chromosome aberrations in Tradescantia 751 



If oxygen is present in the water, as is normally the case in most bio- 

 logical systems, the following reactions can also occur : 



H + 2 -> H0 2 (9) 



H0 2 + H — H 2 2 (10) 



The experimental evidence, particularly that of Bonet-Maury and 

 Lefort (1948) and Allen (1948), indicates that in the absence of oxygen 

 there is very little decomposition of water by X rays, since little or no 

 H 2 , 2 , or H 2 2 can be detected. However, pure water is readily decom- 

 posed by a particles. The amount of H 2 2 formed [presumably by 

 reaction (5)] is directly proportional to the dose. There are few experi- 

 mental data for other radiations, but Allen's results (1948) indicate that 

 neutrons also produce appreciable amounts of H 2 2 in oxygen-free water. 

 When dissolved oxygen is present, H 2 2 is produced by X rays, presum- 

 ably by way of reactions (9) and (10). The influence of such factors as 

 dose rate, pH, temperature, and amount of dissolved oxygen have been 

 investigated by Bonet-Maury and Lefort (1948). With a particles, the 

 yield of H 2 2 when oxygen is present is approximately the same as in the 

 absence of this gas. This absence of an oxygen effect is apparently 

 related (Allen, 1948) to the closely spaced distribution of the H atoms and 

 OH radicals, such that reactions (4), (5), and (6) are favored, and conse- 

 quently H atoms are unavailable to participate in reaction (9). With 

 X rays, the more widely spaced distribution of the H atoms and OH radi- 

 cals does not favor reaction (6) and consequently reaction (9) does occur. 

 When dissolved hydrogen is present in water in place of oxygen, a back 

 reaction to form water, and thereby remove the OH radicals, takes place 

 [reaction (7)]. 



As a result of their observations that there is a marked effect of oxygen 

 on chromosome aberration production by X rays, and little or no such 

 effect with a particles, Thoday and Read (1949) suggested that the active 

 substance responsible for aberration production might be H 2 2 . Further 

 striking parallelisms between H 2 2 production in water under various 

 conditions of irradiation, such as temperature and pH, and chromosome 

 aberration production under similar conditions, are discussed by Giles 

 (1952). Much of the radiochemical and biological evidence may be 

 interpreted as furnishing indirect support to the view that H 2 2 is 

 important in aberration production. There is also direct evidence that 

 H 2 2 is mutagenic in such organisms as bacteria (Wyss et al, 1948) and 

 molds (Neurospora) (Wagner et al, 1950; Jensen et al, 1951). The 

 difficulty of introducing this substance into cells such as microspores has 

 precluded a direct test of aberration production by H 2 2 in Tradescantia. 



Although the H 2 2 hypothesis has considerable evidence to support it, 

 the radiochemical data, especially for X irradiation, suggest that other 



