24 L. H. GRAY 



than the one which is effective. It would not inijDly that one ionization 

 (or one excitation) is the initiating event at the physical level, but only 

 that one particle is involved. 



From the slope of the log survival curves, we know that the target 

 within which the chemical reaction chains leading to loss of reproductive 

 integrity are initiated must contain a great many molecules. Either the 

 molecules themselves must be uniquely important to cellular prolifera- 

 tion, or the reaction chains to which they give rise must involve the 

 inactivation of uniquely important molecules. iVttention is thus 

 focused on molecules of the DNxA.-RNA-protein class. 



A second deduction which can be made from the slope of the log 

 survival curves is less agreeable. It is that, for all organisms as large as or 

 larger than bacteria, the target is a small fraction of the volume of the 

 cell. The total number of ions generated in the cell for each electron 

 transit through the target is therefore large (Table 1). It follows that 



Table I. Fast electron transits per cell at a dose ivhich gives 37 per cent 



survivors 



Drv spores 1 2-5.104 3000 T.IO* 



Vegetative £. co/j 1 5.103 600 10* 



Plant or mammalian cells 10 100 1500 10^ 



when physical methods, such as electron spin resonance spectroscopy, 

 are used to detect free radical intermediates, the chances that the signal 

 is characteristic of the actual radicals which initiate loss of reproductive 

 integrity is correspondingly small. 



BIOLOGICAL SYSTEMS IRRADIATED IN THE DRY STATE 



Loss of reproductive integrif/t/ by spores irradiated in the dry state 



I should like to take the exceedingly beautiful work with dry spores of 

 B. megaterium by Dr. E. L. Powers, and his collaborators (this Sym- 

 ]30sium) as a starting point for my remarks concerning loss of repro- 

 ductive integrity in aerobic and anaerobic bacteria, and the proliferat- 

 ing cells of higher plants and animals, with which I am personally 

 more familiar. Davis and Hutchinson (1952), working with the closely 



