SENSITIZED FLUORESCENCE IN VIVO 



815 



the wave length of exciting Hght between 436 and 578 m^u. This proves 

 that an increased yield of nonsensitized fluorescence of chlorophyll in violet 

 light cannot be offered as alternative explanation of the results of the first 

 two experiments. Excitation transfer from carotenoids to chlorophyll 

 might become possible in sufficiently concentrated solutions; we men- 

 tioned on p. 790 that Duysens (1951) reported excitation transfer from 

 chlorophyll b to chlorophyll a in 10 ~^ M solution in acetone. 



Table 24. III. Direct and Carotenoid-Sensitized Fluorescence of Chlorophyll 

 IN Diatoms (after Dutton, Manning and Duggar 1943) 



° These estimates are rather crude approximations {cf. chapter 22, page 726), but 

 it seems improbable that errors could be large enough to account for all the differences 

 between the calculated and observed ratios in the two last columns. 



'' The proportion of light absorbed by these two chlorophyll components is nearly 

 equal at 436 and 578 m/j.. Hence the yield ratio should be approximately 1.0 despite 

 differences in the fluorescence spectra of chlorophylls a and b. 



The experiments of Dutton and Manning are of importance for the 

 theory of photosynthesis. They indicate that the energy absorbed by 

 some carotenoids may become available to chlorophjdl in almost the same 

 measure as that absorbed directly by the green pigment. Thus, chloro- 

 phyll can play the part of a photocatalyst in photosynthesis, even when 

 another pigment acts as a "primary" sensitizer. Results similar to those 

 in Table 24. Ill have also been obtained by Wassink and Kersten (1946) 

 with Nitzschia dissipata. 



Even more interesting are the results obtained with red algae. Van 

 Norman, French, and Macdowall (1948) first found indications that 

 chlorophyll fluorescence in Gigartina and Iridaea can be excited with 

 equal (if not higher) intensity by light absorbed by phycoerythrin (at 

 560 mju) as by light absorbed by chlorophyll (at 650 m^u). French's (1951) 

 fig, 24.4 clearly shows that light absorbed by chlorophyll (and, partly, by 



