INHIBITORY EFFECTS 



139 



for those of P. dispersa had earlier been recorded. The germ 

 tubes of several other fungi, cited in his report, are indifferent in 

 their reaction to light. It would seem that the restriction of some 

 pathogens to the lower leaf surface instead of both leaf surfaces 

 may be, in part at least, a light response. For example, the downy 



35 



30 



25 



o 

 to 



-2 

 c 



<D 



a 



CO 



c 



-2 



20 



15 



10 



High intensity- 



Dosage (roentgen units) 



Fig. 21. Effect of dosage on mutation rate at low intensity (240 r per 

 minute) and at high intensity (5400 r per minute). (From Sansome, Deme- 



rec, and Hollaender.) 



mildews fructify quite commonly on unexposed surfaces, whereas 

 powdery mildews behave in this respect as though quite indiffer- 

 ent to sunlight. 



Numerous studies have been made to determine which portions 

 of the spectrum are most injurious or possibly lethal. As might 

 be anticipated, it has been found that radiations with shortest 

 wavelengths are most effective in retarding growth. It has been 

 noted furthermore that the ultraviolet regions are more effective 

 than the blue, but in many of these studies intensities are not 

 measured, and consequently the results cannot be satisfactorily 



