EFFECT OP ALTERNATION OF LIGHT AND DARKNESS 539 



falling temperature, and to darkness. The individual eftect 

 of each of these factors is not known and it is therefore 

 necessary to determine the effects of variation of tempera- 

 ture and of light, 



EFFECT OF VARIATION OF TEMPERATURE. 

 Experiment 204. — The plant was enclosed in a glass 

 chamber and exposed to diffuse light. The experiment was 

 commenced at midday, when the leaflets were open ; the 

 light was kept uniform while temperature was artificially 

 increased by means of an electric heater placed in the 

 chamber, and decreased by introducing cold air into the 

 plant chamber. One of the leaflets was aitacbed to the 

 recording lever and its movement, up or down, indicated 

 the movement of opening or closure. The records showed 

 that rise of temperature induces a movement of closure, 

 while that of fall brings about the movement of opening. 

 EFFECT OF VARIATION OF LIGHT. 



Experiment 205. — This experiment 

 was also carried out at midday, when 

 the leaflets were open. The hori- 

 zontal record in figure 191 repre- 

 sents the stationary expanded condi- 

 tion of the leaflet ; a black cloth 

 was put over the glass chamber at 

 1 p.m., and the effect of darkness 

 was recorded for one hour. Darkness 

 is seen to initiate a movement of 

 closure which increased at a rapid 

 rate ; the black cloth was removed 

 after an hour, and the movement of 

 opening under light was completed in 

 the course of five quarters of an 

 hour. It is thus seen that the 

 leaflets are extremely sensitive to the 

 action of light. 



Fig. 191.— Effect of sudden 

 darkening at arrow, produc- 

 es movement of closure 

 Cup-curve). Restoration of 

 light induces opening 

 movement (down-curve). 

 Successive dots at intervals 

 of 15 minutes. (Leaflet of 

 Casfia.) 



