1184 THE LIGHT FACTOR. III. OOLOli CHAP. 30 



Beside the measurements of Emerson and Lewis and of Blinks and Haxo, 

 all earlier observations on the role of phycobilins in photosynthesis have 

 only the weight of corroborative evidence. Most of this evidence pertains 

 to red algae, and has been gathered in connection with Engelmann's theory 

 of "complementary chromatic adaptation" of these algae to the blue-green 

 light that prevails under the sea. (Obviously, this color adaptation can 

 only be useful to the algae if the light absorbed by the red pigments can be 

 used for photosynthesis.) Because of the absorption of red and blue- 

 A'iolet light by water, a full utilization of the central part of the visible 

 spectrum — which is only insufficiently absorbed by chlorophyll — is of 

 vital importance for the plants living deep under the sea. This considera- 

 tion was the basis of the theory of chromatic adaptation, developed by 

 Engelmann in 1884. This subject was almost lost sight of in the first quar- 

 ter of the new century, while new methods of quantitative study of photo- 

 synthesis by green leaves were being developed by Blackman and co- 

 workers and by Willstatter and Stoll. Later, Warburg made the green 

 Chlorella cells the favorite subject of photosynthetic studies. In the 

 work on green plants, the presence of accessory yellow pigments was con- 

 sidered to be scarcely more than a nuisance. These pigments were not 

 prominent enough — both in concentration and in the part they took in 

 light absorption — to make them a desirable subject of independent study; 

 but their presence interfered with the quantitative study of chlorophyll- 

 sensitized photosynthesis in the short-wave region of the visible spectrum. 

 The fact that blue and red algae offer a much more promising field for the 

 study of the part played by the "accessory" pigments in photosynthesis 

 was almost forgotten. Engelmann had noticed, however, as early as 

 1883 (using motile bacteria for the oxygen determination) that the maxi- 

 mum of the photosynthetic efficienc}^ of red algae (CaUithamnion and Cer- 

 amium) lay in the green part of the spectrum, and that of blue algae (Oscil- 

 latoria and Nostoc), in the yellow. As in the case of green plants, the posi- 

 tion of the maximum of photosynthesis coincided roughly with that of the 

 maximum of light absorption. A year later (1884), Engelmann described 

 a "microspectrophotometer," by means of which he was able to show that 

 the parallelism between the absorption spectra and the "photosynthetic 

 action spectra" of the colored algae is quantitative. He concluded that 

 all pigments that contribute to light absorption by the algae also con- 

 tribute to photosynthesis, and expressed this result by the equation 

 -E'abs. = ■E'assim. (£" Standing for energy), w^hich was a direct challenge to the 

 concept of the exclusive sensitizing role of chlorophyll in photosynthesis. 



One of the developments of Engelmann's theory — ^the concept of "chro- 

 matic adaptation" as a factor determining the composition of the pigment 

 system in plants — has been discussed in chapter 15. Here, we are con- 



