chromosome aberrations in Tradescantia 729 



minutes for both chromosome and chromatid breaks. In later experi- 

 ments (Catcheside et al., 1946b), when irradiation was prolonged beyond 

 about 30 minutes, however, the experimental results were not in very 

 good agreement with the theoretical predictions, since there was an 

 unexpected persistence of interchanges even in experiments in which the 

 irradiation extended over several hours. These investigators studied this 

 situation in an attempt to determine whether the discrepancy could be 

 accounted for on the basis that some exchanges are one-hit rather than 

 two-hit aberrations. Although a certain proportion of exchanges were 

 found to be one-hit types, this value was not sufficient to account for the 

 observed discrepancy. Consequently, it was concluded that, in addition 

 to a short-term component (rl), there is also a long-term component (t2) 

 involved in restitution and that the value of t2 is of the order of hours, 

 not minutes. It is suggested that biologically this situation may result 

 from the circumstance that, if two broken ends of a given break do not 

 restitute within a few minutes, they may then become separated beyond 

 the spatial limits of restitution, and a considerable period may elapse 

 before they are accidentally brought together again. 



In addition to experiments with varying intensity, other experiments 

 with fractionated doses have been performed to investigate the mechan- 

 ism of aberration production. In general, the results of these experiments 

 (Sax, 1939, 1940; Faberge, 1940) are in agreement with those of the 

 intensity experiments. When a given dose is divided into two or more 

 fractions, the yield of exchange aberrations decreases, but that of one-hit 

 types is not affected. By utilizing increasing time intervals between 

 doses, Sax (1939, 1940) concluded that some breaks may remain open 

 and capable of reunion for as long as 1 hour, but that most breaks undergo 

 restitution or reunion in a considerably shorter period. 



The results of experiments by Lane (1951) on dose fractionation are 

 similar to those of Sax in showing a pronounced reduction in exchange 

 aberrations when intervals up to 4 hours between doses are employed. 

 However, when longer intervals of 6 or 8 hours were used, a recovery 

 effect was noted, the aberration yield returning, with an 8-hour interval 

 between fractions, to almost the level obtained with an equivalent dose 

 delivered without fractionation. This result, plus the observation that 

 the aberration yield from a given dose delivered in two equal fractions 

 often appeared to be less than twice the yield obtained when half that 

 dose was used, led to the conclusion that the so-called intensity effect 

 was actually to be interpreted in terms of a temporary inhibition of 

 chromosome breakage by radiation, such that an initial dose rendered 

 the chromosomes more resistant to breakage by a subsequent dose. 

 That this inhibition was temporary was shown by the subsequent 

 recovery effect with time intervals between doses longer than 4 hours. 

 These over-all results and interpretations are clearly incompatible with 



