212 CHAP. XIX. RFXURREXT LIGHT AND DARKNESS 



The effects of rise of temperature and of light have been 

 shown to be antagonistic to each other. The opening 

 movement under Hght in the forenoon has to be carried out 

 against the closure-tendency due to rise of temperature. 

 Light, therefore, is the predominant factor in the diurnal 

 movement of the leaflet of Cassia. The closure-effect of 

 darkness at night, on the other hand, overpowers the 

 tendency to movement of opening due to fall of temperature. 



Diurnal Movement of the Leaflet of Cassia alata 



Experiment 121. — I next took the diurnal record of the 

 leaflet from 4 p.m. till i p.m. next day. As the leaflets were 

 open previously from i p.m. to 4 p.m., the record of this 



Fig. 121. Diurnal movement of the leaflet of Cassia alata. 

 Closure-movement commenced at 5 p.m. and was completed 

 by 9 P.M. Leaflet began to open at 5 a.m. 



period is omitted. In the diurnal record (fig. 121) the 

 first thick dot was made at 4 p.m. ; successive thick dots 

 are at intervals of an hour, the thin dots being at intervals 

 of 15 minutes. It will be seen that a rapid movement of 



