714 RADIATION BIOLOGY 



modification of earlier concepts or which have contributed to a better 

 understanding of the basic mechanisms by which ionizing radiations 

 produce chromosome aberrations. 



A complete evaluation of the significance of current research in this 

 field, especially as it relates to earlier results, is not an easy task. As a 

 consequence, many of the conclusions drawn at the present time may 

 require considerable revision in the near future. This is particularly 

 true with respect to the recent discovery of the significance of oxygen in 

 increasing the frequency of aberrations. It seems clear that an under- 

 standing of this effect is fundamental to any interpretation of the physico- 

 chemical mechanism of aberration production by ionizing radiations. 

 At the present time, however, it is obvious that many questions concern- 

 ing the nature of the oxygen effect remain to be answered. 



The first utilization of Tradescantia for quantitative X-ray experiments 

 is that of Sax (1938). Previous workers (Riley, 1936; Husted, 1936) had, 

 however, demonstrated the suitability of this material, especially the 

 developing microspores, for the cytological analysis of radiation effects. 

 The initial quantitative X-ray observations of Sax were soon extended by 

 him and by his students at Harvard and elsewhere utilizing X rays as 

 well as other types of radiation. Sax presented the early X-ray results 

 in terms of a comprehensive theory of the effects of radiation on the 

 chromosomes of Tradescantia in 1940. The Tradescantia investigations 

 were initiated shortly thereafter in England by Catcheside, Lea, and 

 others. These investigators, on the basis of the earlier studies together 

 with their own observations, developed a detailed, quantitative theory of 

 chromosome aberration production in Tradescantia by ionizing radiations. 

 Certain aspects of this theory are presented in a review by Catcheside 

 (1945), and a comprehensive account is given in the volume by Lea (1946). 

 A later review, including additional literature appearing since 1945, has 

 been written by Catcheside (1948). 



METHODS; ABERRATION TYPES AND THEIR RELATIONSHIPS 



GENERAL EXPERIMENTAL TECHNIQUES 



Most of the observations on radiation effects in Tradescantia have been 

 carried out on diploid species in which the haploid chromosome number is 

 6. Several different species have been studied including T. gigantea, 

 T. bracteata, T. canaliculata, and especially T. paludosa. Although cer- 

 tain differences in quantitative results have been noted when supposedly 

 equivalent irradiation treatments have been applied in different labora- 

 tories, such differences appear not to result from marked differences in 

 species sensitivity (Sax and Swanson, 1941), but rather from differences 

 in exposure conditions and dosage measurements. It is possible, how- 



