conducive to the formation of somatic gametes. 

 One of the triploids arose from an induced tri- 

 somic. A vigorous lateral shoot appeared on an 

 abnormal, weak Xj plant of the Maryland, Mam- 

 moth variety. It turned out to be tetraploid and 

 arose from a sector with tetraploid cells. This 

 tetraploid shoot is, doubtless, a by-product of 

 irradiation, since such shoots never appeared in 

 unirradiated plants. High frequency radiation 

 (in this case X rays), through its primary or 

 secondary effects, increases the number of 

 chromosomal injuries and inhibits pairing. This 

 constitutes a source of polysomics and mono- 

 somies which appear as products of the primary 

 action of irradiation, but also as by-products of 

 the secondary action. These by-products in- 

 clude: 1) induced mutations affecting the pairing 

 of chromosomes, and 2) induced chromosomal 

 reorganizations: translocations, duplications, 

 inversions, etc. The induced asynapsis in N. 

 sylvestris is an example of the first type. It 

 showed up in Xg and was traced down to X3 . ^^ 

 Among the plants of Xg and X3 many trisomies 

 turned up, several double trisomies and four 

 tetrasomics. The union of chromosomes, which 

 is followed or preceded by fragmentation, is a 

 typical product of irradiation. What happens to 

 the fragments is very important to the continued 

 existence of the plant. In some cases fragments 

 are retained and become permanent parts of the 

 chromosome set. One derivative type of N. 

 tabacum retained a pair of fragments for 4 or 5 

 generations. Goodspeed observed many frag- 

 ments in some of the unestablished types. One 

 of the consequences of induced fragmentation 

 may be the deficiency of chromatin material. In 

 Nicotiana these deficiencies result in various 

 morphological types. 



However, neither in Nicotiana , nor any of the 

 other plants, have radiation -induced chromo- 

 somal reorganizations been as well investigated 

 cytologically as in Zea mays (Anderson, 1936). 

 Anderson devoted a chapter in Duggar's book to: 

 "Induced chromosomal alterations in maize." 

 This plant has 10 chromosomes, but different 

 varieties or strains have visible differences in 

 their chromomeres, the presence or absence of 

 knobs at the ends, different size and shape of 

 the chromosomes, and also a different appear- 

 ance of chromomeres and knobs. These pecu- 

 liarities permit the identification of each chro- 

 mosome, as has been done for Drosophila . In 

 addition to the normal diploid set of 10 pairs of 

 chromosomes, it is possible to encounter one or 

 more B-type chromosomes. This aberrant type 

 of chromosome behaves less regularly in meio- 

 sis and does not seem to have any hereditary 

 function. When maize cells are X rayed, the 

 same types of changes appear in the nucleus as 

 have already been described for other plants. 



In some strains of maize, some of the chromo- 

 somes were inverted. These so-called inver- 

 sions were observed in the short arm of chro- 

 mosome 8. A short inversion was also found in 

 chromosome 4; this latter included the centro- 

 mere. These inversions occur in the natural 

 state. McClintockl^ observed an inversion in 

 chromosome 2; 2/3 of the chromosome became 

 inverted. Long heterozygous inversions produce 

 loops typical of inversions in the prophase of 

 meiosis. These loops form as a result of the 

 fact that only homologous parts of the chromo- 

 somes synapse. Pollen and egg sterility were 

 observed in the cases of long inversions, which 

 resulted from crossing over in the loop of the 

 inversion. When the inversions are homozygous, 

 fertility is normal. In heterozygous inver- 

 sions partial sterility is observed. Deficiencies 

 (losses of chromosome ends) crop up in maize 

 cells after irradiation, but they are not trans- 

 ferred to the offspring. Zygotes containing 

 chromosomes with deficiencies are viable and 

 vigorous, but spores with a deficiency in their 

 haploid set do not survive to function. Chromo- 

 somes with deficiencies pair up with homologous 

 portions of the corresponding normal chromo- 

 some. As has already been pointed out above, 

 chromosomes in maize can be indentified; conse- 

 quently, when the deficiencies were measured, 

 it was possible to establish the position of 

 certain genes in certain limited locations of 

 specific chromosomes. 



The most noticeable, and apparently the most 

 significant, changes produced by X rays are 

 tiiose in chromosomes. The high sensitivity of 

 the nucleus to X rays (as compared with the less 

 sensitive cytoplasm) has always been noted by 

 investigators. However, the reason for this has 

 not been established. Uber and Goodspeed (1935) 

 assume that the difference can be ascribed to a 

 difference in mineral salt content. There can 

 be no doubt, however, that other factors must 

 play significant roles. According to the investi- 

 gations of Marshak and Bolman (1936), the pres- 

 ence of elements with high atomic weight in- 

 creases the absorption of X rays, and they cite 

 this to explain the increase in transformations. 

 In this connection Stadler established in 1928* 

 that plants from seeds soaked in salts of heavy 

 metals before being irradiated showed a notice- 

 able increase in the frequency of induced muta- 

 tions. The relative weakness of the bonds which 

 connect molecules (i. e. , micelles or genes in 

 chromosomes that are broken by the impact of an 

 electron) may determine the greater X-ray sensi- 

 tivity of the nucleus, as compared to the cyto- 

 plasm. Metz (1934) assumes that the insulating 

 substance of the visible, gelatin-like chromo- 

 some sheath may be destroyed by irradiation 

 which permits an intimate contact between 



^^Editor's note: Should read "xg" according to 

 Goodspeed (1936). 



"Reference given in article by Anderson cited 

 above. 



76 



