X-Ray and Ultraviolet Studies on Pollen Tube Chromosomes. I. the Effect of Ultraviolet (2537 A) on X-Ray-Induced Chromosomal Aberrations.

H E direct approach to the problem of the mechanism of chromosome T breakage and reattachment-namely, an analysis of the end results following radiation-has not as yet yielded a wholly satisfying answer, although our understanding of the limitations within which the mechanism operates has been considerably broadened, and its complexity has been recognized. An indirect approach has been made by determining the effect on breakage of varying the environmental conditions before, during, and after radiation. MICKEY (1938), SAX and ENZMANN (1939), and FA BERG^ (1940) have demonstrated that high temperatures reduce the total observed X-ray breakage, although MICKEY’S results seem doubtful in view of the more recent data of MULLER (1940). The plant data are explained by SAX and ENZMANN as resulting from an increased rate of restitution, thus leaving fewer broken ends available for reattachment. More recently SAX (1943) has shown that centrifuging during radiation increases the number of breaks; this, he feels, is due to the increased stresses placed on the chromosome during the period of breakage and reattachment. With the recognition that a series of canalized events must bridge the gap between the initiating ionization, which is on an atomic level, and the realization of the complete break and its later reattachment (or restitution), which by comparison are on a macroscopic level (DEZBRUCK 1940), the indirect method of approach to this problem assumes considerable importance. By breaking down the series of events into separate units (at present only a theoretical possibility), a more thorough study could be made of the nature of the disturbances produced by ionization as well as of the conditions encouraging or discouraging breakage, restitution, and reattachment of broken ends. KAUEMANN (1941), recognizing this, has instituted investigations with infrared designed to alter if possible the irradiated sperm of Drosophila during the time between radiation and fertilization. The results of KAUFMANN’S study, however, have not as yet been published. A series of preliminary experiments have been carried out to determine the effect of short-wave ultraviolet (2537 8) on X-rayed pollen tube chromosomes of Tradescantia. It will be recognized that the more precise the tool used in such a study, the more informative will be the data obtained. Ultraviolet, in this respect, possesses several unique advantages. In addition to the fact that it can be successfully combined with X-rays by employing the pollen tube technique (SWANSON 1g4oa), any effect that it might have on X-ray breakage