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PHYSICAL AGENTS 



very low, and at still longer wavelengths most organisms survive very 

 large doses (50), although bactericidal action can be detected at high 

 intensity (62). The requirement for high intensity poses serious 

 methodological problems in attempts to demonstrate the mutagenicity 

 of the long ultraviolet (43). Very short ultraviolet — the Schumann 

 region, below 200 m^ — has not been studied, primarily for technical 

 reasons; indications are that lethal action at least is very great (85, 

 111, 159). 



The lethality of ultraviolet radiation is most easily expressed by the 

 fraction of irradiated spores which fail to germinate. Doses that are 

 not lethal delay germination or induce abnormalities (33, 64, 67, 85). 

 Different fungi vary widely in susceptibility; thus, spores of Mucor 

 dispersus require almost 20 times the energy per spore that is sufficient 

 for an equivalent degree of inactivation of Aspergillus meUeus spores 

 (32, 33). In surveys of fungal susceptibility it is often noted that 



100 



Figure 4. The survival of microconidia 

 of three mutant strains (inositolless) of 

 Neurospora crassa as a function of ultra- 

 violet dosage. Redrawn from Giles (51), 

 by permission of the Long Island Bio- 

 logical Association. 



100 200 400 800 



Irradiation time, seconds 



