49G A. piusoN 



Projecting these findings back into the previous conditions of light- 

 dark cycling we must conclude that, in Hydro(Hctij07i, respiration is 

 reduced during illumination; this behavior seems to differ from that 

 of Chlorella (6). The requirement of intermittent darkening in Hy- 

 drodictyon perhaps is based upon the fact that dark respiration, 

 which seems to be inhibited in light, possesses an indispensable func- 

 tion in normal development. 



Although we cannot find a simple l)iochemical mechanism ex- 

 plaining the periodical changes of metabolism in constant conditions, 

 we are able to influence the after-oscillations experimentally and 

 thereby characterize the mechanism of periodicity. Thus it is possible 

 to raise respiration to a higher level by addition of 0.01% glucose and 

 thereby eliminate the after-oscillations. Photosynthetic periodicity, 

 however, cannot be altered by glucose, either during intermittent 

 illumination or ni subsequent continuous light. By interrupting CO2 

 supply during intermittent illumination we observed only very low 

 rates of photosynthesis produced by the stored intracellular respira- 

 tory carbon dioxide of the dark times. If this treatment is followed by 

 continuous illumination with sufficient CO2 supply, after-oscillations 

 are very small. This strengthens the assumption that photosynthetic 

 periodicity is not induced by the change of light and darkness per se, 

 but by a periodically fluctuating activity of the photosynthetic mecha- 

 nism. It is possible, by the way, to let photosynthesis work quite 

 aperiodically for some time (2 to 3 days) . This can be done by treating 

 the alga with constant light for about 48 hours without CO2 and then 

 supplying it with sufficient CO2, still keeping illumination constant. 

 Hence periodical changes of photosynthetic activity need an external 

 induction and do not represent a behavior of photosynthesis fixed 

 a priori in this alga. 



Beside a physiological analysis of the periodical metabolic reactions 

 of Hydrodiciyon, a treatment of periodicity was also started on an 

 enzymological basis. It could be shown that the activity of acid 

 phosphatase obtained from extracts of the algal colonies at different 

 times of the light and dark periods has strong fluctuations similar to 

 the periodicity of photosynthesis and respiration (7). The maximum 

 of photosynthesis in light and the maximum of respiration in dark- 

 ness, respectively, coincide with the minima of phosphatase activity, 

 i.e., the frequency of enzyme periodicity appears to be twice that of 

 the two single metabolic processes (Fig. 3). It would be premature 



