DISCUSSION 751 



any tasc. voii do Iki\i' similar problems in \i.si()ii au(.\ in photosynthesis. 

 You. too. do not undcisiand how a primary process located at any one 

 molecule in the layered structure in the \isual rod can initiate nerve excita- 

 tion somewhere else in the cell. 



The idea that two-dimensional structures are integrating devices in both 

 chloroplasts and \isual rods seems to me sound. In one case, liic mechanism 

 may be energy migration, in the other case, chemical transler. We would 

 like to know which. That is why people like Bill Arnold and I are in- 

 terested in this matter. 



Dr. Walu: Yes, it is all right as long as you keep your field ol \iew a little 

 wide. Hagins is the only man I know of who has tried to find these processes. 

 We would be happv to find them too, but scj far that has not succeeded. 

 There is no logic in the position that they must be there, and that it is up 

 to us to find them. Biologists find lamellated structures all over the place. 

 Mitochondria have them: but I will tell you another place where they are, 

 very much as in rods and cones, and that is the myelin sheath of a nerve 

 axon. In the myelin sheath the layers are arranged radially, so that they 

 extend for long distances. Yet you know that nerve conduction does not 

 depend, as far as we know, on processes ol exciton migration or semi- 

 conduction, but it waits for the chemistry to happen. 



Dr. Platt: To what degree is this ignorance about these relations of struc- 

 ture to chemistry due to a lack of physical micro-tools and chemical micro- 

 tools for operating at this level? 



Dr. Rabinowitch: You can turn the tables, and ask Bill Arnold: did you 

 really prove energy migration in photosynthesis? The answer will be no, 

 w'e did not prove it in chloroplasts either. Incidentally, in the case of the 

 ner\'e sheath, a plausible purpose of concentric wrappings is easy to suggest- 

 good insulation in the radial direction, conductivity in the tangential 

 direction. 



Dr. Wald: But the saving grace in photosynthesis is that you know what 

 you want to do with the absorbed energy. You have a job to do. If I only 

 had a hint of the job I want clone, in terms of mechanism in visual excitation, 

 it would be easier to go ahead. I told you a little earlier that I think that 

 maybe the product of light acting on a visual pigment is an enzyme. Well, 

 what do you want an enzyme to do that will produce a nervous excitation? 

 Rushton suggested, and it has been suggested before, that it may increase 

 the permeability of a membrane. 



Dr Rushto.n: I just wanted to say one thing, and that is that there are 

 two aspects in the problem which Dr. Wald raised. It is not only to turn 

 the light sensation on, but also to turn it off. If you had this carotenoid 

 cap on a protein and a cjuantum of light turns it on and lifts the cap, we 

 know that with rhodopsin that cap will have a 5():.50 chance of being back 

 by 10 minutes later, if the light goes out. 



Dr. Wald: Dr. Rushton raises the very interesting and important problem 

 of the fading of the light sen.sation when the light goes off. From the picture 



