52 • Photoperiodism: Attempts at Analysis 



indicate that the phase of the rhythm can be shifted by low 

 temperatures during part of the cycles used. A recent paper by 

 Oltmanns (1960) suggests that the interactions between tempera- 

 ture, light, and rhythmic phemonena in the flowering of Kalanchoe, 

 and by implication in the flowering of any other plant, are not yet 

 sufficiently understood to be described by any simple hypothesis. 



ENDOGENOUS CIRCADIAN RHYTHMS 

 AND THE RED, FAR-RED SYSTEM 



There appears to be a relationship between the red, far-red 

 system, unquestionably involved in photoperiodism, and endog- 

 enous circadian rhythms in plants. Red is the most effective light 

 in initiating the movements of etiolated bean seedlings, previously 

 discussed, and this effect is far-red reversible (see Bunning, 1959a). 

 More directly related to photoperiodism is the observation by 

 Konitz (1958) that far-red given as an interruption of the main 

 light period of Chenopodium amaranticolor (SDP) inhibits the 

 effectiveness of inductive cycles, just as does red given in the dark 

 period. Since rhythms in plants demonstrably affect many processes 

 under certain circumstances, the particular closeness of their con- 

 nection with the red, far-red system is hard to judge, even from 

 these results. Engelmann (1960) has found that when red light is 

 given to Kalanchoe at various times during a 62-hour dark period, 

 it inhibits induction in what would be predicted to be the scoto- 

 phile phases and promotes it in the photophile phases. Far-red, 

 however, does not show an inverse pattern, but simply inhibits 

 during the first half (30 hours) of each dark period and inhibits 

 less during the second half. 



CONCLUDING REMARKS ON CIRCADIAN RHYTHMS 



AND PHOTOPERIODISM 



If the reader is now confused, he is in good company; no aspect 

 of flowering physiology has given rise to more complex experi- 

 ments, tenuous interpretations, and heated controversy. The contro- 

 versy is not over the existence of rhythms in plants, which is not 

 seriously questioned, but over their usefulness and relevance in 

 understanding photoperiodism. In this situation, even more obvi- 



