546 RHYTHMS IN PLANTS AND ANIMALS 



period mixtures. (By this method each intersection of Hnes that occur 

 at some place other than at a measured point on the plotted curves 

 was inferred.) 



RESULTS 



The effects of exposure to far ultraviolet are shown in Fig. 1 . Three 

 general effects are obvious. ( 1 ) In each category there is an endoge- 

 nous rhythmicity of reactivity with cycles of approximately 23 hr 

 duration. (2) There is apparent phase shift to the right (delay) that 

 is UV dose-dependent. (3) The amplitudes (RM) of the response 

 curves are only slightly affected by low doses of UV. (Higher doses, 

 correlated with some cell death, cause an extreme depression of 

 maxima and a slight elevation of minima (Ehret, 1955b).) 



The results of subsequent exposure of ultraviolet-irradiated cells 

 to visible irradiances are shown in Fig. 2. (1) The approximately 

 diurnal rhythmicity (23 hr) is again evident. (2) There are no 

 conspicuous phase shifts experienced by treated groups independently 

 of the controls. (3) The response curves are either not at all, or only 

 shghtly depressed, and there is a slight elevation of minima in curve 

 D. 



A further analysis of these results shows that there are three rather 

 general superimposing effects that can be distinguished from the more 

 specific UV + dark and UV + light effects on the cellular mecha- 

 nism. The superimposing effects are ( 1 ) an endogenous 23-hr rhythm, 

 (2) the initial exposure of all categories to dim light, and (3) the 

 effect of photoreactivating light itself on controls. In Table I such an 

 analysis is summarized, and the consequent minimum and maximum 

 sensitivity phases are inferred from it. It should be noted in the table 

 that in categories lA and IB the controls are 2.5 hr earlier than 

 expected. This is compatible both in direction and degree with the 

 shift expectations that ensue from foreshortening of the photoperiod 

 (Ehret, 1953) in IB, and from a dim light stimulus applied during 

 the late scotophilic phase in lA (Ehret, 1955a). It is probable that 

 the stimulus in this case was the dim red light to which the cultures 

 were exposed at the start of the experiment. However, in both lA and 

 IB the irradiated categories are late whether compared with the 



