The effects of radiation on the broad bean, Vicia faba, had been studied over a period of years at the Mount Vernon Hospital by the late Dr. J. C. Mottram, and by Dr. Read and Dr. Gray, prior to the investigation which we are about to describe. In a series of papers dating from 1913, Dr. Mottram described the effects of γ rays, β rays, and X rays on the growing tip of the broad bean root, Vicia faba. These papers record a number of interesting observations regarding the extent to which the injury produced by radiation may be influenced by changes in the physiological condition of the cells at the time of irradiation. Thus, by taking advantage of the diurnal mitotic rhythm in roots grown in the light, Mottram showed (1913) that cells at or near division are much more sensitive to radiation than are resting cells. Mottram also showed (1933) that roots irradiated when at 0 deg. C. are markedly more sensitive than those irradiated at 24 deg. C., while roots irradiated anærobically in an atmosphere of nitrogen are markedly less sensitive. Roots immersed in 1/2000 molar hydrocyanic acid for three hours were found to be more sensitive, but five-hour immersion in 1/40 molar solution of urethane was found to be without effect (Mottram, 1935). Interesting as they were, many of these experiments could only be regarded as exploratory and did not lead to any definite conclusion as to the nature of the primary radiation damage.