THE PERIODIC ASPECT 



561 



Fig. 5. Saintpaiilia plants grown at 10°C at different cycle lengths of 

 24 and 32 hr, for 5 months. 



We know that in a number of cases (strawberry, Poinsettia, etc.) 

 temperature strongly modifies the photoperiodic response. It is possible 

 that this must be attributed to interference with the changed internal 

 cycle. 



Now I want to come back to photoperiodism. I have stressed the 

 importance of the periodicity aspect of this phenomenon. We know 

 that the periods of light and darkness have to alternate with a 24-hr 

 cycle, and that it is not a question of a certain amount of Hght or a 

 certain total length of darkness. Only in the exceptional case of 

 Xanthium is a single long dark period sufficient to cause a certain 

 degree of induction. Therefore the extensive work with Xanthium has 

 perhaps made us overlook the significance of the periodicity aspect of 

 photoperiodicity. When we look at the process with this new informa- 

 tion at hand, then we must come to the conclusion that in some way a 

 synchronization has to be achieved between some internal process and 

 the external conditions. 



As a further point we can state that it is not primarily a photo- 

 chemical process we are looking for, since a temperature cycle can 

 achieve the same effects as a light-dark cycle. Although we might be 

 tempted to think of the primary effect as a release mechanism, a 

 stimulus in Pfeffer's sense, a shock effect, it must become clear that 



