RADIATION STUDIES ON FUNGI 449 



is not known. From a phenogenetic point of view it is difficult to decide 

 what is meant by dominant lethal mutations in microorganisms. Reces- 

 sive lethal mutations are not, by definition, lethal except in haploid, 

 effectively uninucleate organisms. Furthermore, it is known from 

 studies with various organisms that radiation (especially at higher doses) 

 may cause cytoplasmic changes, and so the role of the cytoplasm, although 

 probably of secondary importance, cannot be disregarded. Experi- 

 mental means of approaching such problems are rapidly developing as a 

 result of intensified research in microbial genetics, and the application of 

 these new methods may be expected to dispel much of the confusion 

 which now predominates in this area of radiation biology. 



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