148 Applied Biophysics 



Nuclei at resting, or early prophase, stages at the time of 

 irradiation, although delayed in division, recover and show no 

 adhesive tendency when they reach metaphase. Instead, they 

 may show structural changes. These are due to the production 

 of breaks in the chromosomes, which may be followed by the 

 formation of structural rearrangements resulting from the re- 

 combination of the breakage ends in various ways. This subject 

 has recently been reviewed - and space permits the description 

 of only some of the manifold changes. The descriptions refer 

 to the appearance of the afifected chromosomes at the metaphase 

 of the division cycle in which the changes are induced. 



Structural changes are of two kinds : chromosome, where both 

 the chromatids are similarly affected and chromatid, where only 

 one of the two chromatids is affected at a given place. The 

 former are normally produced by irradiation during the resting 

 stage, at which time the chromosomes are simple undivided 

 threads. The latter are produced by treatment at the early 

 prophase, when the chromosomes are divided into two chroma- 

 tids. In flowering plants, the pollen grains in a given anther 

 and bud develop approximately synchronously. In Tradescantia, 

 for example, at 20° C the division cycle, including a prolonged 

 resting stage, occupies about 10 days, all the grains in one anther 

 reaching metaphase within a period of less than 24 hours. The 

 material is thus convenient for radiation work in providing a 

 group of cells all approximately at the same stage of mitosis. 

 Chromosome division occurs about 30 hours before metaphase. 

 A change from chromatid to chromosome structural changes is 

 shown by metaphases observed respectively less than, and more 

 than, 30 hours after exposure of pollen grains to radiations. 



Other convenient material is provided by germinating pollen 

 grains on an artificial medium, and using the nuclear division 

 that takes place in the very thin pollen tube, 7\i in diameter. This 

 is especially valuable where soft, weakly penetrating radiations 

 must be studied. 



Radiations produce breaks in the chromosomes, and the breaks 

 suffer various fates (figure 2). A large proportion, estimated 

 at 90%, undergo restitution, the two fragment chromosomes 



