chromosome aberrations in Tradescantia 733 



trons. It is possible to conclude from these results that a particles need 

 not always traverse a chromosome in order to produce a break, but that 

 breaks may also arise when the particle passes in the immediate vicinity 

 of the chromosome. Such a view implies an indirect action of the radia- 

 tion in producing chromosome breaks, a topic which will be considered in 

 detail later. 



ABERRATION PRODUCTION BY ABSORBED RADIOISOTOPES 



The abundant production by atomic reactors of radioactive isotopes of 

 various elements has stimulated interest in the use of these substances, 

 not only as tracer, but also as radiation, sources. Experiments have been 

 performed with two different isotopes, P 32 and C 14 , both producing /3 rays, 

 to determine the effects of these substances in producing chromosome 

 aberrations in Tradescantia microspores (Giles, 1947; Giles and Bolomey, 

 1948). When cut stems of inflorescences were placed in solutions of 

 various activities, containing one or the other of the two isotopes, aberra- 

 tions similar to those produced by X rays were observed in the micro- 

 scopes. The types, frequencies, and temporal sequence of aberrations 

 were recorded over a period of several days following the initial treat- 

 ments. Quantitative measurements of /3 activity from disintegrations of 

 P 32 molecules in individual half-anthers showed good correlations with 

 the relative frequencies of aberrations detected cytologically in sister 

 half-anthers (Giles and Bolomey, 1948). It has not been possible, how- 

 ever, in the Tradescantia anthers to make sufficiently accurate calculations 

 of dosages from internally distributed isotopes to determine whether 

 aberrations arise as a result of recoil and/or transmutation events in addi- 

 tion to ionization events. 



ABERRATION PRODUCTION BY SLOW NEUTRONS 



The cytogenetic effects of slow neutrons are being discussed separately 

 from those of other radiations because of certain unique features associ- 

 ated with the interaction of this type of radiation with biological mate- 

 rials. Fast neutrons arising from uranium fission can be moderated by 

 elastic collision with graphite or heavy water in a nuclear reactor until 

 their velocities are reduced to thermal energies (average 2200 meters/ 

 second = 0.025 ev). Such neutrons do not behave like fast neutrons, 

 whose principal reaction of significance in biological materials is the 

 ejection of recoil protons from hydrogen atoms. Rather, thermal neu- 

 trons, because of their very low velocities and energies, are normally 

 captured by various atoms in biological materials. Such a capture reac- 

 tion produces an unstable compound nucleus with an excess of energy. 

 This unstable nucleus may then (1) emit a y ray immediately to form a 

 stable isotope, e.g., 



H 1 + n -» [H 2 ] -> H 2 + 7 



