PERIODICITY IN HydrucUctyou. 495 



pletelj' removed; the light-induced principle, on the other hand, 

 would not yet have accumulated because of the previous continuous 

 destruction; however, accumulation would then occur in the following 

 period of illumination so that photosynthesis would then decrease. 

 Possibly light and dark inhibitors are components of the same redox 

 system. At the begiiming of illumination the photosynthetic apparatus 

 would be in an overoxidized state and at the end of the light period 

 in an overreduced one. The maximal value Avould correspond to an 

 optimal relation of the two redox partners. Dark respiration prob- 

 ably is regulated by an inverse mechanism. The increase of respira- 

 tion during the first hours of darkness may be caused by a gradual 

 inactivation of an inhibitory principle (formed in light) by means of 

 the respiratory apparatus. 



In changing from a light-dark cycle to constant conditions, perio- 

 dicity is maintained for over 2 to 3 periods. In continuous illumina- 

 tion photosynthesis goes on reacting periodically, as does respiration in 

 continuous darkness. For the explanation of these after-oscillations a 

 simple biochemical mechanism cannot be taken into account. Here 

 we are facing a phenomenon somehow resembling the after-oscilla- 

 tions, which, since W. Pfeffer's observations on the foHar movements 

 of beans, have been described as rhythmically determined reactions 

 in very different organisms. In regard to their frequency, the after- 

 oscillations described by previous authors were independent of the 

 external periodicity applied during pretreatment and thus indi- 

 cated the presence of an endogenous rhythm, whereas the after- 

 oscillations of photosynthesis and respiration in Hydrodictyon behave 

 quite differently. They follow almost exactly the inductive frequency 

 and thus are as plastic as periodicity under the conditions of light- 

 dark change (cf. Fig. 1). Further investigations will be necessary to 

 demonstrate the importance of this difference and to give evidence 

 as to W'hether the observed metabolic periodicities have a direct 

 value for the analysis of the so-called endogenous rhythms of the 

 Pfeffer-type (5). From all our experimental results, only the after- 

 oscillations of photosynthesis and respiration induced by a periodi- 

 cal induction of 9:9 hours have been selected and are shown in Fig. 2. 

 It is particularly interesting that the after-oscillations of photosyn- 

 thesis in continuous light and those of respiration in continuous dark- 

 ness do not coincide, but alternate with each other. A maxinunn of 

 photosyjithesis generally corresponds to a mininmm of respiration. 



