PHOTOSYNTHESIS 205 



Although this hypothesis played so important a part 

 in the subsequent interpretation of the scheme of plant 

 nutrition, it is remarkable that Sachs makes no mention of 

 it either in his textbooks or in his periodical pubHcations. 

 The hypothesis has undergone several modifications at 

 the hands of later investigators, notably Reinke, Erlen- 

 meyer, Crato, and Bach, and has been adversely criticised 

 by many authors, but no more plausible explanation of 

 the phenomena was forthcoming up to the end of the 

 century. 



The years 1869-71 heralded the entry of two new 

 names into the ever-increasing army of workers on the 

 photosynthetic problem, viz. Pfeffer and Timiriazeff. 

 Pfeffer dealt more especially with the effect of individual 

 rays of the spectrum in carbon assimilation. When 

 Draper examined this aspect of the problem he was 

 faced with two difficulties, viz. to obtain light of sufficient 

 intensity and at the same time to maintain purity of 

 colour. Sachs's double bell jars permitted the use of 

 coloured solutions as light screens, but the light transmitted 

 was not monochromatic. Pfeffer adopted the plan of 

 using fluids which absorbed only one type of ray, and 

 contrasted the results with those given by hght that had 

 passed through pure water. He concluded that each 

 ray of the spectrum, whether combined or otherwise, 

 had a specific power of decomposing carbon dioxide, 

 and that the most effective rays were those in the yellow 

 region of the spectrum, the maximum of carbon assimila- 

 tion coinciding with the maximum of illumination, while 

 the more refrangible rays in the blue violet region were 

 comparatively unimportant and the dark heat rays 

 were quite useless. 



In the same year (1871) Lommel claimed that the 

 greatest decomposition of carbon dioxide took place 

 between Fraunhofer's hues B and C, and he was supported 

 in this view by Miiller. Timiriazeff's first contribution 

 also dealt with the unrehabiUty of Draper's results, for 



