The effect of ionization distribution on chromosome breakage by X-rays

Summary

1. The coefficients of chromatid breakage (compare Table 4) are highest with AgL-radiation (λ=4·1 A.) and fall off through CuK-radiation (λ=1·5 A.) to medium X-rays (λ=0·15 A.) and to AlK-radiation (λ=8·3 A.) where they are least.

2. This is interpreted to mean that only the densely ionizing ‘tails’ of the electron tracks are effective in chromosome breakage, and that the ‘tails’ have a higher efficiency only where they traverse the chromatid; for the track of an AlK-electron which is shorter than a chromatid diameter is relatively inefficient.

3. The probability of survival of chromatid breaks is the same in the pollen-tube nucleus as in the pollen-grain nucleus.

4. The two chromatids of a chromosome are in contact 24 hr. before metaphase in the pollen-grain nucleus. They are slightly separated 15 hr. before metaphase in the pollen-tube nucleus.

5. The probability of interchange in the pollen-tube nucleus is reduced by the different method of packing of the chromosomes in a long cylindrical nucleus as compared with the spherical one of the pollen grain.