EFFECT OF ALTERNATION OF LIGHT AND DARKNESS 541 



thinner dots being at intervals of 15 minutes. It will be 

 seen that a rapid movement of closure was initiated at 

 5 p.m. when the light is undergoing a rapid diminution. 

 The movement of closure is completed at about 9 p.m. 

 The leaflets remain closed till 5 a.m. next morning, after 

 which they begin to open ; this opening may commence 

 even an hour earlier. It shouhl be borne in mind in 

 this connection, that since light and rise of temperature 

 are antagonistic in their reactions, the effects of light and 

 fall of temperature would be concordant ; and the opening 

 in the early hours may possibly be hastened by the low 

 temperature in the morning. The leaflets open to their 

 utmost by 9 a.m., and they remain opeu till the afternoon. 

 The plant is so extremely sensitive to light that any slight 

 fluctuation is followed by responsive movement of the 

 leaflet. Thus the transitory passage of a cloud is marked 

 in the record by a short-lived closure movement. 



DiURNAL MOVEMENT OF THE TERMINAL LEAFLET OF 

 Desmodium (jijrans. 



Both the petiole, and the terminal leaflet of this plant 



Fm. 193.— The day and night positions of the petiole and terminal leaflet of 

 Desmodium gyrans. 



exhibit very marked nyctitropic movement. The petiole is 

 raised and becomes almost erect in the evening, while the 



