Time Relations and Endogenous Rhythms • 45 



show movements 12 hours out of phase with the second group. 

 Finally, and perhaps most important, the rhythm of such move- 

 ments is not exactly daily, not precisely 24 hours long. It may be 

 from 20 to 30 hours; different varieties have rhythms with char- 

 acteristic period-lengths, so that this is a genetically controlled and 

 thus endogenous property. The term "circadian" (Latin: circa, 

 about, and dies, day) has been coined for such rhythms with period- 

 lengths of close to 24 hours. 



The relation of the bean circadian rhythm to temperature is 

 shown by data from Biinning (1959a). In darkness (after a light 

 flash) the period is 28.3 hours at constant 15° C and 28.0 hours at 

 constant 25° C. Thus a 10° difference in ambient temperature has 

 no effect. However, a change in temperature does have an effect. 

 Seedlings moved from 20° to 15° had a period of 29.7 hours, and 

 those moved from 20° to 25° had a period of 23.7 hours, for the 

 first day or so after a change. Later, compensation occurred and 

 the periods in the two temperatures became similar. Thus it is not 

 strictly true to call such circadian rhythms temperature-insensitive, 

 but they are clearly temperature-compensated and arrive at the 

 same period in different constant temperatures. 



In general, the phase and amplitude of circadian rhythms in 

 various organisms are greatly affected by the environment but the 

 basic period-length can only be changed within narrow limits. An 

 organism with a rhythm of 20 or 30 hours will adapt its period to 

 a normal 24-hour day, but may either revert to its endogenous 

 rhythm or exhibit highly disorganized activity under light-dark 

 cycles totaling 12 hours in length. Not only light flashes but transi- 

 tions from light to darkness and abrupt temperature shocks as well 

 can reset the phase or initiate circadian rhythms, but it seems clear 

 that they do not cause them. 



Many processes in an organism generally exhibit the same cir- 

 cadian rhythm, probably manifesting the activity of a single "clock" 

 mechanism. This "clock" may be a. basic property of the organiza- 

 tion of most cells or a particular unknown process, but there is no 

 general agreement even as to its possible nature. A major investi- 

 gator (Brown, 1959) has recently abandoned the hypothesis of a 

 completely endogenous origin, and suggests that organisms may 

 register the passage of time by perceiving certain unknown geo- 

 physical periodicities, although the way in which such an exogenous 



