Time Relations and Endogenous Rhythms • 43 



response to this timing is sometimes surprisingly precise; Xanthium 

 can distinguish clearly between a dark period of 8 hours (non- 

 inductive) and one of 8 hours, 40 minutes (inductive) (Long, 1939). 

 On the reasonable assumption that the main survival value of 

 photoperiodism in an organism is in the seasonal timing of devel- 

 opment that it affords, Withrow (1959) has calculated that to be 

 accurate, the timing mechanism must detect daylength differences 

 of 14 to 44 minutes within a week in temperate latitudes. In addi- 

 tion, it should be relatively insensitive to random changes in light 

 intensity and temperature brought about by local weather. Insensi- 

 tivity to intensity changes is provided by the fact that low intensi- 

 ties are sufficient to bring about most photoperiodic responses, but 

 insensitivity to temperature is more difficult to understand. 

 Although both the accuracy and the temperature-insensitivity (see 

 Chapter Two) of the photoperiodic control of flowering are, in the 

 writer's opinion, often exaggerated, it is true that certain aspects 

 of photoperiodism are less temperature-sensitive than most plant 

 processes. 



The effects of low temperature in lengthening the critical dark 

 period in Xanthium, discussed earlier, indicate that a drop of 

 about 16° C increased the dark period required by only about 3 

 hours, or less than 40 percent (Long, 1939). This contrasts with the 

 general observation that the rates of most ordinary chemical re- 

 actions, and thus of growth or other processes in most biological 

 systems, are at least doubled by a 10° C rise in temperature within 

 a fairly wide range. If the series of events constituting the dark 

 period "timing mechanism" in Xanthium responded in this fashion, 

 one would expect the 16° drop in temperature to bring about at 

 least a 20- or 24-hour dark requirement, but it does not. This and 

 similar evidence, although there is not a great deal of it, suggest 

 that the photoperodic timing mechanism is not a simple linear 

 series of ordinary reactions, but may be more complex. 



Neither timing nor temperature-insensitivity are peculiar to 

 photoperiodism. In mammals and birds, of course, a self-regulated 

 temperature could obviously permit the accurate timing of re- 

 sponses and metabolic events by simple chemical means alone, but 

 it is now well established that probably all plants and animals- 

 even unicells, excluding perhaps the bacteria— have accurate timing 

 mechanisms that are temperature-insensitive, more so, in fact, than 

 most photoperiodic phenomena. Several groups of workers have 



