116 RADIOBIOLOGY 



1.0 



10" 



Lag 



Logarithmic 



I l_ 



Maximum 

 -stationary - 



I 



2 4 6 8 10 



Age of culture (hours) 



12 



Id 1 



Fig. 55. A replotting of some of the data of Fig. 55, showing the cycling of 

 radio sensitivity. The cell concentration is shown as a dashed line. (From Staple- 

 ton, J. Bacteriology, 20, 357, 1955; courtesy the author and Williams and Wilkins 

 Co., Baltimore, Md.) 



numbers simply do not follow the ploidy. Another example is the varia- 

 tion in inactivation curves of bacteria in various stages of their growth 

 cycle. The experimental results are given in Fig. 54, taken from Staple- 

 ton's work. This information can be summarized as in Fig. 55, also taken 

 from Stapleton, which shows clearly the cyclic variation in radio- 

 sensitivity. This variation is not altogether out of the purview of target 

 theory, for rapidly dividing cells could well have a lesser number of 

 nuclei than lagging or stationary phase cells in which nuclear division 

 may have occurred without corresponding cell division. Furthermore, 

 the metabolic activity of the nucleic acids of the nuclear apparatus 

 could also result in different sensitivities at different times in the growth 

 cycle. However, there still remain the problei is of a change in target 

 number from unity to six, which seems outside any reasonable increase 

 in nuclear number. 



Opponents of the use of target theory maintain that such failures 

 show the terrible weakness of the theory, and some even go so far as to 

 say that target theory should therefore really not be used at all. Our 

 point of view is that since there are many instances in which target 

 theory gives well-documented success, there is nothing basically wrong 

 with the theory, but that where the theory fails it must be due to the 



