448 KADIATION HIOLOGY 



tompcraturo-insenhiitivo roMction, and tliat the crt'cct of the cokl treatment 

 was to slow clown these injurious reactions so that the cell might recover 

 (Latarjet, 1913). Another paper notes no effect on this same yeast of 

 holding cells on ice for several days after X or a irradiation (Lacassagne 

 and Hoi week, 1930). 



Old resting cells of S. ellipsoideus have been found to be more sensitive 

 than young dividing cells (Lacassagne and Holweck, 1930; cf. Oster, 

 19341)). Time of irradiation apparently has little effect (Lacassagne and 

 Holweck, 1930), although one might expect that too prolonged an expo- 

 sure might lead to experimental complications. Toxic substances formed 

 in the suspending medium might play an important role if experiments 

 are prolonged, although under ordinary circumstances this has not been 

 found to be a serious factor. 



Experiments in which two other variables, oxygen and humidity, have 

 been shown to be important in modifying the lethal effects of X rays have 

 aroused considerable interest. Dried S. cerevisiae was found to be more 

 resistant to X rays than normal (wet) cells (Dunn et ai, 1948). Wet 

 spores of A. terreus were more susceptible to the killing action of X rays 

 than dry spores (Stapleton and Hollaender, 1952). X rays kill spores of 

 A. terreus more efficiently in the presence of oxygen than in its absence 

 (Stapleton and Hollaender, 1952). A similar observation had earlier 

 been made with the yeast Torulopsis cremoris; viz., in the absence of air 

 a greater dosage was necessary to bring about the same killing action 

 (Anderson and Turkowitz, 1941). It was observed that this yeast, if 

 grown without shaking or aeration and then irradiated, was more resistant 

 than if grown similarly but shaken prior to irradiation. The observation 

 takes on added validity in view of recent results showing that the oxygen 

 effect is general. 



Concbmons. In summary, it may be said that fungi, in common with 

 other organisms, may be killed or mutated by various radiations and that 

 these effects may be intensified or decreased by adjunctive treatments. 

 The results of some of the adjunctive treatments are most easily inter- 

 preted as influencing the indirect radiation action, e.g., the chemical inter- 

 mediates produced by radiation which subsequently react with the bio- 

 logically significant materials. On the other hand, the differences in 

 effectiveness of ionizing radiations of different quality are most easily 

 interpreted in terms of sharply localized effects. These conditions are 

 not, of course, mutually exclusive. 



It seems reasoiuible to ascribe a large role to events in the nucleus, 

 based on the obssorvations of haploid and diploid yeast survival curves 

 (Latarjet and Ephrussi, 1949; Tobias, 1952; DeLong and Lindegren, 1951 ; 

 Pomper, unpublished; Lucke and Sarachek, 1953), and on the differences 

 between uni- and multinucleate conidia of Nenrospora (Norman, 1951). 

 The relation, if any, between these nuclear effects and mutation per se 



