464 PLANT PHOTOPERIODISM 



±10°, the temperature coefficient would have to have a value of 

 about 1.1 in order to insure accurate precision. Since, as we have 

 mentioned, average temperature cycles are probably more reproducible 

 than this, it is likely that a temperature coefficient of 1.2 would be 

 adequate. 



o 



~' 90 

 o 



- 80 

 z 

 u 



a 70 

 o 

 60 



§50 



X 



40 



UJ 



30 



20 

 o 



I 10 







INCIDENT ENERGY 

 It ■ 250 m|/cm' 



200- 



/ 



/ 



o- 



EQUIVALENT . 

 ENERGY^"' 



O 

 / 



/ AT END OF 20 hr 



^^ HOOK ANGLE 



-• MEASURED 



I \ j\L 



250 -^ 



e 

 >- 



200 q: 



\±i 



z 



LlI 



150 i 

 o 



100 ^ 



UJ 



_i 

 < 



50 I 



3 10 30 100 300 



TIME OF IRRADIATION (min) 



1000 



Fig. 11. Time-intensity response function for a photomorphogenic sys- 

 tem, the hypocotyl hook of bean. The solid curve is the magnitude of hook 

 opening as related to the time t, at various irradiances I. I X t = constant, 

 250 mj/cm^. As the time varied from 2.5 min to 1200 min, the irradi- 

 ances were decreased proportionately. In all cases, the irradiation began at 

 zero time, i.e., excision. The dashed curve is a plot of the equivalent 

 energy when the angle of opening is extrapolated over the log-curve of 

 Fig. 9 to the equivalent energy value for a 20-hr exposure to give the 

 same opening response. This is a measure of the true effectiveness of the 

 applied energy. Data compiled from Withrow, Klein, and Elstad (1957) 

 and unpublished data of these authors. 



Not only must the period of the clock be relatively refractory and 

 independent of the common environmental factors, but the phase must 

 likewise be a refractory property and relatively uninfluenced by such 

 factors. In fact, in order for photoperiod to be the sole control of the 

 phasing of the clock, it should be the only factor which has the right 

 key. The work of Buhnemann ( 1955a, b,c) and Biinning (1954) in 

 this respect is quite significant. Both have shown that the period and 

 the phase relationships of the clock are relatively unaffected by temper- 

 ature and a great variety of chemicals, including auxin. The only 



