118 THE FLOWERING PROCESS 



Or does it also show that timing is not reset by a light flash in the 

 middle of the dark period, but that sensitivity to this flash is greatest 

 after 8 hr regardless of how much darkness is to follow ? Obviously 

 we have been led into the problem of timing, the subject to be 

 discussed in the next chapter. 



4. Continuous Threshold Light 



It would be of general interest to know what is meant by darkness 

 in the induction of a short-day plant such as cocklebur. How low 

 must the light intensity be before the plant no longer is able to 

 respond to it ? The question has bearing on responses to twilight and 

 moonlight; in a reciprocity study such a situation represents the 

 lowest eff'ective light intensity for the longest interval; and response 

 to threshold light has implications relating to our understanding of 

 phytochrome action and time measurement. 



The data are relatively simple to obtain. Plants are induced (or 

 inhibited in the case of long-day plants) by placing them at different 

 distances from a point Hght source in a room with dull black walls. 

 Figure 7-8 shows the results of such an experiment, in which prepared 

 cocklebur plants were arranged on concentric circles around a 

 7^ watt incandescent bulb and induced with a single 16-hr dark 

 period. 



Incidentally, the plants which show essentially no response to the 

 light were far enough from it that they could just barely be seen by 

 the moderately dark adapted eye. The minimum red light intensities 

 to which the plants will respond are probably below the red light 

 intensity of full moonlight (see Fig. 3-2 and 3-4), although it is likely 

 that natural shading by other leaves would allow induction to occur 

 readily even on nights illuminated by a full moon in a cloudless sky. 

 Nevertheless, the degree of response might be influenced, so we do 

 have some precedent for plant response to the phases of the moon 

 (see also 48). 



5. Kinetics of the Far-red Reversal of Red Interruption 



In the early part of the 1950's, R. J. Downs performed a series of 

 experiments at Beltsville, in which he studied the kinetics of the 

 far-red reversal of red inhibition of flowering in cocklebur and Biloxi 

 soybean. One experiment is of particular interest, in which he studied 

 the time interval between red interruption and subsequent far-red 



