GENERAL REMARKS ON LIGHT ABSORPTION BY PLANTS 073 



In the last place, if, overcoming these two difficulties, we succeed in obtain- 

 ing a reliable value of a, it is an average absorption coefficient of a mixture 

 of several pigments, whose individual absorption spectra in solution we may- 

 know, but whose bands are variously shifted and deformed, in the living 

 cell, by adsorption and complexing. The task of apportioning the total 

 absorption at a given wave length to the component pigments (which re- 

 (juires the knowledge of their individual absorption coefficients and of their 

 distribution in the cell) often proves impossible of achievement, except by 

 gross simplifications. 



We shall deal first, in part A, with the determination of the amount of 

 light energy absorbed })y plants, and then, in part B, with the spectroscopic 

 properties of individual pigments in vivo and their contribution to the total 

 absorption. 



A. Light Absorption by Plants* 



1. General Remarks 



In working with solutions in plane-parallel glass cells, the detennination 

 of the absorbed light energy (A) requires two measurements: Either one 

 measures the incident light flux (/) and the transmitted light flux (T), or, 

 more commonly, one compares T with the flux To transmitted by a blank 

 cell containing pure solvent. A is calculated by one of the following: 



(22.2a) A = I - T or 



(22.2b) A = To - T 



Both Sive first approximations. Equation (22.2a) neglects all reflections; a 

 second approximation can in this case be obtained by subtracting from / 

 the light flux reflected from the front wall of the absorption cell : 



(22.3) A = 1(1 - r) - T 



where r is the reflection coefficient of the cell material. However, reflec- 

 tion from the front wall is only part of the total reflection in the cell; to 

 make our equation exact, we should write, in place of (22.3) : 



(22.4) A = I - T - R 



meaning by R the total reflected flux. 



Equation (22.2b) is a better first approximation than (22.2a), because 

 it neglects only the difference between the reflections from the solution cell 

 and the blank cell. The fluxes reflected from the front walls of both cells 

 are identical, but those reflected from the back walls are different (because 



* Bibliography, page 736. 



