94 CHAP. XII. EFFECT OF INTERMITTENT LIGHT 



retardation, as will be clearly seen by the marked shifting 

 of the last dot to the right in the series of records, the 

 increasing retardation being associated with decreasing 

 photosynthetic efficiency. 



I next increased the intermission-period to 10 seconds, 

 the record of which is not given in the illustration. The 

 bubbling-period, which was 55 seconds at 5 seconds' inter- 

 mission, was now shortened to 52 seconds, and the photo- 

 synthetic activity increased to 138-4. There is thus a 

 turning-point, which is crucial evidence of the reality of 

 a photo-molecular strain. 



The results show that increasing the intermission-period 

 produces diminution of photosynthetic efficiency ; but a 

 maximum period is attained after which there is a turning- 

 point, any longer period of intermission giving rise to a 

 partial recovery in photosynthetic effectiveness. These 

 results are in perfect agreement with the theoretical con- 

 siderations which have been put forward. The turning- 

 point is sometimes found at an intermission-period rather 

 longer than 5 seconds. The effect would obviously be 

 modified to a certain extent by physiological factors such 

 as the condition of the tissue, temperature, and the intensity 

 of light. 



Intermittent Pointolite Illumination 



Though intermittent illumination is, generally speaking, 

 less effective than continuous light, yet the second line of 

 the automatic record (fig. 27) shows that for very short 

 intermission the effectiveness may be greater ; for the fourth 

 bubble is seen to have occurred in a total period of illumina- 

 tion shorter than 80 seconds. This result was so unexpected 

 that I was at first inclined to believe that it might have 

 been brought about by some accidental variation in the 

 intensity of light from the sky. I therefore employed in 

 the present series of experiments the constant light of 

 Pointolite. The intermission of light was also very accu- 



