542 LIFE MOVEMENTS IN PLANTS 



pulvinus of the terminal leaflet exhibits a sharp curvature 

 downwards (Fig. 193). 



Experimeyit 207. — The petiole was held fixed, and the 

 terminal leaflet attached to the recording lever. I have 

 already explained that light falling on the pulvinus from 

 above, induces an up-movement of the leaflet, which is 

 thus erected under light of moderate intensity. If the light 

 be strong, the transversely conducted excitation induces a 

 partial neutralisation ; very intense light may even cause 

 a reversal into down-movement. Under natural conditions, 

 day-light acting from above induces an up-movement ; 

 darkness, on the other hand, induces a rapid movement of 

 fall. The leaflets sometimes exhibit autonomous pulsations ; 

 but the diurnal movement is very strong and the daily 

 curve appears as a single large pulse on which smaller 

 autonomous pulsations may become superposed. 



The diurnal curve (Fig. 194) exhibits a sudden flexure 

 at about 5 p.m. on the rapid waning of afternoon light 

 till, by r)-30 p.m., it becomes closely pressed against the 



Flu. 194. — Diurnal record of the terminal leaflet of Deamodiiim yyrnns. Up-cnrve 

 represents movement of closure. 



petiole, by the rapid fall of the leaflet. The discriminat- 

 ing test, between effects of variation of temperature and of 



