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DISCUSSION 



Hollaender : I wonder whether the effect of oxygen alone would pro- 

 duce as many breaks in Tradescantia as a metabolic system? 



Swanson: This is one reason for assuming that a metabolic system is 

 involved. With Conger's work on oxygen alone it seems to me that it is 

 much more likely that a metabolic system is involved than it is to relate 

 the effect to hydrogen peroxide or some reactive system of that sort. 

 One can, for instance, follow Ebert's lead in adding hydrogen to an 

 oxygenated system which, if we consider the cell to be an aqueous 

 medium, would lead to an increase in hydrogen peroxide concentration. 

 This will induce a larger proportion of breaks, but our data bear no 

 simple relationship to Ebert's curve. Added hydrogen increases the 

 frequency of aberrations. There must be some saturation phenomenon 

 involved; we have gone through the whole range of hydrogen- oxygen 

 mixtures, but we just get a slight increase and it doesn't matter what 

 the hydrogen oxygen mixture is. 



Alper: In fact Ebert's curve for hydrogen peroxide yield with this 

 mixture is very flat, particularly for low doses. 



Swanson: Then I have misinterpreted Ebert's curves because I 

 assumed from them that a mixture of 25 per cent hydrogen and 75 per 

 cent oxygen gave a very large increase in hydrogen peroxide formation. 



Alper: The hydrogen peroxide determinations were made on the basis 

 of total dose. If you irradiate water in the presence of oxygen alone, 



