Chromosomes and Genes 117 



ations may effect a different distribution of ionization along the tracks 

 and thus affect breakage in different ways. 



The work on oxygen with identical results in root tip chromo- 

 somes of beans (Thoday and Read, 1947), barley seeds (Hayden and 

 Smith, 1949), and Tradescantia cells (Giles and Riley, 1949) leaves 

 no doubt that peroxides are again the intermediary between radiation 

 effect and breaks. We do not intend to discuss the special question of 

 how this result reflects upon the Treffer theory. Giles, as well as 

 Thoday, thinks that there is no contradiction here. To quote Giles: 

 "The essential requirement is that the action of ionizing particles, 

 whether direct or indirect, be relatively localized. If the effect is 

 principally indirect, it appears that a substance such as H2O2 must be 

 produced along the track of an ionizing particle and must have a 

 relatively limited effective diffusibihty (or short half-life). In fact it 

 seems necessary that its effective distribution within the nucleus must 

 correspond in pattern rather closely to that of ionization distribution 

 along particle tracks." 



For our discussion of the nature of the gene the decisive result 

 is that there is still less reason to assume that there is a difference 

 between gene mutation and chromosome rearrangement as a result of 

 irradiation experiments. To this conclusion we may add that an 

 increasing number of facts have come to light which show that 

 different physical and chemical conditions influencing radiation effects 

 act aHke for general radiation damage, point mutation, and chromo- 

 some breakage. 



Muller, while realizing the conclusions just drawn and empha- 

 sizing that the two effects (gene mutation and rearrangement) must 

 have much in common, still warns against assuming their identity. In 

 his opinion, some facts do not support the identity. To quote Muller 

 (1950a); "One of these [facts] is the very great difference in the 

 mode of response to X- and gamma rays of the higher plant [cereal] 

 material studied by Stadler and of the Drosophila material. In the 

 plants the gene mutations are induced with a frequency so low 

 (supposing them to be induced at all) relative to the frequency of 

 breaks and rearrangements, as to have been practically impossible to 

 demonstrate; while in the Drosophila material the two phenomena 

 occur with comparable frequency." I cannot see why these facts must 

 bear upon the mode of production of the two effects by, for example, 

 different types of ionization. I should prefer to conclude that the 

 difference relates to chromosomal structure. In the cereal there may 



