DIURNAL CHANGES 539 



to abolish this inhibition at any time of the dark period, the same 

 energy of far red was necessary. 



If on the other hand, the 10 hr light period (white light) is inter- 

 rupted by light breaks (2-3 hr), it is only far red that causes an 

 inhibition of flower formation. The highest sensitivity was found 5 hr 

 after the beginning of the main light period. This inhibition may be 

 cancelled by red light, and again in any part of the main light period 

 the same energy is necessary to remove the inhibition. 



These facts show that some diurnal antagonistic change in the 

 sensitivity to red and far red is involved in photoperiodism. This also 

 is made clear by measuring the energy of light breaks required to 

 prevent flower formation under suitable experimental conditions 

 (Fig. 4). Perhaps these antagonistic changes in the light sensitivity 



200 - 



> 

 m 

 _ o 



a 0=230 



i5 



t^ 260 



z 



9 



2 80 



z 



~« 60 



^o: to 



o 



11. 20 



......ymmmmmm 



10 12 U 16 18 20 22 2< 

 HOURS 



Fig. 4. Chenopodium amaranticolor. Required energy of light breaks 

 for complete suppression of flowering impulse. The light breaks were 

 offered in the 13-hr light period of 13: 11-hr light-dark cycles or in the 

 14-hr dark period of 10: 14-hr light-dark cycles. (Unpublished experi- 

 ments, K5nitz.) 



are to be explained by diurnal changes in the responsible pigments, 

 but other explanations are also possible. 



REFERENCES 



BUnning, E., and W. Konitz. 1957. Diurnale antagonistische Schwankungen 

 von Hell- und Dunkelrot-Empfindlichkeit einer Kurztagpflanze. Naturwissen- 

 schaften, 44, 568. 



Clauss, H., and W. Rau. 1956. Cber die Bliitenbildung von Hyoscyamus niger 

 und Arabidopsis thaliana in 72-Stunden-Zyklen. Z. Botan., 44, 437-54. 



