EXPERIMENTAL INVESTIGATIONS 5. UPTAKE STUDIES A number of studies which might relate to the uptake of sr® by plants and animals have been run as field experiments. It must be emphasized that these are not controlled laboratory experiments and that the data are subject to more variability than if strict controls were possible. However, they are part of the data available for interpretation of sr*® analyses and can be of some assistance in predictions of Sr® pehavior. 5.1 HASL PASTURE SITE SURVEYS A series of annual samples has been taken at five sites in the United States by Dr. Lyle T. Alexander of the U. 8. Department of Agriculture. The sampling program was begun in 1953, and samples of vegetation, animal bone, and soil have been collected. The intention was to study the relative uptake of Sr*™ from soils to plant to animal under the most favorable conditions possible. The animal bones in most cases represent yearling lambs or calves that grazed in the same pasture from which the soil and vegetation samples were taken. It is believed that the major problem in this uptake study is the variability of the sr® content of the vegetation. The contribution of activity retained on the leavesto the total activity of the plant is quite variable, depending on timing of the collections with respect to the test periods. There is a noticeable general increase in the level of the animal bone from year to year, and this program is continuing. 5.2 CHICAGO MILKSHED AREA SURVEY Dr. Lyle T. Alexander has collected samples of vegetation and soil at a number of farms in the Chicago area since 1955. In addition, milk samples were also taken in 1956. Since these farms are in active use, it is increasingly difficult to sample soils in unplowed areas and ob- tain values for top layers and lower layers of the soil. The vegetation data, as mentioned in the previous section, appear to be more indicative of the Sr® retained on the leaves than of uptake from the soil, since there is no increase with time in the vegetation levels. 5.3. UPTAKE OF Sr® BY BEAN PLANTS During the summer of 1956, three types of bean plants —snap beans, lima beans, and black-eyed peas——-were grown at the Beltsville Laboratory of the U. S. Department of Agriculture. The leaf, stalk, pod, and fruit of each were analyzed separately, and the snap bean and black-eyed peas samples were run in duplicate. The results are given in Table 56. Normalizing the data to stalk = 1, the mean Strontium Unit (S.U.) values for leaf, fruit, and pod become 0.85, 0.52, and 0.54, respectively. The mean Ssr®/sr® ratios are 1.9, 0.2, 0.7, and 1.4 for the leaf, fruit, stalk, and pod. Although such averaging may not be completely justified, it would lead to the following conclusions: (1) The high stalk activity may indicate uptake that is partially blocked from the rest of the plant. (2) The leaves show higher S.U. values than the pod and fruit, indicating some leaf retention. (3) The Sr® values indicate that the leaf and pod activity is younger than the fruit activity. 125