LIGHT ABSORPTION AND PIGMENT CONTENT 1259 



exactly calculated even by means of the exponential formula (32.1), because 

 we are dealing, in plants, with non-homogeneous systems, in which the 

 absorption depends not only on the total quantity of the coloring material 

 in a given volume, but also on its distribution (c/. chapter 22). For ex- 

 ample, doubling the chlorophyll content in a thin, faintly green tissue will 

 have a different effect on light absorption, depending on whether this 

 doubling is achieved by increasing the number of chloroplasts (in this case, 

 light absorption will be nearly doubled, too), or by increasing the concen- 

 tration of chlorophyll in each chloroplast (in this case, the effect on absorp- 

 tion will be much weaker) . The same is true of cell suspensions which are 

 thin enough for some light to be transmitted between the cells (c/. the dis- 

 cussion of the ''sieve effect" in chapters 22 and 37C). Rehable estima- 

 tion of the influence that a certain change in chlorophyll content will have 

 on light absorption by a leaf, or a cell suspension, is particularly difficult 

 when white (or, generally, nonmonochromatic) light is used, since in this 

 case the average absorption coefficient, a, is itself a function of the pigment 

 concentration. 



Because of these complications, experiments on the relationship between 

 chlorophyll concentration and the jdeld of photosynthesis should best be 

 accompanied by direct determinations of light absorption; only then will 

 it be possible to judge what part of the observed effect is "trivial" (i. e., 

 attributable to changes in light absorption), and what part, if any, requires 

 a different explanation. This requirement was not satisfied by past inves- 

 tigations on this subject, and this makes it impossible to use them for any 

 but preliminary, qualitative considerations. 



If the effect of variations in chlorophyll concentration on the rate of 

 photosynthesis were due entirely to changes in light absorption, the general 

 shape of the "chlorophyll curves," P = /[Chi], could easily be predicted. 

 At first, these curves, drawn to appropriate scale, should coincide with the 

 light curves, P = /(/), which correspond to the same parameters (such as 

 [CO2] or T) ; later, the chlorophyll curves should rise more slowdy than the 

 light curves (because absorption increases proportionately with /, and more 

 slowly than proportionately with [Chi ]) . At the end, however, both should 

 approach the same saturation level. These relations are shown schemat- 

 ically in figure 32.1 A, while the solid curves in figure 32. IB show the posi- 

 tion of two light curves corresponding to two concentrations of chlorophyll, 

 one twice as high as the other. The ratio of the initial slopes of these two 

 curves is between 1 and 2, because the more strongly pigmented system 

 absorbs more light, but not quite twice as much as the less pigmented one. 

 The curve that corresponds to [Chi] = 1 continues its linear rise longer 

 than that corresponding to [Chi] = 2, and finally approaches the same 

 saturation level. We expect to find this picture, e. g., in the comparison 



