492 A. PIRSON 



O2 consumption at first is not maximal, Init rises during the first 4 

 hours of darkness and then gradually decreases toward the end of the 

 dark period. Thus we observe, in connection with light-dark changes, 

 a periodically changing activity of photosynthesis and respiration. 

 This periodicity is not confined to periods of 24 hours, but can vary 

 in a broad range with changes in the length of the light and dark 

 period, as detailed investigations of W. J. Schon (2) in our institute 

 have shown (Fig. 1). The maximum of photosynthesis, in these ex- 

 periments, is generally shifted into the first half of the light period, 

 the maximum of respiration into the first half of the dark time. An 

 alternation of 3 hours light and 3 hours darkness acts like continuous 

 illumination. In this case, photosynthesis steadily decreases; at the 

 compensation point, the first symptoms of the above-mentioned 

 growth anomalies appear. To a certain extent the length of light and 

 dark periods can also be varied independently of each other; thus 

 even a periodicity of 10 '/2: 7 hours light and darkness can be im- 

 posed upon the alga. Complete adaptation to a new frequency gen- 

 erally needs a pretreatment over about 6 to 8 periods. 



These experiments show that metabolic periodicity is not directly 

 connected to the natural change of light and darkness, or to the 

 length of the day (daily sun cycle). It is easily possible to induce an 

 inversion of periodicity by doubling the dark time once. As to a 

 right judgment of the induction of photosynthetic periodicity it 

 should be stressed that the measurements were not continuously per- 

 formed on one specimen of a Hydrodictyon colony but that, during 

 one period, the experimental material was supplied several times from 

 a uniformly treated stock. Determinations of the dry weight of the 

 cells growing during the experiment enabled us to eliminate the in- 

 crease of photosynthesis caused by newly formed metabolically ac- 

 tive substance. In this connection it is important to note that, within 

 the whole light period, chlorophyll content (0.8%) and gross com- 

 position of the cells remained fairly constant. This means that 

 the decrease of photosynthetic output occurring in the second half of 

 the light period cannot be based on an accumulation of storage ma- 

 terial. We rather have to conclude that a genuine regulation of photo- 

 synthetic activity by the cell takes place. 



For the analysis of the mechanism of this periodical regulation of 

 photosynthesis not only the above-mentioned shifting of the maxi- 

 mum is imjiortant, but also the course of photosynthesis after dark 



