1144 THE LIGHT FACTOR. III. COLOR CHAP. 30 



twisting experimental evidence : Direct sunlight has an energy maximum 

 in the yellow and not in the red, as the action spectrum of photosynthesis; 

 and the energy maximum of diffuse sky light, which is the second com- 

 ponent of the natural "light field" of land plants, lies even further toward 

 the blue-violet end of the spectrum. In this controversy, too, Engelmann 

 was undoubtedly right. Looking backward, one cannot but admire the 

 unfailing correctness of his conclusions, obtained by means of an experi- 

 mental method most investigators would hesitate to use even for qualita- 

 tive, not to speak of quantitative, purposes. l<]ngelmann not only estab- 

 lished correctly the general parallelism between the action spectrum of 

 photosynthesis and the absorption spectrum of chlorophyll ; he also clearly 

 understood the influence on both spectra of the optical density of the 

 specimen. His conclusions concerning the photosynthetic efficiency of the 

 carotenoids and phycobilins, long neglected, appear well on the way to 

 vindication sixty five years later. 



Much of the controversy between Engelmann and his opponents can be 

 attributed to a lack of understanding of what is meant by "action spec- 

 trum." A primitive definition can be based on the above-mentioned 

 simple experiment of Draper : A spectrum is thrown on a plant or cell sus- 

 pension, and photosynthesis is measured in different spectral bands of 

 equal width. Such an experiment, performed with a prism in artificial 

 light, may easily lead to the belief that the action spectrum of photosyn- 

 thesis has only one maximum, because the energy of most artificial light 

 sources declines rapidly toward the violet end of the spectrum, while the 

 dispersion of the prism increases in the same direction. Both factors co- 

 operate in causing a rapid decline of the yield of photosynthesis (related to a 

 given spectral band width) as one proceeds from the red to the violet ; this 

 decline may more than offset any increase caused by the renewed rise in 

 the absorption capacity of chlorophyll in the blue-violet region. 



Obviously, this definition of the "action spectrum" is arbitrary and ir- 

 relevant. An improved definition can be obtained by using Ught fluxes 

 of equal intensity at all wave lengths, which can be achieved, e. g., by ap- 

 propriate variation of the width of the monochromator slit, or insertion of 

 neutral gray filters. By using such methods. Kohl (1897, 1906), von Rich- 

 ter (1902) and Kniep and Minder (1909) were able to confirm Engelmann's 

 finding of the existence of a second maximum in the action spectrum of 

 photosynthesis, at the short-wave end of the visible region. 



However, even an action spectrum obtained by the use of spectral 

 bands of equal energy is not universal, i. e., it cannot claim validity for all 

 plants, not even for all specimens of a given species (e. g., all Chlorella sus- 

 pensions). The first cause of variability of "equienergetic" action spectra 

 is the varying composition of the pigment system (c/. chapter 15); but 



