718 LIGHT ABSORPTION BY PIGMENTS IN VIVO CHAP. 22 



that the siibmicroscopic structure of the plastids (discussed in Vol. I, 

 chapter 14, and iUustrated by fig. 14.6) does not place some pigments in a 

 different position with respect to light absorption than others. We may 

 hope that studies with the electron microscope and investigations of the 

 optical properties of the plastids (birefringence, dichroism etc.) will reveal 

 more about the arrangement of molecules in these bodies. Pending these 

 developments, all estimates of the relative contributions of different pig- 

 ments to the absorption of light by plants must be based on the assump- 

 tion of an identical proportional composition of the pigment mixture in 

 every point of the cell or tissue. 



2. Apportionment of Absorption in Uniform Mixture 



If it can be assumed that the pigment mixture in the cell or tissue under 

 investigation has uniform composition — meaning that, wherever the mix- 

 ture is present, it has the same relative composition (but not that the same 

 absolute concentration of mixture is present everjovhere)— the contribu- 

 tion of the zth pigment to the total absorption by the mixture at a given 

 wave length, Ai, is proportional to the product CjOJi, where Ct is the concen- 

 tration and «< the absorption coefficient of this component. 



(22.18) Ai = A (CiaifSiCiai) 



Whatever the length and shape of the path of the light beam in the 

 medium, equation (22.18) applies to absorption in every infinitely small 

 element of this path ; therefore, it applies also to the integral light absorp- 

 tion, independently of scattering or other geometrical-optical phenomena. 



The relative concentrations, Cf, can be determined, e. g., by extraction 

 and photometric estimation; the correct value of the total absorption, A 

 (i. e., the value corrected for reflection and scattering), can be determined, 

 for each wave length, by the methods discussed in part A. The applica- 

 tion of equation (22.18) therefore hinges primarily on the knowledge of the 

 true absorption coefficients of all pigments in the state in which they are 

 present in the cell, and here the difficulty comes in. 



In part B, we referred to statistical theories whose application may per- 

 mit the determination of the average absorption coefficients of the pigment 

 mixture, from measurements of transmission and reflection (or two reflec- 

 tion measurements, or two transmission measurements with different opti- 

 cal densities). In figures 22.37 and 22.38 we gave examples of nomographs 

 that could be used for this purpose, provided both the transmitted and re- 

 flected light fluxes are perfectly diffuse; and we suggested that these (or 

 other similar theories) be used in the future in the optical study of cell sus- 

 pensions, leaves and thalli. 



