Time Relations and Endogenous Rhythms • 53 



ously than in most, appeals to expert opinion are useless, since 

 there are accomplished and respected investigators on both sides. 

 The writer is frankly of two minds on the subject. On the one hand, 

 the existence of rhythms and their influence in many processes 

 recommend them as the underlying mechanism of the more particu- 

 lar time-dependent response, photoperiodism. Yet hypotheses on 

 the precise relationship tend to seem vague, or easily disproved, 

 or ad hoc elaborations full of special exceptions. It has understand- 

 ably been argued that they simply confuse the issue, explaining the 

 relatively simple response of photoperiodism in terms of an equally 

 unexplained set of more complex phenomena. Yet, if photoperiod- 

 ism is indeed a special case of a basic biological process, it would be 

 a pity not to recognize it as such. So far, the evidence on both sides 

 consists largely of correlations or the lack of correlations, and these 

 differ from plant to plant. Certainly endogenous circadian rhythms 

 are at least modifying factors in photoperiodism; whether they are 

 more than that, time will undoubtedly tell. 



