VARIATION OF LIGHT 



211 



the evening, on the other hand, will be due to falling tem- 

 perature and to darkness. The individual effect of each of 

 these factors is not known, and it is therefore necessary to 

 determine the relative effects of variation of temperature 

 and of light. 



Effect of Variation of Temperature 



Experiment 119. — The plant was enclosed in the glass 

 chamber and the experiment was commenced at midday, 

 when the leaflets were open, under uniformly diffused light. 

 The temperature was artificial^ raised by means of an 

 electric heater placed in the chamber, and lowered by the 

 introduction of cold air. One of the leaflets 

 was attached to the recording lever, and 

 its movement, up or down, indicated the 

 opening or closing movement of the leaflet. 

 The records showed that rise of tempera- 

 ture induced a movement of closure, while 

 fall of temperature brought about a move- 

 ment of opening. 



Effect of Variation of Light 



Experiment 120. — This experiment was 

 also carried out at midday, when the leaflets 

 were fully open. The horizontal part of the 

 record in fig. 120 represents the stationary 

 expanded condition of the leaflet ; a black 

 cloth was put over the glass chamber at 

 I P.M., and the effect of darkness was 

 recorded for one hour. Darkness is seen to 

 have initiated a movement of closure which 

 increased at a rapid rate ; the black cloth 

 was then removed, and the movement of opening under light 

 was completed in the course of five quarters of an hour. 

 A passing cloud causes an immediate movement of closure, 

 proving how very sensitive is the leaflet to variation of light- 



Fig. 120. Effect of 

 sudden darkening 

 at arrow, produc- 

 ing movement 

 of closure (up- 

 curve). Restora- 

 tion of light in- 

 duces movement 

 of opening (down- 

 curve). Successive 

 dots at intervals of 

 1 5 minutes. (Leaf- 

 let of Cassia.) 



