MANNER OF PRODUCTION OF MUTATIONS 535 



ably all to be accounted for in this case by the eccentricity of the 

 nucleus within the strongly ultraviolet-absorbing cytoplasm of the pollen 

 grains. 



11. POINT MUTATIONS IN RELATION TO QUANTITY AND QUALITY 



OF ULTRAVIOLET 



On treatment of spores of various fungi, namely, Trichophyton, Asper- 

 gillus, Neurospora, and Penicillium, Hollaender and co-workers (HoUaen- 

 der, 1939; Emmons and Hollaender, 1939; Hollaender and Emmons, 

 1941; Hollaender, Sansome, et al., 1945; Hollaender and Zimmer, 1945) 

 found the frequency of phenotypically expressed mutations to rise rapidly 

 at first, more or less in proportion to the dose of ultraviolet, and then to 

 describe a convex curve which reached a maximum and finally, at high 

 doses which allowed the survival of only a small proportion of the spores, 

 dropped off. Since the spores were much smaller and less ultraviolet- 

 absorbent than the pollen grains used by Stadler, the interpretation of 

 differential absorption could not explain much of the progressive decline 

 of the slope of this curve. At least a part of the explanation of this 

 phenomenon came to light, however, in the finding that it was to a con- 

 siderable extent, although by no means completely, prevented by soaking 

 the irradiated spores in saline for several days before culturing them. 

 Since this procedure greatly improved the over-all survival rate at the 

 higher doses at the same time as the frequency of observable mutants 

 among the survivors was raised, it became probable that the mutants, 

 being on the whole weaker anyway, had had their ability to survive more 

 reduced by ultraviolet than the nonmutants had, by the same kind of 

 synergistic action of genetic and phj^siological detriments as has been 

 observed elsewhere, so that the physiologically restorative treatment pro- 

 moted the survival of a higher percentage of the mutants than of the 

 normals. This would allow the assumption that the actual, as distin- 

 guished from the observable, mutation frequency had continued to rise, 

 perhaps linearly, with increasing dose. 



In Drosophila likewise the frequency of lethal mutations, after a pre- 

 liminary rise with dose, was found to fall off rapidly at higher doses when 

 spermatozoa were treated (Sell-Beleites and Catsch, 1942) and, when 

 germ cells in embryonic polar caps were treated, to reach a maximum 

 which was maintained over a range of about fivefold as the dose rose 

 further, but which may at last have undergone a recession (Altenlnirg 

 et al., 1949; unpublished data). The interpretation proposed for the 

 fungus spores is inapplicable in these experiments. The spermatozoa, 

 containing genes which are dormant, are not affected in their survival 

 power or fecundity by mutations occurring in them, and the pole cells, 

 being diploid, cannot be appreciably affected in their viability or multi- 



