A given amount of ionization produced by fast neutrons has an injurious biological action equivalent to that of several times as much ionization produced by x-rays. This has been demonstrated for various biological systems by Lawrence and Lawrence (1), Zirkle and Aebersold (2) and Lawrence, Aebersold and Lawrence (3). This fact indicates that the mechanism of biological action of fast neutrons must differ in some essential way from that of x-rays, and suggests the further possibility that the neutron's superiority in destructiveness may be greater in the production of some biological effects than is the case for others. This possibility, in view of the known differences in distribution of ions produced by neutrons and x-rays, is of great interest in the study of the mechanism of biological action of ionizing radiations in general (1, 2). Moreover, it has important clinical implications. The purpose of the experiments reported below was to investigate this matter with various small biological objects which could be irradiated in large numbers both with neutrons and with x-rays. The biological effects chiefly studied were inhibition of hatching in Drosophila eggs and retardation of growth in wheat seedlings. The irradiation of these two organisms was done under the same conditions of geometry, dosage measurement, and operation of the neutron generator. Hence the results obtained with one organism are directly and accurately comparable to those obtained with the other. A third biological effect studied was inhibition of cell division in the spore of the fern Pteris longifolia. The conditions of irradiation of this organism were not exactly the same as for the other two, but the data obtained, for reasons which will be given below, should be comparable.
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Zirkle et al. (1937) studied this question.