462 



PLANT PHOTOPERIODISM 



is often referred to as the threshold energy in discussions of the Weber- 

 Fechner law, but it should not be confused with the threshold energy 

 of an ungraded response. In no case do these responses have a crude 

 threshold. The response is a linear function of energy up to some low 

 value, which in the hook response amounts to about 10° opening, and 

 from then on the response is proportional to the logarithm of the 

 energy. Differentiating equation ( 1 ) yields DfS = K de/d. This states 

 that the incremental increase in response is proportional to the pro- 

 portionate increase in energy. In the case of the hook opening, a 10- 

 fold increase in energy results in approximately a 20° increase in 

 angle of opening. The fundamental significance of this function is by 

 no means clear. However, it does imply some type of feedback system 

 which keeps reducing the response level and which decreases the rate 

 of the response as the energy increases. It will be noted that in this 

 type of function there is never true saturation. The curve always has 

 a positive slope when plotted on a linear scale as shown in Fig. 9. In 



INACTfVATION , 730m^ 1%) 



+ 



NDUCTION . 660 m^ (•) 



+ 



J 1 1 L_ 



_1 I L. 



-J I I L. 



5 10 15 20 25 30 35 40 45 



INCIDENT ENERGY (m|/cm') 



Fig. 9. Energy-response function for a photomorphogenic system; in- 

 duction and reversal curves of the opening of the hypocotyl hook of bean. 

 The incident energy is plotted on a linear scale. Data from Withrow, 

 Klein, and Elstad (1957). 



this figure the hook opening response is plotted on a linear scale 

 showing the rapid rise in low energy levels and a gradual leveling off. 

 We have not been able to get energies sufficiently high to produce true 

 saturation although, undoubtedly, true saturation would occur at some 

 very high value. This graph also presents the energy relationship for 



