76 MUTATION AND PLANT BREEDING 



all been reported to result from treatment with ionizing radiation 

 (158). Some cases of polyploidy apparently result from inhibition or 

 destruction of spindles as in Trillium (156), others presumably from 

 restitution caused by multiple bridge formation. Aneuploidy could 

 result either from nondisjunction or from abnormal or multipolar 

 spindles. 



4. Binucleate or multinucleate cells 



Either of these effects may result from the inhibition of the 

 spindle. They are commonly seen after irradiation of meiotic or post- 

 meiotic cells in many plants and less frequently in somatic cells. 



5. Effects on centromeres 



One frequently sees lagging chromosomes which behave as if the 

 centromere were nonfunctional or at least delayed in functioning. 

 A second effect of radiation on centromeres is misdivision of the 

 centromere during which it splits at right angles to the longitudinal 

 axis of the chromosome (70). Ideally, one should get two isochromo- 

 somes each time misdivision occurs but both do not always survive. 

 Nondisjunction frequently is reported in irradiated cells. It could also 

 result from an effect on the centromere. 



6. Effect on chiasma frequency and crossing-over 



Radiation can increase or decrease the chiasma frequency, 

 depending upon the time of exposure (92, 99). An effect on crossing- 

 over has also been reported in Drosophila (108). 



7. Inhibition of cell division 



Retardation or inhibition of cell division occurs whenever suit- 

 able doses of ionizing radiations are delivered to a tissue which would 

 normally undergo further cell division. Such inhibition may result 

 from a number of causes which may include genetic damage, inhibi- 

 tion of DNA synthesis, or other unknown physiological disturbances. 

 At low doses the effect may be transient but at higher doses recovery 

 of the ability to undergo further division may be long delayed or 

 may not occur. 



8. Induction of mitotic activity 



This sometimes occurs after irradiation in tissues or cells which 

 would not normally undergo further cell division (62). 



