DIURNAL MOVEMENT OP UMW.^.I 591 



The above explanations having proved unsatisfactory we 

 have to search for other factors to account for the fall of 

 the leaf on the cessation of light. In this connection I 

 was struck by the extraordinary similarity of the diurnal 

 curve of the petiole of Cassia alata with that of Mimosa. 

 DIURNAL CURVE OF THE PETIOLE OF Cassia aJaia. 



Experiment 224. — The leaf of Cassia exhibits as in the 

 leaf of Mimosa a slight erectile movement after the thermal- 

 noon at 2 p.m., there is next a sudden fall after ;') p.m., 

 which continues about 9 p.m. ; after this the leaf exhibits 

 a continuous rise with the fall of temperature, till the climax 

 is reached about • G a.m. in the morning; the leaf then 



Fig. 21.5. — Diurnal record of Cafsia leaf. Note similarity wilh diurnal record of 

 Mimosa. 



undergoes a fall with rise of temperature, there being a 

 number of pulsatory movements in the forenoon, evidently 

 due to unstable balance under the opposing effects of light 

 and of rise of temperature (Fig. 215). 



