1154 



THE LIGHT FACTOR. III. COLOR 



CHAP. 30 



(fig. 22.21), the absorption in the far red was found also in measurements 

 purported to represent "tme" absorption. It may be due either to a 

 genuine broadening of the red chlorophyll band, or to the presence of other, 

 infrared-absorbing components (e. g., ferrous salts). The question of the 

 infrared limit of photosynthesis is closely associated with the interpreta- 

 tion of this infrared absorption "tail." The experimental results of Emer- 

 son and Lewis (1943) and Blinks and Haxo (1950), on the one hand, and 

 Eichhoff (1939), on the other hand, are in extreme disagreement. As 



0.08 



)o 0.06 

 o" 



UJ 



>- 



P 0.04 



< 

 o 



0.02 



246 mm* 

 493 mm' 

 986 mm' 



S 



J_ 



680 700 720 



WAVE LENGTH, m^ 



740 



Fig. 30.3. Decline in quantum yield in far red (after Emerson and Lewis 1943). 

 Points show apparent quantum yield with three different suspension densities, 

 calculated for equal incident light. Solid curve extrapolated for true yield, (com- 

 plete absorption), assuming that differences between the curves obtained with 

 different suspensions is due to incomplete absorption. 



shown in figure 30.1, Emerson and Lewis found a sharp drop in quantum 

 yield above 680 m/x. This part of their curve is reproduced in more detail 

 in figure 30.3. To be sure that the decline was not due to incomplete 

 absorption (we recall that the curve in figure 30.1 was obtained by War- 

 burg and Negelein's method which presupposes total absorption), Emerson 

 and Lewis made determinations with several suspensions of increasing den- 

 sity, and extrapolated the results to infinite density; the result is repre- 

 sented by the solid curve in figure 30.3. It shows that, even with a gener- 

 ous allowance for incomplete absorption, there still remains a drop in 7, 

 from about 0.08 at 680 mn, to as little as 0.02 at 730 m/x. 



