GENETIC AND CYTOLOGICAL EFFECTS 



269 



age of only one of the two sister chromatids, although infrequently half- 

 chromatid deficiencies were observed (Swanson, 1947). 



The frequency of terminal deficiencies increases linearly with increasing 

 dosage if the time of exposure after the pollen grains have germinated is 



15 30 



60 



240 



120 



EXPOSURE, sec 

 Fig. 7-1. The relation of chromatid deficiencies (terminal deletions) to the dosage of 

 ultraviolet (wave length 254 m/u) in the pollen tube chromosomes of Tradescantia. 

 Radiation given at a distance of 20 cm, at an intensity of approximately 10^ ergs/ 

 mm^/60 sec, and at 2 hr after germination. (Sivanson, 1942.) 



5 10 



TIME AFTER GERMINATION, hr 



Fig. 7-2. The relation of chromatid deficiencies (terminal deletions) induced by ultra- 

 violet (wave length 254 lUfx) in the pollen tube chromosomes of Tradescantia to succes- 

 sive prophase stages following germination. Dosage approximates 2 X 10' ergs/ 

 mm^'/eO sec. (Swanson, 1943.) 



kept constant (Fig. 7-1). With increasing condensation of the chromo- 

 somes as they pass through prophase development, the frequency of 

 deficiencies induced by any given dose declines, however, suggesting that 

 internal changes taking place within the chromosome play a role in deter- 

 mining its susceptibility to breakage by ultraviolet (Fig. 7-2). A ques- 



