372 



MUTATION AND PLANT BREEDING 



350 r 



Seedling height distribution. 



Hydrated in presence of 



oxygen ( ) 



nitrogen ( ) 



3 £ 



c 

 o 



Q. 



CM 

 X 



o 

 o 



Q> 

 Q. 



c 



« 

 to 



3 



5 9 



Seedling Height -cm. 



Figure 4. — Relation of X t seedling lieiglit at 7 days to X 2 seedling muta- 

 tion frequency. (X t plants grown from dormant barley seeds subjected to 

 5,000 r of X-rays and stored for 8 days before hydration in the presence 

 of either oxygen or nitrogen.) 



between dose and interchange frequency, but that when they are 

 hydrated anaerobically the relation is linear. Furthermore, they show 

 that the mutation frequency is linear under both conditions of 

 hydration. 



The data suggest a relation between ion density and the role of 

 oxygen in the production of both one- and two-hit events that Avill be 

 presented in the discussion. Also, they indicate the likely reason for 

 the conflicting views that have been expressed by different cytogenet- 

 icists relevant to the nature of the regression of interchanges on dose 

 (4, 30). 



