502 RHYTHMS IN PLANTS AND ANIMALS 



in general, bears a unique phase relation to the light cycle); but it 

 seems likely that the effective control of fi's phase and period is usually 

 due more to its entrainment by A than to direct entrainment by the 

 external temperature regime. This second instance of entrainment 

 (that between A and B) is again predominantly unilateral; at any rate 

 this is clearly the case in Drosophila. But even in the fly there is evi- 

 dence of some feedback of B on ^4, and it may well be that A-B 

 entrainment is more nearly mutual elsewhere, especially in plants. 



It is through entrainment phenomena that the theory of the cell as a 

 self-sustaining oscillatory system bears on the photoperiodic and 

 thermoperiodic control of growth. It seems to us that interpretation of 

 deleterious effects due to the light and temperature regime should not 

 be sought in terms of their direct action as such even when this is 

 coupled to the Bunning hypothesis of a scotophil phase; it should be 

 sought in terms of the success or failure of the light and temperature 

 regimes as entraining agents that mamtain: (1) appropriate syn- 

 chrony of cellular oscillations; and, perhaps, (2) a period sufficiently 

 close to the natural period of the system at which overall metabolic 

 efficiency is likely maximal. 



There can be little doubt that the general coordination of activity 

 between cells which must underlie normal tissue function will include 

 the synchronization of the innate oscillations in the constituent cells; 

 and conversely that failure of cells to be synchronized will lead to 

 tissue and organ disfunction. This is how we would interpret the 

 Highkin and Hanson and Hillman results. The issue does not seem to 

 hinge on the phase relation of the plant's rhythm to environment, as 

 Hillman suggests, but rather on the synchronization of constituent 

 cells. There is good reason to believe that continuous light will induce 

 an asynchrony between cells. Constant light usually has one of two 

 effects in organisms: either (1) the period changes, lengthening in 

 some cases, shortening in others (see summary and examples in Pitten- 

 drigh and Bruce, 1957); or (2) it is, more commonly, lost (Bruce 

 and Pittendrigh, 1957b, for summary of insect and microorganism 

 cases). We have suggested earlier that the loss of an assayable rhythm 

 in a multicellular organism may reflect only a loss of cell synchrony, 

 perhaps due to a differential light-induced period change in the indi- 

 vidual cells. And certainly Hillman's remarkable demonstration that a 



