62 



FLUORESCENCE SPECTROPHOTOMETRY 



microscope. In the normal leaf the much higher concentration of 

 chlorophyll gives an increase in the height of the 730-m/A fluorescence 

 band which is not absorbed to any great extent. On the other hand, 

 the height of the main peak at 685 m/A is nowhere near as high as 

 would be expected from the amount of chlorophyll present. This 

 relative decrease in height is due to selective reabsorption of the 

 fluorescent light near the chlorophyll absorption band by chlorophyll 

 itself. The incident blue light does not penetrate very deeply into 



600 



7S0 HI*. 



WAVELENGTH 



Fig. 11. Fluorescence spectra of chlorophyll a in an albino and in a normal grape 

 leaf. ( El Wakeel, Virgin, and Todd, unpublished. ) 



the leaf. If a very dark green leaf is illuminated with blue light, as in 

 Fig. 12, we find a spectrum which is greatly distorted, but neverthe- 

 less the two peaks are clearly recognizable. If, however, this same 

 leaf is illuminated with green light which penetrates the leaf, then 

 the fluorescence that is emitted within the leaf is largely reabsorbed 

 on its way out. In this case the deeper penetration leads to a higher 

 emission of light at longer wavelengths, but the main fluorescence 

 peak has practically completely disappeared. The fact that the main 

 peak does not show up is presumably due to the weak absorption of 

 the incident light in the outside layers of the leaf. An alternative 

 possible explanation of the shape of these curves might be that there 

 was another pigment present in the leaf which preferentially ab- 

 sorbed green light and fluoresced at longer wavelengths. It appears 



