1880 



SPECTROSCOPY AND FLUORESCENCE OF PIGMENTS CHAP. 37C 



by re-absorption, far toward the longer waves, and become subordinated 

 to the second peak (fig. 37C.54). The fluorescence band of phycocyanin 

 in vivo is distorted in the opposite sense by preferential re-absorption of its 

 long-wave part by chlorophyll. 



The extent of distortion of the fluorescence spectrum caused by self- 

 absorption depends, for a given specimen, on the wave length of the excit- 

 ing light. It is smaller in the case of excitation by red or blue light, which 

 are absorbed in a thin layer in cells, and greater in that of excitation by 



o 



UJ 



cr 

 o 



ID 



PARTIALLY GREENED 

 LEAF 



DARK GREEN 

 LEAF 



r\ 



600 660 700 



WAVE LENGTH IN MJU 

 Fig. 37C.54. Fluorescence spectra of 

 faintly green and dark green leaves 

 showing the effect of self-absorption 

 (after French and Young 1952). 



green light, which penetrates deeper into the tissue (or cell suspension), 

 thus causing the fluorescence to retrace a longer average path before emerg- 

 ing into pigment-free medium (French 1954). 



Virgin (1954) showed that the fluorescence spectrum of leaves can be 

 changed by infiltrating them with water. The elimination of air-filled 

 spaces causes a reduction in scattering and re-absorption, w^ith consequent 

 enhancement of the main fluorescence band at 685 m/i relative to the first 

 vibrational band at 735 m//. The spectrum thus becomes similar to that 

 of an optically thin chlorophyll solution or chloroplast suspension. A 

 similar difference Avas found between the fluorescence spectra of (non- 



