548 RHYTHMS IN PLANTS AND ANIMALS 



controls as they are (early, column 4) or as they should have been 

 without phase shift ("on time," column 3). Therefore, UV-induced 

 phase delay is not entirely (if at all) due to failure of the cells to 

 respond to a phase-shifting stimulus. 



In IC the controls are on time, indicating no response to a short 

 dim light stimulus in the very early scotophilic phase; the irradiated 

 group is late and clearly sensitive to phase delay by UV. In ID the 

 controls are on time, which is compatible with an interruption of the 

 mid-scotophihc phase by dim light (Ehret, 1955a) (before this time, 

 the shift is late; after it the shift is early). The irradiated categories 

 are conspicuously late; however, even here the magnitude of the 

 delay is possibly influenced by pretreatment with dim red light, since 

 the subsequent exposure to white light at this time (2D below) pro- 

 duces an unexpected synergism. 



The photoreactivated categories can similarly be compared. It ap- 

 pears that the control and UV-irradiated populations that have been 

 illuminated post-UV each reacts synchronously. Was this because the 

 UV delays were prevented or because illumination maximized delays 

 in each instance? In three cases this distinction is possible. In 2A and 

 2B, light applied before or at the beginning of the normal photoperiod 

 induces the reaction to occur ahead of schedule. These are not so 

 early as lA and IB controls above because the stronger stimulus 

 occurred 1 hr later. Nevertheless, the UV-irradiated categories in 2A 

 and 2B are just as early as their light controls, although they should 

 have been at least 1.0 to 4.5 hr later than even the lA and IB dark 

 controls had photoreactivation not occurred. 



In 2D the controls are late, which is unexpected since illumination 

 in the late scotophilic phase usually shortens the phase; however, 

 when added to red-light mid-scotophilic interruption (ID above) the 

 effect appears to be that of an early or prolonged scotophil interrup- 

 tion, and the phase shift is delayed. Compatible with this interpreta- 

 tion is the broadening effect of a slightly heterogeneous cell popula- 

 tion. However, the UV-irradiated categories show only 1 hr greater 

 delay, whereas they would have been at least 2 to 5.5 hr later had 

 photoreactivation not occurred. 



These distinctions cannot be made in 2C in which the UV-irradiated 

 categories are no later than the controls, but which were as late as 



