1270 



BIOLOGICAL EFFECTS OF RADIATION 



somal variations involved belong to two sharply separated classes. 

 Variations of one class occur readily under light doses and tend to be 

 lethal under heavier doses; those of the other class occur only under 

 heavy treatment. In genetic experiments with radiation in other forms 

 there has been no indication of such a dosage relation. 



Table 1. — Effect of Grenz Rays, Soft X-rays, and Hard X-rays on Mutation 



IN Antirrhinum majus 



(After Stubbe, 58) 



SPECIES DIFFERENCES IN MUTABILITY 



Although penetrating radiation induces mutation in organisms so 

 diverse as Drosophila and Hahrobracon, Datura and Hordeum, there 

 are large differences in the frequency of induced mutation in different 

 species of the same genus. The much lower mutation frequency of the 

 polyploid species in Avena and Triticum has already been mentioned. 

 On the other hand, Goodspeed (17) states that in Nicotiana the species 

 N. glutinosa and N. sylvestris, which have 12 pairs of chromosomes, are 

 much more difficult to alter by X-ray or radium treatment than the 

 species N. Tahacum and N. rustica, which have 24 pairs of chromosomes. 

 The lower mutability of the polyploid species of Triticum has been 

 questioned by Sapehin (36), who points out that relatively soft radiation 

 was used in the experiments with Triticum by both Stadler and Delaunay. 

 Using somewhat harder X-rays in treating T. vulgare, Sapehin reports: 



Der Erfolg war iiberraschend : Hunderte unter den Nachkommen traten in 

 den verschiedensten Richtungen verandert auf. . . . Die uberwiegende Mehrzahl 

 der Mutanten (nicht alle!) sind Chroniosomen-Aberranten und meist inehr 

 oder weniger steril ; viele sind iin morphologischen Sinne defektiv, docli koinmen 

 einzelne starke, fruchtreiche Mutanten vor, die von praktischem Interesse sind. 



These apparently contradictory results are probably to be explained 

 on the basis of differences not in the quality of radiation applied but 

 in the type of genetic variation found. Germinal variations may result 

 from gross chromosomal modifications as well as from gene mutation, 

 and in the polyploid species the former source is particularly important. 



