sparrow: cytogenetic effects of ionizing radiations 99 



mutations, at least in plants (46, 67, 86). An extensive bibliography 

 (88) on RBE is available for those interested. 



B. Chemical Factors 



A very extensive literature exists relating to the modification of 

 control of radiosensitivity by chemical substances. Variations in the 

 amount of naturally occurring chemical constituents as well as in 

 those which can be added experimentally to the internal or external 

 cellular environment are all potentially suspect and many already 

 are known to be effective (Table 7). While many of the results 

 obtained could have been predicted, the reasons for some of the 

 modifications are still obscure or unknown. However, the study of 

 these factors is important since such studies may suggest some of the 

 chemical steps involved in chromosome breakage and mutagenesis. 

 Moreover, a better understanding should lead to more adequate 

 control of the most important environmental variables and this, in 

 turn, should yield more reproducible data. This also could have prac- 

 tical applications, e.g., one could plan experiments in which the maxi- 

 mum yield of a certain kind of cytogenetic effect could be obtained 

 with the minimum amount of sterility or lethality. Such mutagenic 

 specificity is summarized by Smith (153). 



Due to the complexity of the relationships involved and lack of 

 space, it is impossible to explain here even the best known mecha- 

 nisms by which protection or enhancement is brought about. How- 

 ever, many authors have discussed at length chemical modification 

 and the mechanisms of action involved (2, 5, 12, 22, 38, 71, 91, 125, 

 153, 201). 



C. Biological Factors 



1. Cytological and genetic factors 



a. Chromosome number and polyploidy. — The chromosome 

 number in diploids and the degree of polyploidy are both known to 

 affect radiosensitivity (160, 163, 202). The degree of polyploidy and 

 the kind of polyploidy are also known to affect the type of mutation 

 and the rate (96). Likewise, the higher the degree of polyploidy, the 

 more generations which must be grown to ensure the appearance of 

 recessive mutations. 



b. Chromosome size or nuclear volume. — The size of the nucle- 



