274 RADIATION BIOLOGY 



cell may be able to resume its normal activity and recover; if too long, 

 irreversible damage might be caused. The presence of other substances 

 would raise the inactivating X-ray dose, but ordinary metabolites would 

 have to be present in appreciably higher concentrations to have a marked 

 effect. With regard to reactions which take place on surfaces or are 

 activated by surfaces, too little is known to suggest any well-supported 

 mechanisms. 



An example which may be explained by the dilution effect operating 

 within the cell is an investigation of the effect of varying the water con- 

 tent in germinating seedlings and seeds. The dry seeds are more radio- 

 resistant than those which have taken up water, and the radiosensitivity 

 of seedlings increases with increasing water content (Henshaw and 

 Francis, 1935; Gelin, 1941; Komuro, 1924), whereas retardation of 

 growth by lowering the temperature does not appreciably change the 

 radiosensitivity (Henshaw and Francis, 1933). The effect therefore 

 appears to be due to the water content and not to the higher metabolic 

 activity of growth which is frequently quoted as the cause of increased 

 susceptibility to radiation. Other investigations demonstrate the dilu- 

 tion and protection effect in operation when the concentration of suspen- 

 sions of, for example, spermatozoa (Evans et al., 1942) or of erythrocytes 

 (Halberstaedter and Goldhaber, 1943) is varied or protective substances 

 are added. In the first example the criterion was the fertilizing power 

 and in the second the degree of hemolysis. 



POSSIBLE MODIFICATIONS OF RADIATION EFFECTS 



We shall discuss in this section all those substances which are known 

 to have or are likely to have a modifying influence by their presence or 

 absence. 



Hydrogen Peroxide, HO2, and Oxygen. Most consideration has been 

 given to hydrogen peroxide, which was known to be generated by radi- 

 ation long before the indirect action of radiation was recognized. A 

 great deal of work has been carried out on it by certain French scientists 

 (Bonet-Maury and Frilley, 1944; Bonet-Maury and Lefort, 1948; Loise- 

 leur, 1942, 1943; Loiseleur and Latarjet, 1941; Loiseleur, Latarjet, and 

 Caillot, 1941) and also by Fricke (1934, 1935) and Fricke and Hart 

 (1935). Without enlarging here on the still controversial reaction 

 mechanisms leading to hydrogen peroxide and HO2 formation, it may be 

 stated briefly that X rays produce H2O2 when oxygen is present in the 

 water, but not in its absence, and that a rays generate H2O2 in either case. 



It is important to note, as clearly pointed out by Allsopp (1944, 1948), 

 that hydrogen peroxide is not the primary product of radiation but a con- 

 sequence of the reaction of radicals with each other and with dissolved 

 oxygen. Therefore, without doubt, radicals and hydrogen peroxide can 

 be reactants in oxido-reductions of solutes. There is, however, a tend- 



