SUMMARY 215 



early in the morning at about 5 A.M. This up-movement is 

 also very rapid, and the leaflet assumes its fullest outspread 

 position by 7 a.m. It remains in this position till the after- 

 noon, after which the cycle is repeated. As the leaflet is 

 very sensitive, the position of equilibrium of the leaflet is 

 hable to be disturbed by the slightest fluctuation of light 

 from the sky, which often gives rise to a wavy outline in 

 the record. The leaflet, moreover, has a tendency to exhibit 

 rhythmic pulsations. 



In the leaflets of Cassia and Desmodium the daily move- 

 ments are determined by light rather than temperature, the 

 plant being more responsive to the former than to the latter. 



Summary 



Rise of temperature induces a movement of closure of 

 the leaflet of Cassia, fall of temperature inducing the opposite 

 movement. 



Artificial darkness induces a movement of closure, re- 

 exposure to light brings about the opening of the leaflets. 

 These are so extremely sensitive to light that closure- 

 movement is induced by the transitory passage of a cloud. 



The effect of rise of temperature is antagonistic to the 

 action of light. The movement of opening during the 

 course of the day is due to the response to light overpowering 

 the response to rise of temperature. 



Under daily variation of hght and darkness the move- 

 ment of closure is initiated at about 5 p.m., when the light 

 is undergoing rapid diminution. The movement of closure 

 is complete by 9 p.m. The leaflets remain closed till about 

 5 A.M. next morning, when they begin to open and become 

 fully expanded by 9 a.m. 



The terminal leaflet of Desmodium exhibits a diurnal 

 movement which is very similar to that of the leaflet of 

 Cassia. It begins to open early in the morning and remains 

 outspread during the day ; it exhibits a rapid down-movement 

 after 5 p.m. and becomes closely pressed against the petiole 

 in the course of about 2 hours. 



