Chapter 24 

 FLUORESCENCE OF PIGMENTS IN VIVO * 



Because of close relationship that exists between fluorescence and sen- 

 sitization (cf. chapters 18 and 19 in Volume I, and chapter 23 in this vol- 

 ume), the study of fluorescence of chlorophyll in the living plant can make 

 an important contribution toward the understanding of the mechanism of 

 photocatalytic action of this pigment in photosynthesis. Fluorescence is 

 a property of chlorophyll that can be — and has been — observed simultane- 

 ously with the measurement of photos\Tithetic activity. By measuring 

 the yield of fluorescence, one can obtain insight into the energy exchange and 

 dissipation processes in photosynthesizing cells, without interfering with 

 their life processes. No attempts have been made to observe changes in 

 the fluorescence spectrmn (or, for that matter, in the absorption spectrum) of 

 chlorophyll during photosynthesis, but this, too, may prove possible and 

 useful in future. 



Many plant tissues fluoresce in ultraviolet light; but only those con- 

 taining chlorophjdl, bacteriochlorophyll or the phycobilins show a rather 

 weak, red or orange fluorescence when illuminated with visible light. The 

 fluorescence of the phycobilins (in blue-green and red algae) is more vivid 

 than that of chlorophyll, because it is stronger and the eye is more sen- 

 sitive to orange than to red light. Among green plants, the algae show 

 fluorescence more clearly than land plants, because light scattering (which 

 obscures fluorescence) is much weaker in their water-filled thalli than in 

 air-filled leaves. The fluorescence of leaves is so difficult to observe, that 

 after it was discovered by Stokes in 1852, and also described by Simmler 

 in 1862 and Askenasy in 1867, other investigators, notably Lommel (1871), 

 Hagenbach (1870, 1872) and Reinke (1883), were unable to confirm its ex- 

 istence. Although Hagenbach (1874) and Reinke (1884) revised their 

 views later, the reality of leaf fluorescence remained subject to occasional 

 doubts for another quarter of a century, until the invention of the fluores- 

 cence microscope permitted the observation of the fluorescence of single 

 chloroplasts. (As early as 1883, Engelmann had tried, unsuccessfully, to 

 observe the fluorescence of chloroplasts in ultraviolet light under an or- 

 dinary microscope.) The picture of chloroplasts glowing with a crimson 

 fight on a faint milky background so thrilled the investigators who first 



* Bibliography, page 826. 



805 



