Biological Effects of Jonizing Radiations 



127 



efficiency. Since the diameter of the chromated thread at pro- 

 phase is about O.lu, it is inferred that a break is only hkely to 

 follow when at least 20 ions are formed at one locus within 

 the thread. No other material has been analyzed for chromosome 

 structural changes in such detail as Tradescantia, but a restricted 

 analysis ^^' ^^' -"* of the changes produced by X-rays and neutrons 

 in root tips of the broad-bean, the pea, the tomato, three mouse 

 tumors — sarcoma 180, a mammary carcinoma, and a lympho- 

 sarcoma — and a carcinoma and lymphosarcoma of the rat, showed 



I Or 

 9 

 8 



\ Gross 

 injuries 



Chromosome 



structural 



changes 



X- 



Inhibition 

 of mitosis 



lO 



t 

 Minimum damma 



value rays 



A lOO 



Deep 

 therapy 

 X-rays 



I.OOO 



Neutrons 



t 



Alpha 



particles 



lO.OOO 



FIG. 3. Relative Efficiencies of Ionizing Radiations 



X 



+ 



o 



A 



Chromatid-breaks ) Produced in Tradescantia pollen-grains by 



Isochromatid-breaks j irradiation at prophase. 



Inhibition of growth of wheat-seedlings. 



Mouse tumors rendered inviable by irradiation in vitro. 



Cessation of growth ) t/- • j- ? 



rp _ . u-u-4.- c -^-4. • f yicia jaba roots. 



lemporary inhibition of mitosis j •' 



Abscissae = linear ion density in ions per \\, 

 Ordinates = relative efficiency of radiation 



