EFFECTS OF RADIATION ON FUNGI 893 



violet radiation on Penicillium digitatum within the limits of the intensi- 

 ties that they used. 



It has already been mentioned that deviations from the logarithmic 

 type in the S-shaped survival curves have been attributed by many 

 to other varying factors. The factors which have been considered 

 as possibly responsible have been age of the cultures used and the tem- 

 perature at which the cells are irradiated. The results of Oster (140) 

 and Schreiber (166) are in opposition with regard to the effect of age 

 on the sensitivity of yeast cultures to ultra-violet. Oster found that 

 20 to 50 per cent more incident energy is necessary to produce a given 

 effect in a 15-day than in a 24-hr. culture, whereas Schreiber found that 

 the age of the culture makes little difference in the sensitivity of the yeast 

 cells. The effect of the temperature at which the cells are irradiated 

 was also investigated by Schreiber and by Oster. Oster, working with 

 four different temperatures ranging from 8° to 29.5°C., obtained 1.10 as 

 an average value for the temperature coefficient of lethal action. This 

 value agrees very closely with those obtained for bactericidal action. 

 It suggests a physical rather than a chemical process. Oster noted that 

 the values increase slightly as the temperature is increased. Since 

 30°C. is a critical temperature for the strain of yeast which he used, he 

 attributed the increase to the possible influence of another reaction. 

 Schreiber studied the combined effects of radiation and temperature. 

 He found that the effects are very complex. He did not calculate 

 temperature coefficients, but he made numerous curves which indicate 

 the complexity of the reactions. 



One factor which may markedly affect lethal action and which 

 has been given very little attention is the shielding action of the medium. 

 Any growth below the surface does not receive the full effect of the rays. 

 This has been discussed in some detail by Dillon- Weston and Hainan (34). 

 The composition of the medium and its state of ionization may also 

 alter the effects produced by irradiation. Many investigators have 

 noted that there is no difference in fungi planted on an unirradiated and 

 an irradiated medium. However, they have not considered the more 

 complicated relation of the effects of radiation on the fungus and on the 

 medium in which it is growing at the time of irradiation. Teichler (186) 

 varied the amounts of sodium chloride and calcium chloride in the 

 medium and noted the differences in the number of cells which appeared 

 after irradiation. Hinrichs (73) reported that ultra-violet radiation 

 causes substances to be formed in the ordinary nutrient solution which 

 are toxic to yeast. Smith (169) also noted the effects of the composi- 

 tion of the medium on stimulation in Fusarium. 



The phenomenon of stimulation has been in dispute for some time. 

 It has been considered by many as an irregular sort of process whose 

 occurrence is unpredictable. A number of workers have reported stimu- 



