2l8 CHAP. XX. THERMONASTIC MOVEMENT 



thermal rays being very pronounced. On subjecting the 

 leaf to thermal radiation, the movement of response was one 

 of closure (fig. 125), in sharp contrast with the movement of 



Fig. 124. Fig. 125. 



Fig. 124. Thermonastic responses of petal of Zephyranthes. 



c, closing movement due to cooling ; h, opening movement 



due to warming. 



Fig. 125. R, closing movement due to heat-radiation. Note 

 opposite responses to rise of temperature and to radiation. 



opening under rise of temperature. These experiments 

 clearly demonstrate the opposite effects of rise of temperature 

 and of radiation. 



The following is an example of negative thermonasty. 



Diurnal Movement of Nymphaea 



The Indian White Water-Lily remains closed during the 

 greater part of the day and opens at night. Figs. 126 

 and 127 are photographic reproductions of the day and 

 night positions of the flower. 



The question arises whether the diurnal movements of 

 the flower are predominantly tropic effects of photic or 

 geotropic stimulation or are due to thermonasty. 



Effect of reciiYYent light and darkness. — It has sometimes 

 been supposed that the closure and opening of this flower 

 are mainly due to the alternation of light and darkness. 



