TIMING AND THE I LOWERING PROCESS 



145 



Using the experimental set-up which was used to obtain the data 

 of Fig. 7-8, the critical dark period was determined under light 

 conditions which cause about 50% inhibition of flowering as 

 measured by the Floral Stage system. Plants were allowed to remain 

 under these conditions for various time intervals, while control 



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5-0- 



uj 4-0 

 < 



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 < 



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^ 2-0 



1-0-- 



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, .^Threshold light 



10 II 12 13 14 15 16 



LENGTH OF THE DARK PERIOD IN ' HOURS 



Figure 8-5 

 Flowering response of cocklebur to dark periods of different lengths, 

 given in the presence or in the absence of threshold light (July, 1962). 

 The experiment is typical of four performed with a single dark period 

 and one performed with four dark periods. It is important to note that 

 the actual floral stage of those plants that did flower under threshold 

 light is lower than the Floral Stage of control plants (at night lengths 

 near the critical night, there are vegetative plants in both the controls 

 and the treatments). Thus the inhibition is real and not a statistical 

 artifact produced by more vegetative plants under threshold light. 

 Experiments performed by James Whitmore of the graduate Plant 

 Physiology class. Data previously unpublished. 



plants in the same well-ventilated room (uniform temperature) were 

 placed in total darkness behind a black plastic shade. Results typical 

 of four experiments are shown in Fig. 8-5. Although flowering is 

 inhibited under threshold light conditions, the critical night is still 

 the same — timing is not influenced ! 



