RADIOAUTOGRAPHS SHOWING THE DISTRIBUTION OF SULPHUR* IN WHEAT

The radioactive elements emit rays which are able to affect the photographic plate and it is possible to determine by contact exposure the approximate concentration-distribution of the element in flat sections of plant or animal material, sometimes locating boundaries within a few microns. These exposures are called radioautographs (or more simply autographs). As the different elements vary over a wide range in regard to the nature and intensity of their radiation, the conditions for proper exposure of the plate vary correspondingly. Hamilton states (7) that bombardment by 2 ? IO6 beta particles of at least 0.15 M.E.V. average energy per sq. cm. plate surface is necessary for production of a satisfactory image. X-ray film gives the most rapid results but if it is necessary to magnify the autograph considerably, the graininess of the film is objectionable and slower; fine-grained film should be used instead if possible. Radioautographs of animal structures have been prepared using phosphorus* (12),2 iodine* (8) and others. Hamilton, Soley, and Eichorn (8) in a study of the iodine* content of goiter tissue, made paraffin sections of the tissue, removed the paraffin, and dipped the sections in a dilute solution of collodion which was allowed to dry. The sections were then placed in contact with the film separated only by a layer of collodion approximately one micron thick. Arnon, Stout, and Sipos (1) and also Colwell (6)3 have investigated translocation in plants, partly by means of autographs using radiophosphorus. The activity of radiophosphorus is so great that it is possible to obtain good autographs through several layers of paper after exposures as short as one hour. Autographs employing sulphur* have not been found in the literature. Sulphur* radiation has only about one-fourteenth the energy of phosphorus* radiation and special precautions are necessary to avoid absorption of the former before it reaches the plate. Particularly, it is necessary to place the film in close contact with the sections, with a minimum of intervening material which might absorb a portion of the emanating rays. This paper completes the observations described in the preceding paper (16) by submitting autographs to show the distribution of the sulphur* in the spring wheat.