DISC L' SSI ON 753 



Dr. Franck: It is jusl \vli;il you \v;iiU wliidi makes it cliiriciilt for me. If 

 you have the retinenc between the two hiyers, do you have a diffusion of 

 these things between these layers so {]uickly to the outside that all the 

 solutes in water tan act? Therefore, I think it would be simpler to assmne 

 that if it is on the inside, the energy migrates out and only the pigment on 

 the outside bleaches. Now, we can say that all the rhodopsin goes away and 

 all the retinene is found some place else. I think it might not be between 

 the layers but already outside. That is what I wanted to know. It is, on the 

 other hand, astonishing to me that nature would not use that trick again. 



Dr. W ald: I would like to say a word about the problem that VVilliam 

 Rushton raised. \nil it is not to ilhniiinate it, just to carry the thought on a 

 little. Certainly, a visual excitation must demand more tiiaii converting a 

 single molecule of rhodopsin to metarhodopsin. The whole point is that an 

 amplification is needed. I suggested an enzyme. It woidd be the product of 

 that enzyme-catalyzed reaction which is presumably needed to elicit a re- 

 sponse. One question is how long does that product stay around. How 

 rapidly is the excitatory product removed as the ijasis of the off-effect in 

 vision? I think you can make very simple models that will answer this kind 

 of question, indeed, that will answer everything that one knows about the 

 cessation of \ision: but as to the reality of those models, that is another 

 thing. 



Dr. Arno.n: I woidd like to make just a very brief comment apropos of 

 photosyntliesis and the problem of vision. I agree fully with Dr. Wald that 

 in photosynthesis we do have a different problem. Light is a substrate. It 

 is not a stimidant. But if we wish to picture it, we have excellent examples 

 in plants where we have exactly the same problems as in vision. We have 

 phototropism in which light is a stimulant. It seems to me we are a bit 

 unfair to demand of Dr. Wald an explanation of the photosynthetic phe- 

 nomenon. I think it would be more fair to demand of him an explanation 

 of photostimulation. 



Dr. Wald: This is what you just heard about from Dr. Thimann, and 

 you notice again that in plant phototropism the conduction problem is one 

 of diffusion of auxin and chemical conduction, not of exciton or electron 

 conduction. 



Dr. Arnold: Schrodinger, in the delightful little book What is Life?. 

 points out that a plant does not do photosynthesis to get energy. It has miles 

 of energy around it which it could take in. It docs photosynthesis to get 

 negati\e entropy. .And why do you see? To get information, which is essen- 

 tially the same thing. The problems are not different; they are identical. 

 And the structure is so near identical that you are just sure that you need 

 the same kind of machinery. Biology is not all chemistry. 



Dr. Arnox: I have only one brief retort, Mr. Chairman. There is plenty 

 of energy around, but the plant cannot use it unless it is chemical. 



Dr. Wald: Neither can Bill Arnold. 



