434 PLANT PHOTOPERIODISM 



Table III. Effect of exposure to sunlight on germination of Lepidium virginicum 



seed. Seed were returned to darkness and shifted from 15° to 25° C after exposure 



to radiation. Average results from 4 lots of 100 seed each. 



Germination with 4 min Prior Germination without Prior 



Exposure to Red Radiation Exposure 



degree dependent upon the radiant flux, the balance of those reactions 

 having slow turnover rates in the vital system. The reversible photo- 

 reaction samples this general condition through its dependence on the 

 hydrogen-transferring systems. Upon removal to darkness, the pigment 

 balance is fixed at the condition attained in the high radiant flux. In 

 general, the systems seem to shift toward high amounts of A as shown 

 in reaction 2 — they become relatively more sensitive to red radiation. 



SPECULATION ON POSSIBLE ISOLATION OF THE PIGMENT 



AND ON ITS NATURE 



I can give only one precise bit of information bearing on the nature 

 of the pigment, namely, curves showing variation of molecular absorp- 

 tion coefficients with wavelength (Fig. 6). These are obtained from the 

 reversibility of the reaction, as previously explained, and from action 

 spectra. Values of e at the maxima are indicated as 2 X 10^, which is 

 probably correct to within the range of 1 to 3 X 10^. 



Perhaps you expect a discussion of the possible identification and 

 isolation of the pigment. A short one is, "your guess is as good as 

 mine." That such a strongly colored compound should be unobserved 

 by the eye in plant tissue or not previously observed in preparations 

 indicates that the average concentration must be very low. The physio- 

 logical function would almost require it to be a protein with a strongly 



