Discussion 253 



Koller: I wish to mention that Dr. Aiierbach would Hke to repeat her 

 early experiments, because she does not think tha£ an oxygen effect in 

 the case of mustards really exists. The experimental conditions were 

 not sufficiently stringent. 



Spiegelman : Do these Tradescantia microspores which you treat have 

 an^^ metabolism which is detectable ? Do they respire or glycolize ? 



Swanson: Tradescantia is treated in the bud stage, and it is going 

 through very active division, so I presume it is an actively metabolizing 

 cell. 



Alper: You quoted some data where after seven hours exposure to 

 oxygen you had 73 breaks /1 00 cells at a dose rate of 1 r/min. How 

 many would that be with just seven hours of oxygen? 



Swanson : These data are from a paper by Beatty, Beatty and Collins 

 (Amer. J. Botany, in press). They graciously loaned me the data for this 

 symposium. There are no control data, however, which disturbed us a 

 little, and that is why we have become interested in the problem. Root- 

 tip cells seemed to be the easiest material for us to study at the moment 

 and we obtained a rather high frequency of aberration with long periods 

 of anoxia alone. 



Howard: Is it possible that in your work on Vicia root- tips, where you 

 use all these different treatments — the temperature treatment, blockage 

 of the metabolism of the cell, the chemicals you are using, and radiation— 

 you can really take into account the fact that you must be altering the 

 mitotic cycle, and therefore the moment at which you look at the treat- 

 ment is highly relevant? Would you not need rather complete time- 

 curves to control this? 



Swanson: I expect that you are right here. The only thing I can say is 

 that it is the conviction of Dr. Kihlman, who has done the chemical 

 mutagen work, that this is not a factor. He had worked this out in 

 terms of time relationships and he feels that the mitotic activity is not a 

 controlling factor, at least not to the extent that it throws the data all 

 otit of proportion. 



Koller: On one of your slides you showed two curves, one concerning 

 the interchanges and the other the isochromatid breaks. You pointed 

 out that irradiation and chemical mutagens interfere with the chrom- 

 osome rejoining process which has two components, restitution and re- 

 combination. Did you find a decrease in the interchanges and a similar 

 decrease in the isochromatid or in the open breaks ? 



Swanson: With the mutagens there are relatively few breaks that do 

 not undergo recombination. We don't find many incomplete exchanges 

 or the non-sister reunion tyge of isochromatid deletions. The propor- 

 tions of the two do not change with treatment, although they do change 

 with time. This, of course, is a function of the spatial relationships of 

 the chromatids, but there is no appreciable change in the proportions of 

 the two tj^es with any particular treatment. So I think that they are 

 comparable types of aberration. 



Van Bekkum: With regard to the metabolic inhibitors such as dinitro- 

 phenol, have you any data on the effect of ATP, for instance ? 



Swanson : I can quote only the data of Wolff, to the effect that ATP 



