808 



FLUORESCENCE OF PIGMENTS IN VI VO 



CHAP. 24 



close coincidence of the fluorescence band in vivo (No. 7) with the corre- 

 sponding absorption band (No. 6), and the "red shift" of the fluorescence 

 band in the Uving cell compared to its position in solution (Nos. 2 and 3 

 compared to No. 4). 



... I ii_ o o o O o o 



Wave lenqth, mu: mo mo m o 



Spectrum: ^ 

 Reference 



Fluorescence 



Reference 

 Absorption 

 Fluorescence 

 Reference 



o o o o o o o 



If) O ID O tn O m 



r^ h- <0 ^O tf> tf> sf 



o 



o 



o 

 o 



Fig. 24.1. Fluorescence spectrum of living leaves of Pelargonium (Nos. 

 2, 3, 7) and of chlorophyll in ether (No. 4), compared with the absorption 

 spectrum (No. 6) (after Dhere and Raffy 1935). 



Dhere and Raffy (1935) suggested that the second fluorescence bands of chlorophylls 

 a and b in vivo, situated in the near infrared, may account for the striking brightness 

 that green vegetation exhibits on landscape photogi'aphs on infrared-sensitive plates 

 (c/. fig. 22. 31 A). However, the fluorescence of living loaves is much too weak to produce 

 such a spectacular effect. Mecke and Baldwin (1937) disproved Dhere's theory by 

 showing that the vegetation remains dark when illuminated with infrared-free light and 

 photographed through a filter that transmits only the infrared. The brightness of green 

 plants in infrared light is thus due to lack of absorption, and not to fluorescence. 



Dhere and Biermacher (1936) photographed the fluorescence of Pelar- 

 gonium leaves on plates whose sensitivity extended far into the infrared, 

 and foimd a new band, with an axis at 812 m/x, extending to 830 m/i. In 

 Table 24.11, the wave lengths of the peaks of the four known fluorescence 

 bands of the chlorophylls a and h in living cells are compared with the 

 wave lengths of the corresponding bands in ethereal solution. 



Table 24.11 shows that, in the living cell, the fluorescence bands are 

 shifted by 5-15 m/x toward the infrared from their positions in ethereal 

 solution — i. e., by about the same distance as the corresponding absorption 



