66 



FLUORESCENCE SPECTROPHOTOMETRY 



with hot water before measurements. It is of course necessary to 

 heat the leaves to prevent further transformation by the Hght used 

 for exciting the fluorescence and this heating itself shifts the fluo- 

 rescence peak of protochlorophyll from 655 to about 638 m/i. 



Chlorophylls b and c 



The fluorescence band of chlorophyll b at 655 m/x in several different 

 species of plant has been reported ( Rabino witch, 1951, p. 807). This 



o 



CMerofbTli a 



PrttecbUrophyll 



600 



7S0in» 



650 700 



WAVELENGTH 



Fig. 15. Fluorescence spectrum of a partially greened barley leaf. (Young, un- 

 published. ) 



band has never been found in any of our curves of living plants. We 

 have measured very pale leaves with the hope of finding chlorophyll 

 b fluorescence which might have been reabsorbed in more highly 

 colored material, but it has never appeared. There are several possible 

 reasons for its lack. One may be that chlorophyll b is appreciably less 

 fluorescent and that it occurs in smaller amounts than chlorophyll a. 

 Another reason may be that the efficiency of energy transfer from 

 chlorophyll b to chlorophyll a is very high. This transfer has been 

 found by Watson and Livingston (1950), Young (1952, Carnegie 

 Institution Year Book No. 51), and Duysens (1952) in ether solutions 

 of the mixed pigments. A less interesting but also possible reason may 



