894 BIOLOGICAL EFFECTS OF RADIATION 



lation of the growth rates of fungi with ultra-violet radiation. Nadson 

 and Phihppov (134) observed in a yeast colony that there is much greater 

 growth around the edges of an irradiated zone than outside the zone 

 whereas growth in the middle is retarded. The stimulation has been 

 attributed to small amounts of scattered radiation. However, when 

 small doses of radiation have been used in an attempt to obtain stimula- 

 tion, the results have been on the whole quite unsuccessful. Wyckoff 

 and Luyet (196), Schreiber (166), and Smith (169) obtained no evidence 

 of stimulation with small doses of ultra-violet. Schreiber made very 

 extensive experiments with a large number of different wave-lengths 

 and intensities. Chavarria and Clark (24), however, found that short 

 exposures to ultra-violet always produce a stimulation in the growth 

 rates of Montoyella cultures, whereas" longer exposures are lethal. The 

 difference in the results obtained by Chavarria and Clark from those of 

 other workers may be due to the difference in the times at which measure- 

 ments of the amount of growth were made after irradiation. That this 

 might be a real factor is brought out by the experiments of Smith (169) 

 who found in Fusarium cultures only temporary stimulation, which was 

 never obtained without a previous retardation. After a short period of 

 time the growth rate returned to normal. Smith considered stimulation 

 of vegetative growth as merely an indirect effect of radiation and a 

 direct effect of retardation, since other agents which produce a retardation 

 also produce stimulation. Smith also found that temperature conditions 

 and nutritional conditions which favor the growth of the fungus favor 

 also stimulation. The idea was suggested that those conditions which 

 favor the formation and accumulation of labile products favor also 

 stimulation following retardation. This is not in agreement with the 

 work of Hutchinson and Newton (80) who obtained the greatest stimula- 

 tion in slow growing cultures of yeast. Hutchinson and Newton also 

 studied the effects of individual wave-lengths from a monochromator 

 on the growth rate of yeast. With some wave-lengths they obtained 

 stimulation and with others retardation. However, differences in the 

 effects produced by different spectral lines may be caused entirely or 

 partially by differences in intensity. This factor was not considered 

 in the investigation and a final evaluation of the results is provisional. 

 The effects of radiation on the growth rates of fungi are of some prac- 

 tical as well as theoretical importance. Its effects on yeast are of 

 importance to the brewing industry and radiation may also assume some 

 importance to the plant pathologist in the control of certain plant 

 diseases. Hey and Carter (71), for example, found that it is possible 

 to give wheat seedlings an amount of ultra-violet which gives a fairly 

 effective control of Erysiphe graminis without causing damage to the 

 host. The effects of radiation on fermentation will be discussed more 

 fully in the discussion of physiological properties. 



