14 



L.N.M. Buysens 



system 2 and the other compounds mentioned, and suggest that 

 QB is the primary photoreductant of system 2. 



It was observed (Duysens and Kamp, unpublished experiments) 

 by means of an oscillographic technique that by illuminating a 

 suspension of Chlorella by means of an electronic flash the 

 yield of the fluorescence at 685 m^i mainly due to chlorophyll 

 a2 could be tripled. 



10 '°einstein/cm* 

 I 



Fluorescence yield ^ at 685 mu as a function of total in- 

 cident energy , for flashes of relative intensities 

 varying from 2.6 to 100. The initial increase depends upon 

 the total energy but not upon the intensity of the flash. 

 For intense flashes for which the risetime of the fluo- 

 rescence yield is of the order of 10 ^l8ec the maximxim 

 level attained decreases. 



Fig. 4 shows that the initial increase in a rather wide range 

 of intensities studied, is proportional to the absorbed energy, 

 and independent of the intensity of the flash. If the intensi- 

 ty is sufficiently high that the risetime becomes shorter than 10 

 jisec, then the yield does not increase up to the maximum at- 

 tained at lower flash intensities. The shape of the time 

 course depends on the conditions of the algae, but the appro- 



