1284 BIOLOGICAL EFFECTS OF RADIATION 



region in 4 out of 20 cases, thus indicating that this is a point of weakness. 

 The immediate origin of fragments was observed, in that, frequently, 

 detached chromosome segments lay close together, indicating their 

 recent derivation from the normal unit. That fragments lacking an 

 insertion region are not propagated through a series of mitoses was 

 indicated (a) by the complete absence of certain specific chromosome 

 segments and (6) by the absence of "undifferentiated" fragments in 

 mitoses observed a month after irradiation. Some fragments apparently 

 possessed newly formed constrictions, but as these were not known to 



represent points of functional fiber 



- *^ /(-^ \ attachments, no final evidence as 



• ^ ^^ * / ^^^Rr-1 \ to the possibility of a de novo 



origin of an insertion region was 

 V^'t^^^aT^^ / provided. In addition to frag- 

 mentation, the shortened condition 

 of one chromosome and the elong- 

 „ „ ^, , ,. "~~Xir„ „„, ation of another, in Vicia and 



Fig. 2. — Chromosomal disruption appear- ' 



ing in first meiotic anaphase of PMC of Crepis, was interpreted in terms of 



Nicotiana Tabacum var. purpurea 48 hr. translocation. The "association" 

 after heavy X-radiation. (a) Photomicro- 

 graph; (6) camera lucida drawing of same of twO chromOSOmeS and the 



^ translocation of fragments from 



two chromosomes to a third were also reported. Yamamoto (50) has 

 also figured fragmentation and fusion of chromosomes in PMC 

 (PMC = pollen mother cells, EMC = embryo sac mother cells) of 

 Rumex acetosa following X-radiation. 



One of the most significant recent descriptions of the character of 

 induced chromosomal alterations in plants is that of Navashin (32), 

 who examined mitoses. in root tips of young plants of Crepis tectorum 

 grown from soaked seeds subjected to X-radiation. In this unusually 

 favorable cytological material it was possible to determine the extent 

 and character of the chromosomal reorganizations produced. Fragmen- 

 tation was often followed by fusion of detached chromosome segments. 

 Frequently chromosome fragments possessing "kinetic constrictions" 

 were conspicuous features, a bisatellited fragment formed by the fusion 

 of satellited fragments of two chromosomes being especially striking. 

 Increases in chromosome length and changes in chromosome form 

 could be specifically related to the fusion of "distal fragments" which 

 lacked insertion regions with other chromosomes which possessed them. 

 Navashin (33, 34) has, also, reported the finding of similar alterations 

 in roots of plants grown from aged seed. 



In Nicotiana Tabacum, heavy X-ray or radium treatment of sex cells, 

 seeds, or the growing points of seedlings produces an extreme degree 

 of fragmentation (Fig. 2) and a certain amount of fusion, accompanied 

 by aberrations in the spindle mechanism. The cytological evidence 



