TIMING AND THE FLOWERING PROCESS 141 



approach, loses a great deal of its impact. Rhythms which will main- 

 tain themselves for a hundred cycles or more under constant environ- 

 mental conditions surely seem to indicate the presence of a biological 

 oscillator. Furthermore, the experiments of Hamner, and other 

 workers seem to indicate that there is a relationship between this 

 oscillator and timing in the flowering process. 



3. The Sharp Critical Dark Period 



It is rather difficult to reconcile the shape of a half-time decay curve 

 with a sharp critical dark period. As can be seen from Fig. 8-2, a 

 decay curve is quite flat by the time only 3 or 4% of the original 

 material is left. A slight variability in decay rate or response to 

 critical level might make timing very inaccurate. For example, if 

 the half time for decay is 2 hr, and the critical night is 9 hr, the critical 

 level of F-phytochrome must be 4.42% of the original. If response 

 varies within the range 4.42 plus or minus 2%, then critical night 

 would vary from 7 hr 20 min to 10 hr 45 min. Thus the argument of 

 simplicity for phytochrome decay timing again loses impact. Such 

 a system must be so accurate and temperature compensated that it 

 could hardly be very simple. It is probably just as easy to imagine 

 that phytochrome decays to a low level very rapidly (within 1 or 2 hr), 

 and that this then couples the flowering process to a highly tempera- 

 ture insensitive biological timer of the oscillator type. 



4. Cobaltous Ion and Clock Resetting (68) 



An oscillator clock would allow but does not require resetting by 

 a flash of light, but an hour glass clock implicitly contains the 

 assumption that inhibition by a red light flash is eff'ective because 

 the clock is reset. If the clock is not reset by a light interruption of 

 the dark period, then the hour-glass type of clock becomes difficult 

 to accept. In the last chapter. Fig. 7-7 showed results using cobaltous 

 ion which seemed to indicate that the flowering clock is not reset by 

 a light interruption. The curve obtained from cobaltous ion treated 

 plants is shifted to the right and crosses the control curve, so that 

 after 5 or 6 hr of darkness, plants treated with cobaltous ion are less 

 sensitive to a light interruption than are control plants. A light 

 interruption given in the middle of a 16-hr dark period may not 

 completely inhibit the flowering of cobaltous ion treated plants, 

 although control plants are always inhibited under these conditions. 



