1146 THE LIGHT FACTOR. III. COLOR CHAP. 30 



wave lengths, the quantized action spectrum can be expected to parallel 

 exactly the absorption spectrum; the "equienergetic" action spectrum, 

 on the other hand, will always be askew, with blue-violet light appearing 

 ceteris paribus less efficient than red light. 



If the quantized action spectrum determined from measurements in 

 low light, differs markedly from the absorption spectrum, this is a definite 

 indication that quanta of different wave length have different photochemi- 

 cal effects in photosynthesis. Recent determinations of the quantum 

 yield of green and colored algae in monochromatic light, carried out by 

 Emerson et al., Button and Manning, and Blinks, have established 

 the existence of such differences, and made speculations as to their origin 

 legitimate. Similar conclusions have been drawn previously from experi- 

 ments under badly controlled conditions, in which broad spectral bands 

 (isolated by means of colored glass filters) and unknown light intensities 

 were used. Conclusions drawn from experiments of this type, e. g., by 

 Montfort, have sometimes proved partially correct, but comparison of 

 Montfort's confused discussions with the concise presentation of Emerson 

 and Lewis, and Button and Manning gives a most eloquent demonstration 

 of the progress that can be achieved in plant physiology by the use of better 

 physicochemical tools. 



The explanation of the effect of wave length on the maximum quantum 

 yield of photosynthesis can be sought in three phenomena: (a) in the 

 composite nature of the pigment system and the (qualitatively or quantita- 

 tively) different photochemical functions of the individual pigments; (h) 

 in the multiplicity of the excited electronic states of chlorophyll, two (or three) 

 of which are involved in the light absorption in the visible spectrum {cf. 

 fig. 21.20) ; and (c) in the influence that vibrational energy, acquired by the 

 sensitizer together with electronic excitation, may have on its sensitizing 

 action. The first factor causes the rate of photosynthesis in different spec- 

 tral regions to be affected by the apportionment of the absorbed light 

 energy to the several pigments, while the second and third factors can 

 cause changes in efficiency with wave length, even in light absorbed by a 

 single pigment. 



Thus, the study of the effect of wave length on photosynthesis should 

 aim, first, at the qualitative and quantitative determination of the role of 

 various pigments in sensitization and, second, at the analysis of the rela- 

 tion between wave length and photochemical efficiency for each pigment. 

 Needless to say, we are far from having achieved these aims. Even now, 

 the greater part of newly pubUshed work on photosynthesis in colored 

 light remains purely descriptive and unsuitable for quantitative interpreta- 

 tion. 



