CHAPTER XIX 



EFFECT OF RECURRENT LIGHT AND DARKNESS ON 

 MOVEMENTS OF PLANTS 



For the demonstration of the effect of recurrent change of 

 light and darkness in the course of day and night, the 

 experimental plants must be such as have organs which 

 readily respond to photic stimulation. One of these is 

 the leaf of Cassia alata, the petiole of which carries a 



Fig. ii8. Leaflets of Cassia alata : open in daytime, and closed 



in evening. 



number of paired leaflets, each of which is about 5 cm. 

 long and 2-5 cm. broad. At night each pair of leaflets fold 

 themselves in a forward direction (fig. 118). With the 

 appearance of light they open at first in a lateral direction ; 

 later on there is a twist of the pulvinus by which the inner 

 surface of the leaflets faces light coming from above. The 

 diurnal movements of the leaflets will be shown to be due 

 to the predominant effect of variation of light. 



It will be convenient to begin with a general description 

 of the experimental methods employed, and of the apparatus 

 by which diurnal movements are recorded. 



