500 RHYTHMS IN PLANTS AND ANIMALS 



temperature range. The cell, in this view, comprises a whole class of B 

 oscillators coupled to and entrained by a master A oscillator that alone 

 imposes that temperature independence of period universally charac- 

 teristic of the steady-state rhythm. 



Further discussion of detail in our scheme must await more experi- 

 mental data; but its relevance to thermoperiodism and photoperiodism 

 needs comment. The photoperiodism we here have in mind is that dis- 

 cussed in Highkin and Hanson's ( 1954) and Hillman's ( 1956) papers. 

 Highkin and Hanson showed that tomato plants exposed to Yighi-dark 

 cycles with periods of 12 (6:6), 24 (12:72), and 48 (24:2^) hr 

 differed markedly in their growth although they received equal total 

 light per 48 hr. Only those plants grown on a light regime with a 

 period of 24 hr escaped a radically stunted growth. In a separate ex- 

 periment a 2-hr light supplement was given to greenhouse plants either 

 at the end of the day (photophil fraction of the cycle) or in the middle 

 of the night (scotophil); in the latter case growth was measurably de- 

 pressed. The authors relate their findings to Biinning's ( 1937) hypoth- 

 esis that endogenous daily rhythms are involved in "photoperiodic" 

 responses. The essential feature of this hypothesis, which Highkin and 

 Hanson seize, is the division of the plant's daily cycle into photophil 

 and scotophil fractions; light falling in the scotophil fraction is opera- 

 tive either in affecting flowering or, in this case, inhibiting growth. 



Hillman also found that tomatoes could be injured by an abnormal 

 light regime. They developed necrotic or chlorotic spots in continuous 

 light or in light-dark cycles with periods of 8, 10, 12, 15, or 16 hr. 

 No injury resulted, however, from light-dark cycles with periods 

 closer to 24 hr (viz., 20, 24, 30 hr). Hiflman's most remarkable result 

 was demonstration that the injury caused by continuous light in a 

 constant-temperature regime of either 30°C or 17°C was avoided 

 when the continuous light was accompanied by a cycle of temperature 

 with a "daily" (24-hr) period (30° day, 17° night). Hillman also con- 

 siders his results in terms of the Biinning hypothesis: ". . . it is possi- 

 ble that all the results with tomato are due to the existence in the 

 plant of a rhythm whose period of 24 hours must be in phase with 

 environmental changes for normal development." This comes much 

 closer to the present writers' view than does the Highkin and Hanson 



