G76 



LIGHT ABSORPTION BY PIGMENTS IN VIVO 



CHAP. 22 



Measurements of T and R must include both the colhmated and the diffuse 

 components. 



If the leaf is sufficiently thick, and not glossy, it acts as an ideal scatterer, 

 i. e., the intensity of light scattered in a given direction is proportional to 



o 



UJ 



u. 



UJ 



o 

 a: 



LiJ 



I- 



UJ 



< 

 < 



10 20 30 40 50 60 70 80 90 

 ANGLE, degrees 



Fig. 22.1. Angular distribution of light transmitted and reflected liy a 

 leaf of Coleus blumei (after Loomis, Carr and Randall 1941). Reflection is 

 diffuse and obeys the cosine law; transmission is only partly diffuse and 

 therefore deviates from the cosine law. 



II0J00_901_80 70 



Fig. 22.2. Scattering of light by a dense suspension of Chlorella (in vessel 

 ()) (after Noddack and Eichhotf 1939). Direction of incidence A ^ B; 

 area a represents backward scattering, or reflection (R); area b forward 

 scattering or transmission (7'). 



the cosine of its angle with the direction of the incident light. Figure 22.1 

 shows the angular distribution of the light scattered by a comparatively 

 thin leaf of Coleus. This leaf transmits some colhmated light (as evidenced 

 by the deviation of the angular distribution of T from the cosine law), but 

 its reflection is entirely diffuse. Thicker leaves, with similarly dull sur- 

 faces, obey the cosine law with respect to both reflection and transmission. 



