694 



LIGHT ABSORPTION BY PIGMENTS IN VIVO 



CHAP. 22 



dioxide. Altogether, green plants absorb comparatively little light above 

 700 m/x — in the region where absorption would probably be useless for 

 photosynthesis, but could cause strong heating. 





Cell suspension 



640 



700 

 WAVE LENGTH, m/x 



800 



840 



Fig. 22.28. Absorption spectra of pigments of green sulfur bacteria in the red 

 and infrared region (after Katz and Wassink 1939). 



600 



1000 1400 1800 



WAVE LENGTH, m/i 



2200 



2600 



Fig. 22.29. Transmission and reflection of near infrared by potato leaf {Sol- 

 anum tuberosum) (after Loomis, Carr and Randall 1941). Note absorption bands 

 due to water, > 1300 m^, but very low absorption (reflection + transmission = 

 90-100%) in the 800 to 1300 mju region, which contains most of the near infrared 

 rays of sunlight. 



Figure 22.31 illustrates the "whiteness" of leaves and conifer needles in 

 the near infrared, by a comparison of the reflection of infrared light by 

 leaves with that by a sheet of white paper. According to Mecke and Bald- 

 win (1937) this lack of absorption (rather than infrared fluorescence) 

 causes the striking brightness of vegetation on infrared landscape photo- 



