214 CHAP. XIX. RECURRENT LIGHT AND DARKNESS 



Experiment 122. — The petiole was held fixed, and the 

 terminal leaflet was attached to the recording lever. Under 

 natural conditions, daylight 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, so that the daily curve appears as a single 

 large pulse on which smaller autonomous pulsations may 

 be superposed. 



The diurnal curve (fig. 123) exhibits a sudden fall at 

 about 5 P.M., resulting from the rapid waning of afternoon 



Fig. 123. Record of diurnal movement of the terminal leaflet of 

 Desmodium gyrans. Up-curve represents down-movement. 



light, till by 6.30 p.m. the leaflet becomes closely pressed 

 against the petiole. 



The question arises whether or not variation of tempera- 

 ture has any marked effect on the diurnal movement of 

 the leaflet. It has been explained that when an organ of 

 a plant is sensitive to variation of temperature, the record 

 exhibits a flexure at about 2 p.m., when there is a change 

 from rise to fall of temperature. No such flexure was, 

 however, observable at that period. But the sensibility 

 of the leaflet to variation of light is seen in the rapid closure 

 movement about 5 p.m. The leaflet remains tightly closed 

 throughout the night, and begins to open and spread out 



