Discussion 117 



Gray: It is clear that the position of the bend in now very much in need 

 of being looked at again. 



Previous studies with X-rays by Read, who observed root growth in 

 Vicia faba, by Giles, who observed chromosome structural damage in 

 Tradescantia microspores, and by Baker and others, who observed 

 chromosome structural damage in Drosophila sperm, all showed this 

 bend to occur when the partial pressure of oxygen in the gas phase was 

 about the same as in air, corresponding to a molarity of oxygen dissolved 

 in water at room temperature of around 250 (jiM/litre. This contrasts 

 with the situation indicated by Miss Alper for bacteria, where the bend 

 occurs at about 50 piM/litre dissolved oxygen. The contrast is the more 

 striking if expressed in terms of the molarities of dissolved oxygen which 

 confer half the difference between the anaerobic and the fully aerobic 

 sensitivities; these are around 7 [xM/litre for bacteria and 120 piM/litre 

 for the other materials. Each of the other materials referred to has been 

 irradiated as tissues, not as single cells, and the observed relation be- 

 tween sensitivity and oxygen tension must in each case have been in- 

 fluenced in some degree by the gradient of oxygen tension throughout 

 the tissue arising from cellular respiration. The point has been con- 

 sidered by the authors concerned and believed not to be one of major 

 importance, though definite experimental evidence is lacking. This 

 factor cannot have been operative, however, in the experiments of 

 Conger, who irradiated ascites tumour cells in a suspension in equili- 

 brium with known gas mixtures and obtained a relation essentially the 

 same as that which had been obtained for the roots, microspores, and 

 Drosophila sperm. The procedure used in Conger's experiments was 

 that the fluid in which the cells were suspended was stirred vigorously 

 by a jet of gas of known composition. The molarity of the dissolved 

 oxygen was not measured. In view of Miss Alper's results it becomes 

 a matter of importance to check Conger's observations. It will be re- 

 called that, using the same experimental procedure. Conger found only 

 a very slight influence of oxygen on the sensitivity of ascites tumour 

 cells to neutron radiation which, by analogy with the relatively slight 

 influence of oxygen tension on the chemical reactions induced in aqueous 

 solutions by high ion density radiations, lent support to a chemical — as 

 against a biochemical — type of explanation. In other cases the evidence 

 is less clearcut. Dr. Laser referred to the influence of carbon monoxide 

 on the X-ray sensitivity of his bacteria. It will be recalled that Sachs 

 found that carbon monoxide at four atmospheres pressure in the 

 presence of air increased damage to Tradescantia microspores. I found 

 similar effects in roots but only when the carbon monoxide pressure 

 was increased to about 20 atmospheres. Moreover, I found a much 

 smaller influence of carbon monoxide on growth inhibition induced by 

 exposure to neutrons. While these facts would be most readily accounted 

 for in terms of radiation chemistry, the fact that the enhanced radio- 

 sensitivity was a function of the period of carbon monoxide pretreat- 

 ment, which was only fully effective if applied for 30 minutes prior to 

 as well as during irradiation, suggests that metabolic factors were 

 involved. 



