902 BIOLOGICAL EFFECTS OF RADIATION 



wave-lengths in the ultra-violet may increase rather than decrease the 

 percentage germination. This was show^n by Hutchinson and Ashton 

 (79) who worked with Colletotrichum phomoides and a quartz monochro- 

 mator. However, since there were no measurements made of the inten- 

 sities used, it is impossible to tell w^hether the variations produced by 

 different spectral lines were caused entirely by changes in w^ave-length. 



Color may form a protective screen against the harmful effects of 

 light. The results of Bie (7), Bovie (14), Dillon- Weston (32), 

 Rabinovitz-Sereni (151), and Chavarria and Clark (24) in which the 

 sensitivities of different colored spores were compared all support the 

 theory advanced by Chavarria and Clark that pigment in colored spores 

 protects the spores against the destructive action of light by partial 

 absorption of the destructive rays. 



This same theory has been advanced as a possible explanation for the 

 differences in susceptibility of mycelium and spores. The comparisons 

 made by Schulze (167), Fulton and Coblentz (54), and Elfving (39) 

 indicate that the mycelium is more sensitive than resting spores. Smith 

 (168), however, reported that this does not apply to Fusarium Eumartii 

 since the mycelium is less sensitive to ultra-violet than the spores, and 

 germinating spores are no more sensitive than resting spores. This 

 fungus, however, has colorless mycelium and spores. This does not 

 imply that pigment has no effect, but merely that some other factor 

 must be assumed to explain the data which have accumulated on the 

 relative sensitivity of spores and mycelium. 



There is some question as to the importance of age in the susceptibility 

 of spores to radiation. Sibilia (cf. Rabinovitz-Sereni, 151) and Smith 

 (168) both noted no effects of age, but Luyet (109) found that young 

 spores are prevented from germinating with much smaller doses than 

 the older ones. Possibly these differences are characteristic of the 

 different species used. 



The medium in which the spores are suspended influences to a large 

 degree the amount of inhibition. Both the optical and chemical proper- 

 ties of the medium are important. Dillon-Weston (32) reported that 

 spores of Puccinia graminis tritici are killed more easily by ultra-violet 

 from a quartz-mercury-vapor lamp when irradiated in water than w'hen 

 irradiated dry. This might be explained by the fact that a larger amount 

 of light is scattered from dry spores, whereas this effect is largely elimi- 

 nated when the irregularities on the spore surface are filled with water. 

 Houghton and Davis (cf. Fairhall and Bates, 44) could not obtain more 

 than 20 per cent destructive action of ultra-violet on aqueous suspensions 

 of Aspergillus spores whereas Fairhall and Bates (44) readily killed 

 Aspergillus spores suspended in an oil emulsion with an exposure of 

 1 min. to ultra-violet. Both of these investigations were made with a 

 quartz-mercury-vapor lamp. Most oils fluoresce and the secondary 



