2()4 RADIATION BIOLOGY 



they represent the distribution of tissue resulting from chjinoe variations 

 in cleveh)pment, when one cell in the two-celled pro-endosperm is deficient. 



With X rays, the endosperm deficiencies affect the entire endosperm in 

 most cases. There is a substantial minority of fractionals, but these in 

 general are strikingly different in pattern from those observed with ultra- 

 violet. In most of them, the deficient portion co\'ers the entire surface of 

 the endosperm except for one or more islands of tissue, usually amounting 

 in total to only a small fraction of the entire area. These cases in the 

 X-ray material cannot be regarded as merely the extremes of a range of 

 patterns resulting from random variations in development of the half- 

 deficient endosperm; there is no corresponding frecjuency of half-and-half 

 mosaics and their variants. The distribution of tissue in the fractionals, 

 as observed in the mature endosperms, suggests that in the ultraviolet 

 cases the nondeficient tissue is the sector derived from one cell of the two- 

 celled pro-endosperm, but in the X-ray cases it is the sector derived from 

 only one or a few cells at a more advanced stage of the pro-endosperm. 

 The distribution of tissue is as would be expected if a chromosome frag- 

 ment could occasionally escape elimination through one or more cell 

 divisions, and then be restored to normal mitotic distribution. 



The elucidation of the breakage-fusion-bridge cycle in mechanically 

 broken chromosomes by McClintock (1941) suggests plausible hypotheti- 

 cal mechanisms by which such endosperm patterns could be produced, 

 though the sequelae of mechanical l)reakage do not parallel those of 

 either X-ray or ultraviolet alteration of the chromosome. 



A comprehensive study of the chromosomal effects of ultraviolet and of 

 X rays, as shown by the mosaic patterns in the maize endosperm, is being 

 made by Faberge (1951). Using the endosperm marker genes /, Sh, Bz, 

 and Wx, all located in a single chromosome arm, a variety of chromosomal 

 effects may be recognized, including rings, dicentric translocations, and 

 inversions, if accompanied by a breakage-fusion-bridge cycle. Ultra- 

 violet treatment produces all these aberrations in large numbers, as does 

 X-ray treatment. 



Cytological studies of the chromosomal effects of ultraviolet in maize 

 have been made by Singleton (1939), Singleton and Clark (1940), and 

 De Boer (1945). The accounts of these studies have been published only 

 in abstract form. Singleton (1939) examined Fi plants identified by the 

 loss of dominant characters present in the treated male parent and found 

 cytologically demonstrable deficiencies of the corresponding chromosome 

 regions in four plants. In another series, cytological examination of Fi 

 plants identified by segregation for defective pollen showed deficiencies 

 in several cases, including one plant with deficiencies for parts of two 

 chromosomes. All the deficiencies observed appeared to be terminal. 

 No translocations were found. Singleton and Clark (1940) found, among 

 16 Fi plants with segregation for defective pollen, 8 with observable 



