180 THE FLOWERING PROCESS 



slightly shorten the critical night. Is this because a small amount of 

 hormone is normally produced before the end of the critical night, 

 but the natural auxin normally causes it to disappear, while 

 application of an antiauxin preserves it ? 



Second, high carbon dioxide partial pressures following the dark 

 period will shorten the critical dark period (allow flowering on 

 shorter dark periods). Does this treatment also tend to preserve a 

 small amount of hormone normally produced before the end of the 

 critical dark period ? 



Third, low temperatures following the dark period will shorten the 

 critical night. These experiments were originally done by workers at 

 the California Institute of Technology (67) and by workers in 

 Holland (78), using a number of inductive dark periods. We repeated 

 them with our growth chambers in Colorado, using either a single 

 dark period or five dark periods. The results, shown in Fig. 9-15, 

 show that the effect can be observed only with more than one cycle. 

 This seems to complicate the interpretation, but it may mean that 

 low temperature following a single dark period tends to inhibit 

 translocation so that flowering is inhibited, even though hormone is 

 not as rapidly destroyed. If a number of cycles are given, preserva- 

 tion of the accumulating hormone may become more important than 

 other effects, and flowering on subcritical dark periods can be 

 observed. Other explanations also come to mind, such as an eff*ect 

 of low temperature on the phytochrome system. Or perhaps we are 

 observing an eff'ect on the long-day inhibitor discussed earlier in this 

 chapter. In Schwabe's experiments, low temperatures made the long 

 days less inhibitory. Perhaps the failure of low temperatures to be 

 effective following a single dark period is because the low temperature 

 only acts on the following dark period (by inhibiting synthesis of an 

 inhibitor, such as auxin ?). 



7. Cumulative Effects of Repeated Cycles of Induction 



So far all of the discussion in this chapter has been directed towards 

 understanding the processes taking place during the inductive dark 

 period, assuming that repeated inductive cycles are only additive but 

 do not interact with each other. It has been suggested that such 

 cycles do indeed interact with each other. R. G. Lincoln and K. A. 

 Raven, two of Hamner's students (58), suggest that the ability of a 

 cocklebur leaf to synthesize flowering hormone increases as the 



