714 



LIGHT ABSORPTION BY PIGMENTS IN VIVO 



CHAP. 22 



member that the statistical theory deals with random distributions of large 

 numbers of small colored particles, and therefore does not take into account 

 the "sieve effect," which may be caused by regular alignment of the com- 

 paratively large chloroplasts. The result of this effect is the admixture 



0.8 

 0.7 

 0.6 

 0.5 h 

 > 0.4 



en 

 o 



0.3 

 0.2 

 0.1 



O 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 



log (Tq/T^)- loglTo/r,) 



Fig. 22.38. Nomograph for calculation of absorption and scattering coefficients in 

 turbid media from two transmission measurements (after Wurmser 1941). 



(22.17) To/T = cosh dVaia + a) + „ " , " ^ sinh dV«(a + <r) 



1a{a + cr) 



of white light to the transmitted flux, i. e., a decrease in absoption at all 

 wave lengths. In the determination of the apparent absorption coefficient, 

 this decrease must have the largest effect in the absorption peaks, and a 

 lesser effect in the regions of low absorption. It thus helps to make ab- 

 sorption less selective. 



The sieve effect is negligible in Chlorella suspensions, since the cells in such a sus- 

 pension — unlike the chloroplasts in a leaf — are distributed at random. The suspensions 

 used in the experiments of Noddack and Eichhoff contained about 1 X 10' cells/ml. in a 

 plane-parallel vessel 3.9 cm. thick. The average diameter of a Chlorella cell is about 5 m, 

 and its average cross section is 10 "'^ cm. 2, so that 4 X 10' cells would cover a surface of 

 1 cm. 2 with eight complete layers. Statistics predict that the probability of a beam's 

 traversing such a suspension without striking a single cell is e~^ = 0.0003 and thus 

 negligible. 



According to page 683, the number of chloroplasts in a fully green leaf 

 is sufficient to form five or ten continuous layers; the statistical probabil- 



