123 



J. L. Rosenberg and Tevfik Bigat 



not only less than the sum of the fluorescences with separate 

 excitations but was less than the fluorescence with the green 

 beam only. In a typical experiment, for which the green beam 

 intensity was 97 aJ^d. the blue 90, in units of 10"11 einstein/cmS 

 sec, the relative steady-state fluorescence intensities for 

 green, blue, and green plus blue excitations were I30, 25, and 

 100 respectively. The attainment of these steady values in 

 changing from one type of illumination to another was achieved 

 by way of interesting transients. Fig. 1 summarizes the trans- 

 ients accompanying addition or removal of blue excitation to a 

 sample under continued green irradiation. The transient 



SO- 

 TO - 



t 60 



50 



U w 



"2 



40- 

 30 - 

 20- 



ro- 



-BLUEi-GREEN- 



-GREEN- 



gStc. 



Time 



Fig. 1. Chlorophyll a fluorescence of Forphyridium 

 showing transients due to shifting from green to green 

 plus blue illumination and the reverse . The regime of 

 illumination was not changed \antil the steady-state of 

 fluorescence was reached for each type of illumination. 

 Incident intensity for the green beam was 97 a^d. for 

 the blue, 90 (in 10~11 einstein/cm^-sec ) . 



accompanying the shift from blue to green excitation is shown 

 in Fig. 2. Here the initial instantaneous rise in fluorescence 

 is followed by a slower sigmoid rise to a new steady-state at a 

 rate increasing with Increasing green light intensity. ¥e 

 failed to observe the maximum found by Duysens for Forphyridium 

 but not found by him to a large extent in other species . We 



