242 C. P. SWANSON AND BeNGT KiHLMAN 



some of the answers, and it is obvious that the comphcations 

 are many. 



The studies of Wolff and Luippold (1955; see also Wolff and 

 Atwood, 1954) have been particularly instructive in focusing 

 attention on the involvement of metabolic systems in the 

 final expression of radiation damage in terms of chromosomal 

 aberrations. WolfP and his co-workers have demonstrated, 

 as have others before him, that breakage is oxygen-dependent, 

 but in addition his data also support the idea that the time 

 period of rejoining is similarly governed by the amount of 

 cellular oxygen. The older arguments concerned with the 

 "breakage versus restitution" controversy can now be dis- 

 pensed with since both are shown to be oxygen-dependent. 

 Based on fractionation techniques, and the use of inhibitors 

 of oxidative metabolism, these studies have been interpreted 

 as a demonstration of the fact that the rejoining of broken 

 ends is an energy-requiring event, and Wolff has proposed 

 that the radiation injures the metabolic system upon which 

 the repair of broken chromosomes depends. The greater the 

 dose of radiation, the greater the damage to this system, and 

 the longer the delay before rejoining can take place. The 

 breaks initially induced remain open during this period. 

 Treatment of root-tip cells between radiations with low 

 temperatures, KCN, CO in the dark, and DNP inhibit the 

 rejoining system, and so prolong the period between breakage 

 and rejoining; ATP, but not AMP, shorten the period. The 

 role of externally apphed ATP in effecting the rejoining system 

 must remain questionable for the time being since it is unlikely 

 that it penetrates the cell to act as such. However, the fact 

 that it is the time period of rejoining that is affected rather 

 than the rejoinability of broken ends itself is shown by pro- 

 viding the cells with these same agents as a posttreatment 

 after only a single dose of radiation is given. The final 

 frequency of aberrations induced by single doses of radiation 

 remains unaffected by any posttreatment. 



Wolff's conclusions are supported by the data of Beatty, 

 Beatty and Collins (1956). Using a total dose of 400 r of X- 



