INTRODUCTION 1143 



ceptions. He pointed out that "luminosity" is an anthropomorphic no- 

 tion, without meaning in objective photometry, that utihzation of light 

 energy is the essence of photosynthesis, that this utilization cannot take 

 place unless light is absorbed by a sensitizing pigment, and that this pig- 

 ment cannot be anything but chlorophyll. Timiriazev was the first to 

 use the concept of sensitization, a phenomenon then recently discovered 

 by Vogel and Becquerel, in the discussion of photosynthesis. 



A similar point of view was taken by several physicists, e. g., Jamin, 

 Becquerel and, particularly, Lommel (1871, 1872). The latter pointed out 

 that the basic principle of photochemistry, known as Herschel's law ("no 

 photochemical action without light absorption"), requires that the spec- 

 tral maximum of photosynthetic efficiency coincide with the absorption 

 maximum of the sensitizing pigment. Timiriazev (1869, 1875), Miiller 

 (1872), Engelmann (1882) and Reinke (1884) gave experimental proofs of 

 this coincidence, by showing that the photosynthetic efficiency of green 

 plants decreases steadily from red through yellow to green, parallel with 

 the decline in absorbing capacity of chlorophyll. The error of Draper, 

 Sachs and PfefYer was attributed by Timiriazev to the use of spectrally 

 impure light. (Timiriazev himself employed light isolated by a mono- 

 chromator with a narrow slit, and used microanalytical methods to com- 

 pensate for the weakness of illumination.) Engelmann suggested that the 

 error may have resulted from the use of thick leaves or thalli, which ab- 

 sorb light practically completely even in the minima between the absorp- 

 tion bands of chlorophyll. (He worked with microscopic plant objects, 

 using motile bacteria for the detection and determination of oxygen.) 



Engelmann (1882) noticed that, in addition to the main maximum in 

 the red, the photosynthetic "action spectrum" of green plants has a second 

 maximum in the blue or violet, which he associated with the strong ab- 

 sorption band of chlorophyll in this region. This perfectly natural conclu- 

 sion became the subject of one of the most vitriohc controversies in the 

 history of photosynthesis; it was contested even by such enlightened plant 

 physiologists as Reinke (1884) and Timiriazev (1885). Particularly violent 

 were the criticisms Pringsheim (1886) directed against Engelmann's method 

 and his results; and Engelmann (1887) answered these attacks in language 

 seldom encountered in the pages of scientific journals, even in the quarrel- 

 some nineteenth century. At the same time, Engelmann also sharply 

 rebuked Timiriazev for his attempt (1885) to identify the main maximum 

 of spectroscopic efficiency of photosynthesis with the energy maximum of 

 the solar spectrum. Timiriazev saw in this alleged coincidence a striking 

 example of adaptation of organisms to the prevailing conditions, and thus a 

 triumph of the Darwinian theory. Engelmann answered that a coin- 

 cidence of the two maxima cannot be postulated without deliberately 



