464 



S. S. Brody and M. Brody 



As the Chlorella cells age (to Ig" weeks) the fluorescence maximum of the 

 main band shifts to about 687 - 690 mja and the secondary band increases 

 in intensity relative to the main band. (Figure 4.) Ageing in E ugle na results 

 in a similar effect, but with relatively greater increase in the secondary band 

 in a much shorter period of time (see Fig. 5). By one week the main band 



550 600 650 



WAVELENGTH, m/i 



Fig. 4. Fluorescence spectra excited 

 at 436 mi-L. Room temperature spectra 

 are given by solid curves. Spectrum 

 at 77°K is given by dotted curve (for 

 2 day old cells). 



600 SSO 700 



WAVELENGTH. m;< 



Fig. 5 Fluorescence spectra of 1 1/2.- 

 day and 1 week old cells excited at 

 436 mfjL at room temperature. 



has shifted and the secondary band has increased in intensity until only one 

 wide band - with fluorescence maximum at about 710 m|j. can be seen (Fig. 5). 

 As the cultures age, the fluorescence of the main band - the monomer 

 (68 5 m\i) increases in intensity, but the long wavelength - the aggregate - 

 fluorescence increases at a more rapid rate. 



The above experiments show that nnuch more of the form contributing to 

 fluorescence at 720 mfx is developed in Euglena than in Chlorella. Ochro- 

 monas danica and Monodus sp. were found to have properties similar to 

 Chlorella , whereas Porphyridium cruentum was much more like Euglena . 



That the shape of the emission spectrum in older Euglena does not arise 

 primarily from re-absorption of fluorescence, is based on evidence of two 

 types, firstly on relative fluorescence yield measurements, and secondly 

 on the shape of the ennission spectrum obtained upon excitation with various 

 wavelengths of light. Relative yield measurements indicate that with ageing, 

 the fluorescence of the monomer band (685 m|jL) increases in intensity, but 

 the fluorescence of the aggregate band (720 m|a) increases at a faster rate. 

 As for the evidence based on the shape of the emission spectrum as a func- 

 tion of the wavelength of exciting light, the following was noted. Exciting 

 the fluorescence of very young Euglena and Chlorella with various wave- 

 lengths of light does not seen to affect the shape of their emission spectra. 

 Again, with older Chlorella, the shape of the emission spectra excited with 



