PHOTOSYNTHETIC MECHANISM OF PURPLE BACTERIA ] 65 



(graphs I a, II a) until a steady-state value is reached within a min- 

 ute; upon darkening the change is reversed (I e and II e). At very 

 high intensities more complicated curves were observed (graphs III 

 and IV). A decrease in absorption a was followed by an increase h, 

 which in its turn was followed by a decrease c. Upon darkening a de- 

 crease d occurred followed by an increase e. 



If the light was left on, the decrease c went on for minutes. In 

 the dark this decrease was slowly reversed. The much faster changes 

 a, b, d and the fast part of e took place also after long illumination 

 had produced an appreciable, not yet reversed, "bleaching" c. 



It seems that the changes a and the fast part of e of graphs III 

 and IV are caused by the same pigment as the changes a and e in 

 graphs I and II; h and d are presumably caused by a different pig- 

 ment. Since the change c is slow, it is probably not caused by a 

 photosynthetic catalyst, and, since the other changes seem to take 

 place independently of c, it may be left out of consideration in the dis- 

 cussion of the changes a, b, d, and the fast part of e. 



The bottom part of Fig. 1 shows that at 530 m/z changes corre- 

 sponding to a and e are small (graph V) and that the changes b and d 

 are pronounced (graph VI). It should not be concluded, however, 

 that at 530 niyu the changes b and d are greater than at 430 m/x, 

 since at 430 m/x these changes are counteracted by a and e. 



DIFFERENCE SPECTRA 



Under most experimental conditions the time course of the change 

 in absorption is a monotonously increasing or decreasing function of 

 the time, which approaches a steady-state value. Examples of such 

 changes are graphs I and II of Fig. 1. If this steady-state value (at a 

 certain constant exciting intensity) is plotted as a function of the 

 wavelength of the measuring light, a "difference" spectrum is ob- 

 tained. 



The shapes of the difference spectra of Rhodospirillum appeared to 

 depend on the suspension medium. 



Three different spectra are represented: 



1. The difference spectrum in anaerobic peptone (top curve of 

 Fig. 2). 



2. That in aerobic distilled water (bottom curve of Fig. 2). 



3. That in anaerobic distilled water (Fig. 3). 



