PHOTOREACTION AND ASSOCIATED CHANGES 



431 



These values approach the maximum possible value, about 2.0 X 10\ 

 for an oscillator strength of unhy. 



ASSOCIATED REACTANTS 



With constant cr and efr the variations of €(j)X must arise from varia- 

 tions in (f>X. In the absence of materials absorbing specifically at wave- 

 lengths greater than 6000 A, as is the case for seed germination, X 

 changes monotonously inversely as the fourth power of the wavelength. 

 Changes in X are not more than 1.5-fold for 6500 A compared with 

 7300 A for scattering, and are in the same direction for both Lepidiiim 

 and lettuce seed. The approximately 70-fold greater value of e(/)Z (red 

 absorbing) for lettuce seed compared with Lepidium and the 18-fold 

 smaller value of e^Z (far red absorbing), giving a 1200 overall 

 change, must chiefly be due to variations of the quantum efficiencies. 

 The great changes in quantum efficiencies require the presence of other 

 effective molecules than the pigment in the photoreaction which could 

 only be quenching agents or reactants. The fact that tr^Z (red absorb- 

 ing) varies oppositely to efr4>X (far-red absorbing), as discussed in part 

 by Dr. Toole, for the seed germination indicates that the change is due 

 to another reactant. The high a priori probabilities for the reactions, 

 which can approach a quantum efficiency of 1, indicate that either 

 electrons or hydrogen atoms are transferred in the photoreaction, as is 

 usual for photoreactions in condensed systems. 



% Rel. Response 



— - Lepidium 

 80 Germination 



Bean 

 Internode 

 ?o Length 



J % Pigment Conversion 

 to for red absorbing 

 form 



Rel Energy^ 



Fig. 5. Relationships between the degree of response, the percent pig- 

 ment conversion, and the relative energy. 



