PERIODICITY IN Tlydrodictynfi 403 



intervals longer or shorter than the light times. After a pretreatment 

 of 48 hours darkness the rise of photosynthesis during 12 hours light 

 is so much prolonged that the maximal value is not reached until the 

 end of the light time; in a following 12:12 light-dark change, how- 

 ever, the normal periodicity will rapidly return. If the dark time has 

 been chosen shorter than the light time, e.g., 3, 6, or 9 hours with a 

 light period of 12 hours, photosynthesis does not reach its full effi- 

 ciency; the maximal value then depends almost proportionally upon 

 the duration of darkness. When, during the dark time, the temperature 

 is raised by 10° C. over that of the light time (30° C. compared with 

 20°C.), the increase of photosynthesis is retarded and the effect is 

 similar to that of very long darkness. If, however, dark respiration is 

 not raised by temperature but by addition of glucose, the normal 

 periodicity of photosynthesis is maintained. In this case, the presence 

 of glucose does not affect the rate of apparent photosynthesis; hence 

 it must be concluded that the marked increase of dark respiration 

 caused by glucose does not take place in light and that the consump- 

 tion of glucose in light must occur in a way different from that of 

 oxidative dark assimilation. This concept is supported by findings 

 in Chlorella, which have been obtained in another way (3,4), On alter- 

 ing the carbon dioxide concentration no change of the photos3aithetic 

 periodicity could be induced. Higher light intensities (2000 lux, in- 

 stead of the normally applied 1000 lux) accelerate the increase of 

 photosynthesis to the maximum; the following decrease is steeper 

 and leads to a smaller final value. At low intensities (150 lux) perio- 

 dicity disappears; photosynthesis then keeps nearly constant at a 

 low absolute value throughout the whole light period. Hence it may 

 be concluded in the conventional way that periodicity does not affect 

 the photochemical reaction but affects the enzymatic component 

 of photosynthesis. 



The hitherto-mentioned data may formally be explained by the 

 relatively simple working hypothesis that respiration and photo- 

 synthesis, respectively, develop an inhibitory principle reducing the 

 rate of photosynthesis; it must further be assumed that both princi- 

 ples may inactivate each other. According to this conception, photo- 

 synthesis at the beginning of illumination would be limited by 

 the dark principle; the inhibitory factor formed in light would in this 

 phase be canceled out by reacting with the dark component. At the 

 maximum of photosynthesis the dark principle would be com- 



