GENETICS: B. McCLINTOCKper cent. Other experiments have been performed to determine whether the oxygen effect is exerted by way of the initial breakage mechanism or on the reunion process. These consisted of comparative exposures to a single dose of 300 r in one minute of inflorescences in a vacuum or in oxygen with the addition or removal-of oxygen either immediately after or during part of the irradiation period. In addition, buds were pretreated in the presence and absence of oxygen before exposure to X rays. These experiments show that the presence of oxygen during the actual exposure to X rays rather than during the pre-or postirradiation period is the important factor, thus indicating that oxygen alone does not influence the recovery process. It seems likely that the oxygen effect is an indirect one, resulting from the production during irradiation in oxygen of some substance such as hydrogen peroxide. Although it appears probable that the effect of such a substance on aberration frequency would result from an increased production of chromosome breaks, the alternative possibility, that such a substance might modify the restitution process, cannot yet be excluded.Acknowledgment. PROCEEDINGS, 35, 640 (1949). 2 Sax, Karl, Ibid., 25, 225 (1939). 8 Marinelli, L. D., Nebel, B. R., Giles, N. H., Jr., and Charles, D. R., Am. J. Botany, 29, 866 (1942).4 Lea, D. E., and Catcheside, D. G., J. Genetics, 44, 216 (1942). 6 Lea, D. E., Actions of Radiations on Living Cells, Cambridge University Press, Cambridge, England, 1945. 6 Thoday, J. M., and J. Reed, Nature, 163, 133 (1949). 7 Bonet-Maury, P., and M. Lefort, Nature, 162, 381 (1948 In the course of an experiment designed to reveal the genic composition of the short arm of chromosome 9, a phenomenon of rare occurrence (or recognition) in maize began to appear with remarkably high frequencies in the cultures. The terms mutable genes, unstable genes, variegation, 344 PROC. N. A. S.
