GENETIC AND CVTOLOGICAL EFFECTS 



271 



aberrations in the pollen tube chromosomes of Tradescantia at successive 

 prophase periods. A comparison can, therefore, be made with the ultra- 

 \iolet data obtained under similar circumstances (Fig. 7-2). Isochroma- 

 tid deficiencies and chromatid translocations are readily induced by 

 X rays, and the close parallelism of the terminal deficiency and translo- 

 cation curves supports the generally accepted hypothesis that a transloca- 

 tion owes its origin to the illegitimate fusion of the broken ends of two 

 independently induced deficiencies. It would appear that the lack of 

 translocations in the pollen tube following exposure to ultraviolet cannot 



2 4 6 8 10 



TIME AFTER GERMINATION, hr 



Fig. 7-3. Relation of chromatid aberrations induced by X rays (370 r) in the pollen 

 tube chromosomes of Tradescantia to successive prophase stages following germination. 

 Curve I, terminal deficiencies; II, translocations; III, isochromatid deficiencies. 

 (Swanson, 1943.) 



be ascribed to a lack of deficiencies. This is more clearly indicated in 

 Table 7-2, where the frequencies of terminal deficiencies induced by the 

 two types of radiation are similar, but those of translocations are not. 



The great majority of broken ends of chromosomes induced by ultra- 

 violet clearly do not possess the capacity for subsequent reunion. This 

 may result from a more rapid healing of the broken ends, or it may stem 

 from a lack of maneuverability of broken ends imposed by the surrounding 

 chromosomal matrix. The matrix seems not to be disrupted b}^ ultra- 

 violet, and it may, following heavy doses, actually become more promi- 

 nent in appearance (Swanson, 1942, 1943). 



The terminal deficiencies produced by both radiations in the pollen tube 

 chromosomes of Tradescantia are indistinguishable in microscopic appear- 

 ance, but there is good evidence here, as in maize endosperm, for believing 



