DIURNAL THERMONASTIC MOVEMENT 



551 



morning with the appearance of light. But the flowers 

 often remain open till ten or eleven in the forenoon. I 

 have sometimes succeeded in keeping the flower open for 

 greater part of the day by lowering the temperature of 

 the plant-chamber. The movement of the flower thus appeared 

 to be associated with variation of temperature rather 

 than of light. 



Action of light : Experiment 208. — I investigated the 

 effect of light on the movement of opening or of closing 



of the flower. One of the petals 

 was attached to the recording 

 lever ; light from an arc lamp 

 was made to act diffusely on the 

 petal ; this was done by means of 

 two inclined mirrors by which 

 the divergent horizontal beam of 

 light was thrown on the upper 

 and lower sides. The record in 

 figure 197 shows that light induced 

 a movement of opening, followed 

 by closure in darkness. Since light 

 induces a movement of opening, 

 and darkness brings about a 

 closure, the opening of the flower 

 at night could not be due to darkness. We have therefore 

 to look for a different cause for the diurnal movement of 

 the flower. 



Effect of variation of temperature. — I have already 

 described an experiment which proves that rise of tempera- 

 ture induces a movement of closure of the floral leaves of 

 Nympluea. lowering of temperature producing the opposite 

 effect (p. 311). 



From the study of the action of light and of varia- 

 tion of temperature, it •will be seen that the flower of 



40 



Fig. 197.— Response to light 

 applied successively for 1 

 minute. Down-curve shows 

 movement of opening followed 

 bv recovery in darkness. 

 (Nttmphfea). 



