130 COMPENDIUM OF GENERAL BOTANY 



MULLER). All observers agree that assimilation is much less 

 active in the more strongly refrangible half of the spectrum uni- 

 formly designated as ' ' chemical rays ' ' (actinic rays) because they 

 induce certain chemical processes than in the less refrangible half. 

 The above coincidence of light-absorption and assimilation in the 

 chlorophyll-bodies harmonizes with the supposition l that (1) there 

 are certain atomic groups in the chlorophyll which are set in strong 

 vibrations by the red, and less strongly by the more refrangible, rays 

 of the spectrum, and (2) it is these atomic groups which do the work 

 of assimilation fiy the transformation of light- waves into chemical 

 activity. In connection with (1) we might mention the phenom- 

 enon that an alcoholic solution of chlorophyll fluoresces with a red 

 light, while the living green plant s does not fluoresce ; that is, it 

 does not emit a red light, because the necessary vibrations are 

 being transformed into chemical activity. The coloring substance 

 chlorophyll and living plasm work together in the processes of 

 assimilation: chlorophyll acts perhaps after the manner of a 

 ferment. 



The history of assimilation also contains the investigations of 

 PRINGSHEIM 2 which created considerable interest at the time. 

 Pringsheim' s hypothesis has, according to my knowledge, no firm 

 adherents. The peculiarity of this hypothesis is the original con- 

 ception that the coloring matter of chlorophyll is only of physical 

 importance, not chemical, and that it is the colorless plasm which 

 is active in assimilation. According to Pringsheim, chlorophyll 

 regulates the respiration of oxygen in plants by the absorption of 

 the so-called "chemical" rays (blue, violet, ultra-violet), so that 

 the activity of such respiration is reduced below the activity of 

 assimilation. The absorption-bands in the red therefore cannot 

 have the significance mentioned above. The optimum of assimila- 

 tion, according to Pringsheim ; in agreement with SACHS and 

 PFEFFER, does not lie in the red spectrum but in the yellow. In 

 this matter we are far from having uniformity of opinion. But 

 we will for the time being adhere to the opinion expressed above, 

 which is based upon the results of Engelmann's and Reinke's 

 experiments. 



1 See HOPPE-SEYLER, Botanische Zeitung (1879), p. 819. 



2 Sitzungsberichte der Berliner Akademie, 1879. 



