. — In my present experiment the possibility of variation 

 of mechanical movement 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 evapora- 

 tion. The intense stimulus caused by amputation induced 

 the excitatory fall of the leaf, but it recovered its normal 



activity after a period of 

 three hours or so. The 

 diurnal record of the leaf 

 was commenced shortly 

 after 1 p.m. ; it will be 

 noticed that the leaf, though 

 deprived of the weight of 

 its sub-petioles, still exhibit- 

 ed a sudden fall at about 

 5 p. m. (Fig. 214). The fall 

 of the leaf cannot therefore 

 be due to increased mecha- 

 nical moment. The effect 

 of weight was, moreover, 

 eliminated in torsional res- 

 ponse {Erpt. 221). In spite 

 of this the leaf exhibited 

 a sudden movement after 

 5 p.m. 



Fig. 214.— Record of leaf of Mimosa 

 after amputation of sub-petioles. The leaf 

 fell up to 2-30 p.m., and rose till o p.m., 

 after which th»e is a spasmodic fall. 

 (Successive dots at intervals of 15 

 minutes.) 



Pfeffer has in his ' Entstehung der 



Schlafbewegung ' 

 (1907) offered another explanation of the sudden fall of the 

 leaf of Mimosa. This, according to him, is not the direct 

 effect of diminished intensity of light in the evening, but 

 is due to the release of the leaf from the phototropic action 

 of light, which, so long as it is sufficiently intense, holds the 



