PERIODICITY IN Hijdrodictijon 



491 



ment. When growing the alga in continuous light, characteristic 

 growth anomalies can be observed after a few days. These lead to 

 hourglass-shaped cells and later to irreversible damages. An alter- 

 nation of light and darkness (e.g., 12:12, 24:24, 6:6 hours) com- 

 pletely prevents such anomalies. 



+°2/ 



Dw 



40 

 20 



lO 



-02 



.7^-.. 



>. 



x. 



12 



20 



12 



20 



12 



20 



12 



20 



Jz/dv 



40 

 20 



10 



-O; 



/ ^-4 



» f ^^^^"^* 



^ 



^^— ^"^^^^^ 



12 16 20 24 



1 



'^. 



L 



K 



4 



time 



8 12 16 20 24 4 8 tim« 



*-02/dw 

 / 40- 



20 



lO- 



-O2 



■^02/c 



r 



h 



12 



17 21 



2 6 



n 15 20 time 



Dw 

 40 



20 



(O 



-O 



1^. 



i 



a 



.r\ 



12 16 20 24 4 8 time 



Fig. 1. Time course of photosynthesis and respiration of Hydrodictyon at dif- 

 ferent lengths of light and dark periods. Ordinate: O2 production and respirator \- 

 ()2 consumption expressed in mm. galvanometer deflection per time and dry weight. 



As Hydrodictyon is not suitable for manometric measurements, 

 photosynthesis and respiration were determined by the polarographic 

 method (dropping mercury electrode). Photosynthetic O2 production 

 during the normally applied 12-hovn- light period is not at all con- 

 stant. Maximum production is reached about 4 hours after the start 

 of illumination, whereupon follows a marked decrease to about one- 

 third of the maximal rate. After switching off the light, the respiratory 



