1290 BIOLOGICAL EFFECTS OF RADIATION 



and nondisjunction, the sources of polysomics and monosomies. After 

 heavy dosages, complex chromosomal reorganizations, in Nicotiana 

 Tabacum at least, often accompany these processes so that their results 

 cannot be recognized. On the other hand, to them may be ascribed a 

 larger role than they actually play. Thus, Mavor (29), in Drosophila, 

 assigned all exceptional offspring of X-rayed females to nondisjunction 

 of the X-chromosome, whereas other factors were undoubtedly responsible 

 for certain of the eliminations of this chromosome. Again, the plants 

 of the Xi N. Tabacum progeny, above referred to (page 1286, Fig. 3), and 

 90 per cent of which were strikingly altered in external morphology in 

 contrast to control, possessed chromosome complements so thoroughly 

 disrupted and reorganized that the effects of nondisjunction or non- 

 conjunction could not be followed. In other cases, however, simple 

 monosomic or trisomic individuals were found in Xi cultures. Some 

 appeared as the only variants in progenies otherwise equivalent to control, 

 while others occurred along with a considerable proportion of altered 

 sister plants. It is to be noted that in N. Tabacum, monosomies and 

 trisomies in Xi have followed X-ray or radium treatment not only of 

 buds in meiotie stages but also of mature pollen and dry or soaked 

 seeds. 



Gager and Blakeslee (10) found 17.7 per cent of chromosomal variants 

 in the immediate progeny of buds of Datura treated with radium, while 

 only 0.47 per cent occurred in untreated cultures. Most of the variants 

 were simple trisomic types, but some were secondaries, and others such 

 compound chromosomal types as Nubbin (Blakeslee, 4, 5). Delauney 

 (9) found that the majority of the variant wheat plants from ears sub- 

 jected to X-radiation were chromosomal mutants. In half the cases he 

 reports fragmentation as well as addition or subtraction of chromosomes 

 was involved. In Xi and Ri progenies of N. Tabacum a far higher 

 proportion of monosomies and trisomies occur than in untreated popula- 

 tions. Only a few have been examined cytologically or genetically. 

 Sixteen monosomies and 8 trisomies in Xi have been checked by both 

 methods and found to be simple chromosomal types, and 8 other Xi 

 variants have genetically been shown to belong to one or another cate- 

 gory. Trisomies have also been obtained in Xi progenies of N. sylvestris 

 and A'^. Langsdorffi,i. 



The Xi monosomic and trisomic derivatives in Nicotiana Tabacum 

 owe their occurrence to primary effects of high-frequency radiation. 

 Such types may also arise in plants as by-products of treatment — secondary 

 effects. The latter include (a) induced mutations (possibly extragenic) 

 affecting chromosome conjugation, and (6) induced chromosomal reor- 

 ganizations such as translocation, attachment, etc. An illustration of 

 the first class of secondary effects is furnished by, largely unpublished, 

 evidence as to an induced asynaptic condition in N. sylvestris (Goodspeed, 



