THE PERIODIC ASPECT 563 



sufficiently affected by temperature to serve the perception of tempera- 

 ture fluctuations. Tlie cellular response at the other end of the scheme 

 can be considered in terms of mitotic activity in the stem meristem, 

 which according to BUnning (1952) has a clear-cut 24-hr rhythm. On 

 the basis of Wetmore's anatomical analysis (p. 255) of the major 

 events in the stem tip as it changes from vegetative to generative, we 

 can assume that properly timed cyclic stimuli could activate one or the 

 other group of cells, benefiting leaf primordia or axillary structures. 



Upon closer analysis the P-I-G and T-I-C systems have shown a 

 remarkable similarity; in both, the receptor responds to periodic stimuli 

 and is periodic itself with the rhythm impressed upon it by the environ- 

 mental stimuli, and in both, the reactor has an endogenous periodicity, 

 which must be synchronous with the period of the receptor to respond 

 properly. We can only envisage such synchronization when the inter- 

 vening processes, I, are rhythmic, probably with the period imposed 

 upon them by the receptor. The synchronization of the complete system 

 could then be expected to occur at the I-C or I-G transfer point. 



If the previous analysis is correct, we should expect not a specific 

 substance to mediate between perception in the leaf and response in the 

 growing parts of the plant, but the intermediary processes would be of 

 the nature of a pulse, perhaps in the form of rhythmic concentration 

 changes in some chemical constituent. 



Now I leave these thoughts with you. I want to stress that I do not 

 reject a single fact in the extensive photoperiodic lore; I claim only 

 that they give a one-sided picture of the problem because all experi- 

 ments were slanted toward the photoside of it, neglecting the perio- 

 dicity and the temperature aspects. Whether I have overemphasized 

 them, will have to be seen in the future when a proper balance between 

 these views has been achieved. 



REFERENCES 



Arthur, J. M., and E. K. Harvill. 1937. Plant growth under continuous illumina- 

 tion from sodium vapor lamps supplemented by mercury arc lamps. Contribs. 

 Boyce Thompson Inst., 8, 433-43. 



BUnning, E. 1932. Untersuchungen iiber die autonomen tagesperiodischen 

 Bewegunsen der Primiirblatter von Phaseoliis nmltifiorus. Jahrb. wiss. Botan., 

 75, 439-80. 



