254 Discussion 



externally applied, shortens the rejoining period. He also has unpub- 

 lished data which indicate that adenine, added to the solution in which 

 the roots are immersed, does it even more rapidly. 



Bracket: Is there any cytological effect of the inhibitors alone, dinitro- 

 phenol for instance? 



Swanson: They don't appreciably affect the mitotic cycle or the 

 appearance of the cells at the concentrations and times employed. 



Brachet: In Acetabularia, where the nucleolus is very conspicuous, it 

 changes quickly and considerably in shape and RNA content in the 

 presence of dinitrophenol and similar substances. Regarding the part of 

 the cell where these poisons may be acting, it is interesting that, in 

 Acetabularia, Stich got the same transformation as with dinitrophenol 

 by leaving the algae in the dark for several weeks. Of course, since the 

 chloroplasts are only present in the cytoplasm, the effect is primarily the 

 reaction of the nucleolus to metabolic events taking place in the cyto- 

 plasm. In eggs also, dinitrophenol produces big cytological changes and 

 mitotic inhibition ; but it seems that it induces disturbances of the 

 nuclear RNA, rather than changes in the chromosomes. 



Swanson: We have looked at them when stained with Feulgen and 

 there appears to be no difference between them. 



Pirie: You said that dinitrophenol reduced this effect of mutagenic 

 chemicals. Yet it is not a blocking agent to metabolism, it is an upset- 

 ting agent, an uncoupling agent which is surely very different to blocking; 

 respiration will increase, and your ATP will go down. 



Swanson: It should be emphasized that we have only reached the 

 point of asking biochemical questions. With EOC, for example, the 

 toxic effects are not interfered with by dinitrophenol ; if the temperature 

 is raised up 28°, the major action of EOC is one of reducing root growth. 

 The roots actually turn black. This is not interfered with by dinitro- 

 phenol, so that the mutagenic effect on the chromosomes must be 

 quite different. 



Holmes: Did Kihlman find any change in mitotic index with dinitro- 

 phenol ? 



Swanson: There is no change in mitotic index with dinitrophenol. 

 This has been checked very carefully. 



