sparrow: cytogenetic effects of ionizing radiations 93 



64, 126, 158, 198, 199). In addition to these, position effect has been 

 described by Catcheside (20) in Oenothera blandina. In spite of some 

 of the practical difficulties sometimes associated with certain types of 

 aberrations, e.g., reduced fertility, the record clearly shows that 

 appropriately designed (or even accidental) cases of chromosome 

 engineering can yield results of great potential value. An outstanding 

 example is the work of Sears (140) in which leaf rust resistance was 

 transferred from the wild glass, Aegilops umbellatum L., to common 

 wheat, Triticum aestivum L., by means of a short interstitial trans- 

 location. A similar transfer of stem rust resistance has since been 

 reported by Elliott (36) from Agropyron elongatum to T. aestivum. 

 A more extensive discussion of the usefulness of chromosomal aberra- 

 tions in plant genetics and plant breeding is given in earlier publica- 

 tions and elsewhere in this volume (46, 47, 64, 86, 97, 123, 158). 



VIII. Factors which Modify Cytogenetic or Other 

 Radiobiological Responses 



One of the difficulties with the status of modern radiobiology is 

 a plethora of facts and a deficiency of general principles. This regret- 

 able state of affairs is well illustrated in the case of the literature 

 on factors which modify radiation response. It is possible to catalogue 

 these facts (Tables 6, 7, 8), but unfortunately the exact reason or 

 mechanism by which modification comes about is often unknown. 



Since it is important that radiation experiments be as repro- 

 ducible as possible, the maximum number of variables should be 

 controlled carefully. Some things which may seem to be trivial details 

 often turn out to be important modifying factors, i.e., moisture con- 

 tent of seeds, exact age of seeds or seedlings, growth rate, and tem- 

 perature. A diagram of some possible post-radiation events and of 

 various stages at which modifying factors may act is given in Figure 13. 



In the interest of providing a brief summary of the known 

 modifying factors and to provide a guide to investigators desiring 

 uniformity of results, most of the major modifying factors are 

 described or listed below. 



A. Physical Factors 



1. Miscellaneous factors 



Many physical factors other than total dose can modify the degree 



