738 RADIATION BIOLOGY 



45 minutes immediately before exposure to 300 r of X rays. Subsequent 

 to irradiation, both experimental and control roots were kept in a colchi- 

 cine solution and a cytological analysis was made 48 hours later. There 

 was a marked reduction, by about two-thirds, in the frequency of X-ray- 

 induced chromatid aberrations in the root tips pretreated with colchicine; 

 whereas, chromosome aberrations were about equally frequent in the 

 experimental and control roots. These results are interpreted as arising 

 from an effect of colchicine in reducing the amount of chromosome move- 

 ment in prophase, where chromatid effects are induced, thus favoring 

 restitution as opposed to recombination. Since chromosome movement 

 at the resting stage is presumably at a minimum, the absence of an effect 

 of colchicine on chromosome breaks is as expected. 



ULTRAVIOLET 



The investigations of Swanson (1940, 1942) utilizing pollen tube 

 chromosomes (in Trade scantia) have shown that ultraviolet differs from 

 X rays in that this radiation produces only chromatid breaks and no 

 isochromatid breaks or chromatid exchanges. Subsequent to these 

 studies of the comparative effects of ultraviolet and X radiation indi- 

 vidually, experiments were performed (Swanson, 1944) in which ultra- 

 violet was used in combination with X rays. Pretreatment with ultra- 

 violet (2537 A) 1 hour before X irradiation produces an inhibition of all 

 types of visible X-ray breaks. Chromatid breaks and translocations are 

 relatively more affected than are isochromatid breaks. The degree of 

 inhibition depends on the dosage of ultraviolet, but the nature of this 

 proportionality was not established. The suggestion is made that the 

 inhibition resulting from ultraviolet pretreatment arises from an effect on 

 the chromosome matrix. This effect may result from a greater resistance 

 of the matrix to subsequent X-ray breakage, or from the failure of broken 

 ends having an ultraviolet-treated matrix to undergo new reunion, thus 

 presumably favoring restitution. 



Posttreatment with ultraviolet has no inhibitory effect on isochromatid 

 breaks. However, translocations are inhibited, the effect decreasing as 

 the time after radiation at which the ultraviolet is applied increases (up to 

 1 hour). Chromatid breaks are inhibited even when ultraviolet is 

 applied as late as 1 hour after X irradiation. These posttreatment 

 inhibitory effects on chromatid translocations and deletions are inter- 

 preted as indicating an effect of ultraviolet in facilitating restitution as 

 opposed to reunion. 



Similar experiments in Drosophila (Kaufmann and Hollaender, 1946) 

 involving posttreatment with ultraviolet of X-rayed spermatozoa also 

 resulted in a decrease in the frequency of gross chromosomal rearrange- 

 ments. Dominant lethals, however, were not affected. These results 

 are interpreted in the same way as those obtained in Tradescantia — as 



