722 RADIATION BIOLOGY 



later shown (Sax and Swanson, 1941), that the sensitivity apparently was 

 greatest shortly before mid-prophase, somewhat less in early prophase, 

 and only about one-third the prophase maximum during the microspore 

 resting stage. Comparative studies with diploid microspores from tetra- 

 ploid Tradescantia species indicated a similar sensitivity cycle, but also 

 demonstrated that the diploid spores were only one-half to one-third as 

 sensitive as the haploid, in terms of aberrations per chromosome. Fur- 

 ther evidence from the tetraploids indicated that meiosis was much more 

 sensitive than mitosis. Chromosomes in root tip mitoses in the diploid 

 were found to be less sensitive than those in the haploid microspores. 

 Bishop (1950) has studied the radiosensitivity of chromosomes in the 

 postmicrospore mitosis (in the generative nucleus of the pollen grain) 

 which is initiated after the microspore division and completed in the 

 pollen tube, at which time cytological analyses at metaphase can be 

 made. He finds evidence for two sensitivity peaks, one at 3 days and 

 one at 5 days before the dehiscence of the anthers. These peaks are con- 

 sidered to correspond, respectively, to the time of chromosome doubling 

 during prophase of the pollen tube mitosis (cf. Roller, 1946), and to 

 metaphase-anaphase of the microspore mitosis. The range of sensitivity, 

 from the low point at the postmitotic resting stage, is estimated to be 

 approximately 2.4 times as great for the prophase maximum and ten 

 times as great for the metaphase-anaphase maximum. 



The extensive studies of Sparrow (1951) on the radiosensitivity of 

 Trillium chromosomes are in general agreement with the Tradescantia 

 data. The more detailed data on Trillium, particularly at meiosis, indi- 

 cate that late prophase and metaphase of the first meiotic division are 

 over fifty times as sensitive as the least sensitive stage, early interphase in 

 the microspore. Complete data are not yet available for the entire 

 microspore cycle. 



The factors responsible for such marked changes in sensitivity during 

 the nuclear cycle are not yet completely elucidated (Sparrow, 1951). It 

 is clear that any change in sensitivity (i.e., any change in the frequency 

 of aberrations produced by the same dose) may be attributed either 

 (1) to an actual difference in the number of initial breaks, or (2) to a 

 change in the proportion of breaks which undergo reunion and/or restitu- 

 tion, or (3) to both these factors. There is evidence in Trillium (Sparrow 

 and Maldawer, 1950), from a comparison of the ratios of fragments to 

 dicentrics and rings induced at metaphase of meiosis and at microspore 

 interphase, that a greater amount of reunion occurs at interphase, the 

 stage of lowest sensitivity. However, the increase in reunion is not of 

 sufficient magnitude to account for the observed decrease in fragmenta- 

 tion. It thus appears that both factors mentioned may be involved in 

 changes in sensitivity. 



Many possible reasons for variations in the incidence of primary break- 



