26 L. H. GRAY 



properties closely resembling those inferred from the l)iological obser- 

 vations, have been most beantifnlly demonstrated by Dr. Powers and his 

 collaborators by means of electron spin resonance (ESR) spectroscopy. 

 Further research may well lead to a further subdivision of these 

 classes. As they stand, we may assign relative probabilities to the 

 recognized classes of initiating event as: A-0"24, B-0'14, C — 0*38, 

 and D — (V2-t. Relative probabilities of reaction chains in which the 

 participation of oxygen is essential to those to which it is not, are thus 



C + D 0-62 



= = 1-63 



A + B 0-38 



The sensitivity of the spores irradiated in oxygen to that when partici- 

 pation of oxygen is excluded, l)oth during and after irradiation, viz: 



1 



= 2-63, a figure close to the values usually observed for the 



A+B ^ 



relative sensitivities of plant and animal cells irradiated aerobically 

 and anaerobically. 



BIOLOGICAL SYSTEMS OF INTERMEDIATE WATER CONTENT 



The radiosentivity of seeds, with special reference to wafer content 



The influence of varying amounts of water on the sensitivity of cells 

 has been studied in seeds (see reviews by Ehrenberg 1955, Caldecott, 

 1960, Nilan et al. 1960, Konzak et al, 1960, Sheldon Wolff, 1960, and 

 Davidson, 1960). In evaluating this work it must be borne in mind that 

 the l)iological damage induced in the seeds (as also that in spores) has 

 always been examined after hydration and germination. 



When seeds are irradiated dry, intermediates of short and long life- 

 time are produced which have much in common with those that lead 

 to loss of reproductive integrity in irradiated spores. H2S and nitric 

 oxide present during irradiation decrease the extent of the injury. This 

 is largely due to the influence of these treatments on the course of those 

 reactions which involve participation of oxygen. By ESR spectroscojjy 

 it has been established that long-lived radicals induced by the irradia- 

 tion of dry Agrostis seed disappeared fairly rapidly in the j^resence of 

 nitiic oxide (Sparrman et al., 1959). 



If irradiated seeds are hydrated in water which is free from dissolved 

 oxygen, the long-lived intermediates which are present at the end of ii-ra- 

 diation are stabilized against subsequent reaction with oxygen, or with 

 any other molecule so far tested. The level of damage is approximately 



