The Effect of Ionizing Radiations on the Broad Bean Root—Part IX

The accurate distribution of genetic material to the daughter cells in mitosis depends upon the movement of the centromeres. Irradiation of the cell with γ or X rays, neutrons or α particles, results in the breakage of, or exchange of parts between, chromosomes or chromatids and thus leads to the detachment of chromatid parts from their centromeres, and sometimes to the attachment of centromeres one to another. Detached chromatid parts may not reach the appropriate daughter nuclei (Plates g and h), which are thus left deficient with respect to part of their gene complement. If these deficiencies are important the daughter cells are incapable of functioning normally, and it is therefore probable that the breakage of chromosomes will contribute to the damage caused to tissues by irradiation. When, however, a meristematic tissue such as that in the root tip of Vicia faba is irradiated, a considerable number of different kinds of change may be induced. Many of these may be damaging, and many of them may perhaps affect growth rate without having lethal effects. For example, ionizing radiations destroy growth hormones, decrease their production in the tissue and increase the rate at which they are destroyed in the tissue (Skoog, 1934, 1935; Smith and Kersten, 1942); enzymes may be inhibited (Forssberg, 1947); nucleic acids, proteins and nucleoproteins may be depolymerised and the formation of new desoxyribose nucleic acid may be slowed down (Svedberg and Brohult, 1939; Mitchell, 1942; Sparrow and Rosenfeld, 1946; Errera, 1946, 1947; Hevesy, 1947; Koller, 1947).