180 COMPENDIUM OF GENERAL BOTANY, 



Mullek). 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 ' that (1) there 

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

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

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

 of assimilation hy the transforination of light- waves into chetnical 

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

 enon that an alcoholic solution of chlorophyll fluoresces with a red 

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

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

 being transformed into chemical actiWty. 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* which created considerable interest at the time. 

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

 adherents. The j^eculiarity 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 Pringsheiin, 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 Keinke's 

 experiments. 



» See Hoppe-Seyler, Botaniscbe Zeituug (1879), p. 819. 

 "^ Sitzuugsberichte der Berliner Akaderaie, 1879. 



