1162 THE LIGHT FACTOR. III. COLOR CHAP. 30 



curves, we consider those in figures 30.7 and 30.8. In figure 30.7A, the 

 relative position of the curves for green and red light is as predicted in 

 figure 30.6B. In figure 30.7B, the curve for white light, lies, as predicted, 

 below that for the more strongly absorbed red light, but the difference is 

 much larger than expected. 



It was mentioned in chapter 29 (page 1098) that the ratios between photochemically 

 equivalent intensities of red and white light, given by Eichhoff, appeared remarkably 

 small. (Satui'ation in red light was reached at an intensity equivalent to <2000 lux 

 of white light!) If one arbitrarily multiplies the "monochromatic" intensities by a 

 factor of 2.5 (this would reduce quantum yields from 0.25 to 0.10), fig. 29. 7B would be 

 changed as indicated by the dotted line, and acquire a much more plausible appearance. 



Figure 30.8A and Table 30.III show the results of Gabrielsen (1935). 

 The difference in the quantum yield in the green and in the red is notable 



Table 30.III 



Photosynthesis in Colored Light (after Gabrielsen 1935) 



Max. quantum Max. P, mg. CO2/ Reached at / = 

 Light Av. X, ni^ yield cm.^ hr. 11 cal/cm.^ sec. 



Red-orange 650 0.100 0.192 1.67 



Yellow-green 540 0.083 ca. 0.12 1.85 



Blue-violet 430 0.071 ca. 0.05 0.83 



(for a discussion of similar results by Emerson and Lewis, cf. page 1148). 

 The maximum rates in the blue-violet, and particularly in the green, also are 

 considerably smaller than in the red; however, figure 30.8A shows that 

 the observed maximum rates may be still far from saturation. Figure 

 30. 8B, taken from a later publication by the same author (1940), shows the 

 coincidence of the maximum rates in the three spectral regions very clearly. 

 It is obvious from the preceding discussion that the action spectra of 

 photosynthesis, obtained by illuminating plants with light of partly saturat- 

 ing intensity, are difficult to interpret — even if precaution has been taken 

 to use the same incident intensity (or, better still, the same number of 

 incident quanta) in all spectral regions. For example, in an optically thin 

 system, the yield per incident quantum should be smaller in the green than 

 in the red, because of weaker absorption (cf. fig. 30. 6B); while in a dense, 

 completely absorbing system the relation could be reversed, because of the 

 better utilization of the more uniformily absorbed green light (cf. fig. 

 30.6A). Working with systems that are not too dense, and in light that 

 is not too strong, one may obtain a "quantized" action spectrum resembling 

 more or less closely the absorption spectrum of the pigments. An example 

 is given in figure 30.9, which shows the action spectrum of wheat as ob- 

 served by Hoover (1937) and "quantized" by Burns (1937-38,1942). How- 



