FLUORESCENCE YIELD 7A^ VIVO 



813 



in a suspension of diatoms. In Chromatium (a purple bacterium) Vermeu- 

 len, Wassink and Reman (1937) at first found a much smaller yield — 

 between 0.005 and 0.01%. These values are incredibly low for a "fluores- 

 cent" material. (Theoretically, the lowest yield to which fluorescence 

 could sink even in a "nonfluorescent" pigment is about 0.001%, since the 

 ratio of the period of a molecular vibration and the life-time of electronic 

 excitation is lO^^^-yiO"^-^ = 10~^) A redetermination of the yield of 

 fluorescence of Chromatium by Wassink, Katz and Dorrestein (1942) in 

 fact gave considerably larger figures — of the order of 0.1%, i. e., similar to 

 those found in algae. Self-absorption may have been the cause — or at 

 least, one cause — ^of the error of the earlier determinations. 



500 550 SCO 



WAVE LENGTH, m/x 



650 



Fig. 24. 5A. Yield of fluorescence of Chlorella suspensions in relation to wave 

 length of exciting light (after Vermeulen, Wassink and Reman 1937). 4 sets of 

 measurements. 



lOOp 



450 



700 



750 



500 550 600 650 

 WAVE LENGTH, m/i 



Fig. 24. 5B. Yield of fluorescence of Chromatium suspensions in relation to 

 wave length of exciting light (after Vermeulen, Wassink and Reman 1937). 4 

 sets of measurements. 



The yield of fluorescence in live blue-green algae (Chroococcus) was esti- 

 mated by Arnold and Oppenheimer (1950) by a rather crude method (visual 

 comparison with the intensity of light scattered by a block of magnesium) ; 

 they found it to be of the order of 1.5% — about ten times higher than the 

 yield of fluorescence in green cells. Presumably, this fluorescence origi- 

 nates predominantly in phycocyanin, although chlorophyll, too, may con- 

 tribute to it. 



Interesting results were obtained in the study of the effect of wave 



