562 RHYTHMS IN PLANTS AND ANIMALS 



this is not the case. For the darkness or the differing temperature has 

 to act for a definite length of time, indicating that there is a definite 

 energy level which will interfere with the photoperiodic induction. It 

 seems very significant that both the lower temperature and the darkness 

 have to last for the same length of time to prevent continuous-light 

 injury in tomatoes. 



The previous analysis has turned up as the principal conclusion, to 

 which I feel most of the participants at this symposium had come al- 

 ready, that one of the major attributes of the photoperiodic response 

 is its periodic nature. Remarkably enough, this had been recognized 

 right from the beginning by Garner and Allard, when they coined the 

 term photoperiodism, but it was largely neglected by investigators in 

 the following years. I also want to stress the second major attribute of 

 the photoperiodic response, namely its 24-hr period, which is only 

 slightly temperature-dependent. And in the third place, I want to 

 record my conclusion, that the photoperiodic response need not be 

 solely based on a photoreceptor, but can proceed also with a thermo- 

 receptor, in which not temperature as such, but a 24-hr alternation of 

 temperatures is effective. 



How do these three attributes affect our theoretical considerations 

 about the mechanism of the photoperiodic process? None of these is 

 at present incorporated in a general theory of the process, parts of 

 which were presented by Bonner (p. 245) and Hendricks (p. 423). 

 Let us first consider Galston's P-I-G scheme. In this the photoreceptor 

 part is not changed at all and Dr. Hendricks' discussion stands un- 

 altered, with the suggestion that even more stress be laid on the natural 

 decay of the far-red modification of the pigment which can make it an 

 effective oscillating system with a 24-hr cycle. The growth response 

 end of the scheme is very definitely periodic in nature as the experi- 

 ments with the tomato have shown. These experiments have also shown 

 that when the endogenous period of the growth response end of the 

 system does not mesh with the externally induced period of the photo- 

 receptor pigment, growth is reduced, and when no periodic signal 

 comes from the photoreceptor, growth is equally reduced. 



An analysis of the parallel T-I-C scheme leads to approximately the 

 same picture. The thermoreceptor must also be an oscillating system, 

 and it would be very attractive if the red, far-red pigment could be 



