After exposures lasting a few minutes to doses of γ radiation up to 250 röntgens, X radiation up to 140 röntgens, and of α radiation up to 36 equivalent röntgens, the rate of growth of roots of Vicia faba falls to a minimum and then returns to normal. The γ and X-ray growth inhibition is dependent on the duration of exposure. The α ray inhibition is independent of exposure times between 10 minutes and 24 hours. The minimum value of the growth rate relative to control roots, which occurs at about the sixth day after irradiation, is exponentially related to dose for α radiation. It is also exponential for γ radiation delivered in 12 or 24 hours and tends to exponential for long duration exposures to X radiation. The relation is sigmoid for both γ and X radiation at higher dose rates. The relative biological efficiency of α radiation compared with X radiation varies from about 4·5 to 12·5 according to the dose level and duration of exposure at which the radiations are compared. Histological changes seen in roots from 2–10 days after exposure to 140 röntgens of X rays have been examined in detail. As a preliminary to these studies, a quantitative method has been devised for measuring the duration of mitosis, mean intermitotic time within the meristem, the rate of elongation and the duration of the elongation process in cortical cells in roots growing either at constant or varying rates, provided there is no cell death within the meristem. When applied to data derived from irradiated roots, the analysis shows that at 4 days after 140 röntgens of X radiation the root is much disorganised, fully elongated cells are smaller, the rate of elongation is about a quarter of normal and, despite the high mitotic index within the meristem, both mitosis and intermitosis are probably very greatly slowed down. These changes are correlated with cytological observations of Thoday (Part IX) made in roots exposed to several doses of X and α radiation. It is concluded that chromosome structural damage constitutes a major factor leading to growth inhibition by all three radiations. The histological observations indicate that after exposure to 140 röntgens of X rays, part of the growth inhibition arises from the fact that meristematic cells have been so damaged that they are unable to maintain the volume of the meristem by repeated division, but it appears that only a proportion of the injured cells die. The remainder differentiate and form part of the permanent root structure.