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In a previous paper (2) the germination and subsequent growth of deuteron-irradiated lettuce seeds was described, with special emphasis on the influence of penetration depth. It was shown that depth of penetration is the predominant factor in eliciting the effects described, i.e., germination percentage and root length are affected at different depth levels. The general phenomena observed in our experiments do not differ essentially from effects of radiation described by other workers. It was possible, however, to pin down the radiation effects locally, and to make a more detailed analysis of the radiation data by following the energy loss through the various layers. The results obtained suggest that this application of techniques of deuteron bombardment to a biological problem is of value in the study of multicellular organisms where a localization of certain physiological activities is required. Since range accuracy and dosimetry are of utmost importance in work of this kind these aspects will be discussed in this article. A method by which the energy lost in the outer seed layers was determined with the deuterons themselves will be described. It will be shown that germination induced by ionizing radiation can be ascribed solely to effects produced on the endosperm. DETERMINATION OF RADIATIONENERGY Loss IN THE OUTER SEED LAYERS: Determinationi of the relative stopping power of the outer seed layers with respect to air (N.T.P.): In order to obtain the mean ranges of deuterons in an average protein of density 1.3, the rate of energy loss per unit distance was calculated with the Bethe-Bloch formula, and a numerical integration of the reciprocal of this rate was performed. The average excitation potentials used for carbon, hydrogen, nitrogen, and oxy-gen were those listed by Pollard (3). It was found that the mean range of deuterons in proteinaceous material, measured in microns, is five times their mean range in aluminum, expressed as mgm/cm2. In stopping power, 1.53 mgm/cm2 of aluminum is equivalent to 1 cm of air at N.T.P. (4). Values for protein were used when calculating depth of penetration into the embryo. They could

Depth Controlled Deuteron Irradiation of Lactuca sativa Seeds. II. Energy Loss in the Outer Seed Layers