442 H\I)I\|-1()\ KIOLOGY 



ill siiliinuta^cnic concent r.it ion when adniinisterod with ultraviolet radi- 

 ation (2r)87 A), has Ween toiind to jiixc an iiici'casc in imitation rate of A. 

 Iirrciis of liOO 100 per cent oxci- that caused hy the iiiadiat ion aionc^ 

 (Swaiisoii and (ioodgal, 1948). This work was iiniitcd to oIiscia at ion of 

 morph()l()j>;ical \ariants. A microeoiiidial strain of M earuapora has been 

 employed, both morphological and nutritionally exacting mutants being 

 scored, in similar experiments leading to the same results, i.e., an increase 

 in mutagenicity of ulti-aviolet radiation after a 30-min pretreatment with 

 0.1 per cent aciueous solution of nitrogen mustard (Swanson d al., 1949j. 

 Pretreatment with dinitrophenol also increased the morphological muta- 

 tion rate of A. terreus, whereas potassium cyanide had no effect (Swanson 

 and Goodgal, 1950). Posttreatment with dinitrophenol and submuta- 

 genic levels of nitrogen mustard also increased the mutagenic effectiveness 

 of the radiation, suggesting to these authors an indirect action of the 

 radiation. An interpretation based on direct action is, of course, e(iually 

 tenable, since in our present state of ignorance it is as reasonable to 

 assume semistable intermediate states of the genie material as extragenic 

 chemical intermediates. 



A photodynamic sensitization by methylcholanthrene of S. cerevisiae to 

 hght in the 3450-4500 A range has been noted (Hollaender d al., 1939). 

 Pretreatment with infrared radiation has been found to raise slightly the 

 mutation frequency at high doses of ultraviolet with .4 . terreus (Swanson 

 et al., 1948). Prior exposure to heat did n(jt sensitize yeast cells to the 

 lethal action of subsequent ultraviolet radiation, although reversal of the 

 sequence did have a sensitizing effect (Anderson and Duggar, 1941). 

 Older yeast cells have been found to be more resistant to killing than 

 younger cells (Oster, 1934b) and a similar result has been reported for the 

 spores of Rhizopus suinus (Dimond and Duggar, 1941). There appears to 

 be a slight temperature effect with yeast, since for the same killing more 

 energy is required at 8°C than at higher temperatures (Oster, 1934b). 

 No effect of humidity was noted with .4. niger spores (Zahl ct al., 1939). 

 The significance of such isolated observations remains obscure, but it is 

 likely that satisfactory interpretations and comparisons of experimental 

 data have been greatly hindered by lack of awareness of the influence of 

 seemingly irrelevant factors on the final outcome. 44iere is a very con- 

 siderable need to explore more thoroughly and under controlled genetic 

 conditions the effects of age, nutritional state, and other eiu'ironmental 

 factors upon the sensitivity of cells exposed to ultraviolet radiation. 



IONIZING RADIATIONS 



The ionizing radiations are free from the dependence on specific absorp- 

 tion which is an outstanding characteristic of ultraviolet. The elemen- 

 tary physical processes which they bring about within the cells involve 



