THE NATURE OF PHOTOSYNTHESIS 109 



light. Draper," using a prismatic spectrum, concluded that the plant re- 

 duced carbon dioxide most readily in yellow light. A number of in- 

 vestigators endeavored to establish the region in the spectrum of maximum 

 photosynthesis. Most of these concurred that maximum activity is in 

 the red portion of the spectrum though they differed as to the precise 

 frequency. Most of these investigations contributed some data of a quali- 

 tative rather than of a precise quantitative nature, also a number of inter- 

 esting demonstration experiments and much experience in the experimental 

 difificulties entailed. 



Against the conclusion that photosynthetic activity exhibits a single 

 maximum in the red-yellow portion of the spectrum were the results pub- 

 lished by Timiriazeff, Englemann and later by Kohl. Timiriazeft' used 

 the solar spectrum and determined the rate of photosynthesis by means of 

 gas analyses. In practical agreement with these were the results of Engle- 

 mann who employed his very sensitive bacteria method in a microspectrum. 

 These investigators maintained that besides the photosynthetic maximum 

 in the red there was a second maximum in the blue. We shall not enter 

 into the lengthy discussions arising from these different results. The ex- 

 perimental methods employed contained numerous errors and the variant 

 results can in part be ascribed to errors due to dispersion and to the unequal 

 absorption of the different wave-lengths in the screens and prisms. 



In most of the older investigations not sufficient attention was devoted 

 to the distribution of energy in the various s^^ectral regions studied. The 

 impetus for most of these researches was the desire to determine whether 

 the regions in the spectrum of maximum or high photosynthetic activity 

 correspond to the absorption bands of chlorophyll. While it is true that 

 there can be no photochemical action without absorption of light, it does 

 not follow that absorption of light results in photochemical action. 



Within recent time several very thorough investigations have ap- 

 peared which elucidate most of the problems with which the older investi- 

 gators struggled with only partial success. The earlier experiments there- 

 fore have largely only an historical interest and we shall confine our dis- 

 cussion to the more recent publications. A new attempt to study this 

 subject was made by the investigations of Kniep and Minder.'^ 



These investigators started with the realization that it was essential 

 to determine accurately the energy of each region of the spectrum they 

 were using and the relation of the different wave-lengths to each other 

 and to the source of light. When sunlight is used as a source of light it 

 must be borne in mind that the intensity of different portions of the 



"Draper, J. W., "Scientific Memoirs," New York, 1878. p. 184. Phil. Mag.. 25, 

 169 (1844). Doubenv, C, Phil. Trans. Roy. Soc. London, 126, 149 (1836). Lommel, 

 E., Pog. Ann., 143, 568 (1871). Sachs, J., Bot. Zeitg., 22, 353-358, 361-367, 369- 

 372 (1864). Pfeffer, W., Arbeit, bot. Inst. Wiirzburq, 1, 1-76 (1871). Timiriazeff, 

 C, Ann. Chim. Ph\s., Ser. V, 12, 355 (1877). Reinke, J., Bot. Zeitg.. 42, 1, 16, 

 2>i, 49 (1884). Richter, A., Rev. gen. Bot.. 14, 151-169, 211-215 (1902). Engle- 

 mann, T. W., Bot. Zeitg., 40, 419 (1882); 42, 81, 97 (1884). Kohl, F. G., Ber. 

 Bot. Ges., 15, 111 (1897) ; 24, 39 (1906). 



"Kniep and Minder. Zeit. f. Bot., 1, 619' (1909). 



