CHAPTER XII 



EFFECT OF INTERMITTENT LIGHT ON PHOTOSYNTHESIS 



Further evidence of the intervention of the molecular factor — Effect of 

 intermission on photosynthetic activity under sky light and under 

 Pointolite — Diminution of photosynthetic activity under moderate 

 frequency of intermission — Increased activity under higher frequency 

 — The curve of variation of activity under intermittent light. 



Consideration of the results given in the last chapter 

 leads to the conclusion that the prolongation of the induc- 

 tion-period under interrupted light is principally due to 

 the undoing of the preliminary work of inducing the photic 

 stress which precedes dissociation. Since the absorp- 

 tion-respiration factor acts relatively slowly, its effect 

 would practically vanish on diminishing the period of 

 intervening darkness in the intermittent illumination. 

 But molecular recovery, by which the preliminary work 

 is partially undone, would commence almost immediately 

 after the cessation of light, and the effect of intermittent 

 light would therefore in general be relatively ineffective as 

 compared with that of continuous light. 



Crucial evidence of the existence of molecular stress 

 preceding actual photosynthesis would be obtained by the 

 observation of the effect of exposing the plant to rapidly 

 alternating light and darkness of equal periods of, say, 

 0-5, i, 2, 3, 5 and io seconds. The results would afford an 

 independent demonstration of the relative importance of 

 the absorption-respiration and of the molecular factors. 



(i) If the photosynthetic efficiency is to any great extent 

 modified by the absorption-respiration factor, the fact 

 would be exhibited by retardation in the rate of bubbling 

 increasing with the duration of the intermittent periods of 

 darkness ; that is to say, the effective rate would be lower 



