268 RADIATION mOT-OOY 



former show no evidence of deliciency, while the hitter Iik ludc dearly 

 detectable deficiencies. The evidence of deficiency here is genetic rather 

 than cytolot!;ical. The only critical evidence is from the induced muta- 

 tions of A studied by Stadler and Roman (1948), and here the number 

 of cases is too small to imply the absence or extreme rarity of short 

 deficiencies among ultraviolet mutations. The fact that no interstitial 

 deficiencies have been found in cytological studies of ultraviolet progenies 

 in maize cannot be considered evidence of their extreme rarity, for 

 here again the only critical evidence, that of De Boer (1945) on Bz 

 deficiencies, relates to a relatively small number of cases. It is dear 

 that ultraviolet radiation induces terminal deficiencies in maize, but it 

 is possible that it may induce interstitial deficiencies also, and that some 

 of these may be included among the alterations genetically identified as 

 mutations. 



It should be noted also that the alterations identified as mutations in 

 Drosophila and in maize may not be analogous classes. The sex-linked 

 lethals spontaneously occurring in Drosophila appear to be qualitatively 

 identical with sex-linked lethals induced by X rays, and the ultraviolet 

 lethals are not dearly distinguishable from either class. In maize, the 

 evidence of difference in the type of mutation induced by the two agents 

 comes from studies at a specific locus, at which the X-ray mutants are 

 found to be distinctly different from the spontaneous mutants. Here the 

 ultraviolet mutants are found to be dearly distinct from the X-ray 

 mutants and similar to the spontaneous mutants. 



DIRECT EFFECTS OF ULTRAVIOLET RADIATION ON CHROMOSOMES 



The pollen-tube technique, as employed with Tradescantia pollen, per- 

 mits a study of the direct effects of ultraviolet on chromosomes before 

 inviable aberrations can be eliminated. As discussed earlier, the tech- 

 nique has certain inherent limitations in that only those aberrations 

 realized by the onset of metaphase can be recognized; any which would 

 form at later stages of cell division, or during the process of fertilization, 

 would escape detection. The chromosomes in the generative nucleus are 

 effectively double to both X rays and ultraviolet, and the aberrations 

 induced are consequently of the chromatid types. 



In the course of these studies, over 50,000 chromosomes have been 

 examined, and approximately 700 terminal deletions have been identified 

 (Swanson, 1940, 1942, 1943). With the exception of occasional iso- 

 chromatid deletions or chromatid translocations, which were no more fre- 

 quent than in untreated nuclei, the aberrations were all terminal. No 

 interstitial deficiencies have been identified with certainty, but it is 

 realized that the method of analysis is such as to preclude their positive 

 identification. The great majority of the deficiencies involved the break- 



