260 RADIATION BIOLOGY 



The ahsonco of gross deficiency or translocation could Ix' assumed it' all 

 the Fi plants were free from segregation for defective pollen. It was 

 found, however, that, among about lOOO Fi jjlants examined, almost 4 

 per cent, showed segregation for defective pollen, ascrihahle to deficiency 

 or to mutations affecting pdjlcn dcxclopment. Later studies showed 

 that mutations affecting pollen development are a frecjuent result of 

 ultraviolet treatment, and lh;it there are also cytologically demonstrable 

 deficiencies induced by the treatment. 



No translocations were found among the Fi plants characterized by 

 defective pollen segregation. In later studies, however, translocations 

 were found, though in very low freciuency. The rarity of translocations 

 was given special study because of their very high frequency in compara- 

 ble X-ray progenies. Since the frequency of deficiency was lower under 

 the ultraviolet doses used than under the X-ray doses commonly applied, 

 it is possible that the difference in effect on translocation is incidental to 

 dosage. If translocations result from chromosome breakage followed by 

 reattachment of broken ends in new combinations, the rarity of transloca- 

 tion following ultraviolet treatment might be due to the smaller number of 

 iireaks produced by the ultraviolet dose applied. A further trial was 

 made (Stadler and Sprague, 1937) in which a maximal dose of ultraviolet 

 was compared with a rather low dose of X rays, these doses being approxi- 

 mately equal in total frequency of induced deficiencies for the endosperm 

 genes A and Pr. The frequency of translocation was determined for each 

 treatment by direct cytological examination of about 100 unselected 

 plants of the Fi progeny. Only one translocation was found in the ultra- 

 violet progeny, while 44 per cent of the plants of the X-ray progeny showed 

 translocations, several of them two or more independent translocations. 



Since it w^as subsequently found that the frecjuency of deficiencies in 

 endosperm and embryo is very different under ultraviolet treatment, this 

 comparison represents doses very unequal in frequency of induced defi- 

 ciencies in the embryos. It therefore does not test the possibility that 

 the rarity of translocations under ultraviolet treatment may be due to the 

 smaller number of chromosome breaks produced. It is clear, however, 

 that, for doses equal in frequency of induced mutation, the frequency of 

 induced translocation is much greater with X rays than with ultraviolet 

 radiation. 



Preliminary comparisons of w^ave-length effectiveness were made in the 

 early maize experiments (Stadler and Sprague, 193Gb, c), using the radia- 

 tion from a commercial mercury-vapor arc with three filters of mercuric 

 chloride solution of varying concentration, and using also the radiation 

 from a commercial mercury discharge tube. The relative effectiveness of 

 the various wave lengths was inferred from the frequency of induced 

 endosperm deficiencies, in terms of the spectral distribution of energy in 

 the filtered radiation applied. The indications regarding wave-length 



