142 THE FLOWERING PROCESS 



When cobaltous ion treated plants are interrupted in the middle of a 

 16-hr dark period, there are not enough hours either before or after 

 the interruption to allow for completion of the flowering process — 

 yet flowering does occur in most experiments. The effectiveness of 

 the red light interruption seems to depend upon some time-measuring 

 mechanism within the leaf, and this mechanism is slowed by 

 cobaltous ion. 



In some of our experiments (but not all) a 6-sec light interruption 

 of the dark period (sufficient to convert all the phytochrome to 

 F-phytochrome) will change the leaf movement pattern. This would 

 normally be considered an excellent example of resetting the clock. 

 Yet in flowering it appears that the 6-sec light flash only inhibits 

 flowering to an extent determined by the timing mechanism, but the 

 clock is not reset. Just how much light is required to reset the clock; 

 that is, to cause all of the dark processes to start over ? 



Hamner caused soybeans to flower with seven 48-hr cycles (8 hr 

 light, 40 hr dark). The cyles were interrupted at various times with 

 short or with long light periods. Results are shown in Fig. 8-4 (in 

 a similar but more complex experiment low and high intensity light 

 interruptions were used with comparable results). Near the middle 

 of the dark period short light periods failed to inhibit but long 

 periods (or high intensity light) actually promoted far above the 

 control level. Hamner's explanation involving Biinning's theory of 

 phases may apply only to short light periods which fail to reset the 

 clock, but also fail to inhibit during the less sensitive phase of the 

 cycle. Long light periods (or high intensity light) may reset the clock, 

 thus dividing the night into two cycles, so that plants receive 14 

 instead of just 7 inductive cycles, and thus they are promoted ! In 

 preliminary cobaltous ion experiments at Colorado we also have 

 indications that 15 to 20 min of full sunlight may be necessary to 

 reset the clock in cocklebur. Could this be the High Intensity 

 Reaction of Mohr ? 



What is the biochemical action of the cobaltous ion in slowing the 

 rate of timing ? At present we have only one clue. The eff'ect of the 

 cobaltous ion can be reversed by applying it along with various 

 substances known to form complex ions with cobaltous ion. Some 

 of these will completely reverse the cobaltous ion effect on flowering 

 as well as a slight cobaltous effect upon vegetative growth. Ethylene 

 diamine tetra-acetic acid (EDTA) is most effective, causing some 



