1106 THE LIGHT FACTOR. II. QUANTUM YIELD CHAP. 29 



a sharp beam was now thrown on the bottom of the vessel (cross section 

 of the beam, about 3 cm. 2; bottom area, 8.3 cm. 2). The total light flux 

 (red light, 630-650 m/x) was 0.2-0.6 Meinstein/min., i.e. 0.07-0.2 /xeinstein 

 per cm. 2 min. — ca. ten times higher than the intensity at which quantum 

 yields of 0.25 had been obtained in 1948. Because of the extremely high 

 density of the suspension, practically all this light was absorbed within a 

 1 mm. thick bottom layer (0.3 cc.) of the suspension; thus, at any given 

 time, >95% of the cells were in darkness, while <5% were exposed to 

 light, the incident intensity of which was close to the saturating value 

 (the photosynthesis of light-adapted Chlorella is saturated, in red light, 

 in a flux of about 0.5 jueinstein/cm.^ min.). 



Intermittency Effect. The finding of the highest quantum yields 

 ever observed when the incident light was of almost saturating intensity 

 appears startling. Warburg, Burk and co-workers explained this paradox 

 by the intermittency of illumination : because of fast shaking, individual 

 cells remain only for a very short while in the illuminated zone, and then 

 plunge into darkness. (Assuming uniform stirring, each cell must spend 

 >95% of the total "illumination time" in darkness, and less than 5% in 

 Hght). Warburg and Burk proclaimed as a "new principle" that this 

 type of intermittency of illumination permits maximum light utilization. 

 This assertion is not easily reconciled with the results of experiments in 

 flashing light, to be discussed in chapter 34: 



According to these experiments, intermittent illumination cannot in- 

 crease light utilization above the maximiun value possible in steady low 

 light. All that intermittency can do is to bring the quantum yield in parti- 

 ally or even completely saturating light close to— but never quite up to — 

 the quantum yield in low steady light. 



For the quantum yield increase caused by intermittency to be at all 

 significant, the light periods must not be longer than the "Emerson-Arnold 

 period" (0.01 sec. at 20° C, c/. chapter 34), allowing the limiting catalyst to 

 work in the dark, after the flash is over, for a period of time which is sig- 

 nificant compared to the duration of the flash itself. It is doubtful, however, 

 whether shaking at the rate of 2.5 swings per second could lead to illumina- 

 tion flashes of 0.01 sec, or shorter. 



Certainly, the "dark periods" in Warburg and Burk's experiments 

 must have been :»0.01 sec; therefore, the assumption of Warburg and 

 Burk, that under the conditions of their experiments all cells are engaged 

 uniformly in photosynthesis throughout the "illumination period," requires 

 revision of the major conclusions derived from experiments in flashing light. 

 However, this assumption is not necessary for the validity of their argument 

 (while the duration of the light period, mentioned in the preceding para- 

 graph, is of crucial importance). 



