94 E. L. POWERS 



can be maintained by means of constant-boiling liquids in the surround- 

 ing vessels. The temperatures during irradiation are monitored by 

 means of thermocouples placed at the bottom of the cylinder. 



The measure of radiation sensitivity in all the studies described here 

 is the slope of the survival curve expressed in reciprocal kiloroentgens. 

 The response curves usually have small shoulders in the low-dose 

 regions. A convenient expression for describing them is 



Fraction surviving = 1 — (1 — e -^'Dyn 



in which the two constants have the following meanings : k is the slope 

 of the curve in kr~i and is the measure we use of radiation sensitivity ; 

 w is a measure of the size of the shoulder in the particular experiment, 

 and in some other papers is referred to as the "hit number". We refer 

 to the slope as the "inactivation constant", and to 7i as the intercept 

 number, because of the fact that extrapolation of the straight line por- 

 tion of the response curve on a semilog plot gives the value of w as the 

 interce]5t on the y axis. All of the experimental variables are tested with 

 complete survival curves, and all I'esponse curves are reduced to their 

 respective slopes. The values of n in these experiments vary about a 

 mean of 1-30. They are regarded as constant and without significance 

 in the studies we review in this paper. The biological response being 

 measured in these experiments is the ability of the irradiated spores 

 to germinate and to produce visible colonies; no other endpoint is 

 being considered. 



THE LONG-LIVED RADICALS (CLASS III) 



The radiation response can be divided into a number of categories, 

 that is, the individual cell may be damaged in a number of ways. The 

 first of these that we shall consider is that damage which is brought about 

 as the consequence of the production of chemical sjiecies with unpaired 

 electrons, termed "free radicals'". In our system these have appreciably 

 long lifetimes, and are related to oxygen in a particular way in the 

 production of the radiation damage. 



Thennal evidence 



We repeated with the spores the study of the basic tem])erature re- 

 sponse of radiation sensitivity in an oxygen-free environment studied 

 earlier in T 1 virus (Bachofer et al., 1953), and noted that, as the tem- 

 perature is varied from very low temperature (5°K) to liigher tempera- 

 tures, radiation sensitivity does not change until the temperature 

 reaches 12r)'K (Webb et a).. 1958; Webb and Powers. 19(U). At that 



