672 LIGHT AND LIFE 



Dr. Commoner: Of all the natural metabolic pigments, which we studied 

 in the ESR spectrometer, flavins and particularly FMN stand out as being 

 exceedingly sensitive to light with respect to free radical formation. This 

 suggests that flavins may participate very readily in light-sensitive biological 

 processes. 



Dr. Gordon: If 1 understood correctly, you are proposing that light affects 

 the rate of auxin transport. How is it then that light has virtually no effect 

 on the amount of auxin obtained by basipolar diffusion irto agar? 



Dr. Thimann: I think this problem is too long to discuss. I will only say 

 that I was not proposing that the total amount of auxin transported is 

 affected. Only the direction in which it is transported is affected. If the 

 auxin moves laterally instead of longitudinally, there need be little or no 

 change in the amount diffusing out. 



Dr. Gordon: I think it should be pointed out that, as far as I am aware, 

 it has never been shown rigorously that light alters the direction of auxin 

 transport. This has only been inferred from inequalities of auxin con- 

 centration. 



Dr. Jagendorf: From what I remember of the cytological work, when you 

 illuminate cells with dim light the plastids line up on the walls perpendicular 

 to the illumination, but this includes the wall on the side towards the light 

 beam and the wall on the dark side. If plastids are localized on both walls, 

 how do you propose that they cause a directional How of auxin? 



Dr. Thimann: Briefly, it is this way. At low light intensity the plastids line 

 up on the wall facing the light. At high light intensities the plastids align 

 themselves along the walls which are parallel to the incident light. There 

 are complicated effects varying with the dosage, but there is a directional 

 movement at low light intensity. 



Dr. Wald: In what organism does that occur? 



Dr. Thimann: One of the most popular groups has been the mosses, such 

 as Fontinalis or Mnium. 



Dr. Wald: Do things like this happen in the organisms you use? 



Dr. Thimann: We are working on it. 



