[Illustration: _Plant ecologists “tagging” experimental forest plots with radioactive cesium for long-term studies._] Natural radionuclides find their way into plants’ metabolic processes. Man-made radionuclides also are so incorporated—even some, such as uranium or radium, that have no known metabolic role. The man-made nuclides, whether they reach the earth in fallout or by other means, mix with the stable nuclides to which they are chemically related, increasing by small fractions the total amount of each element available to participate in plant growth cycles. Because artificial radionuclides behave so typically, they present, on the one hand, a possible long-term hazard and, on the other, the expectation that their detectability will reveal much about the biological courses of minerals and nutrients. The disposition of man-made radioactivity on land is determined in part by such factors as topography and the presence or absence of water. Topography may influence the distribution by setting patterns of drainage and exposure of surface soils to wind and rain. Water may affect dilution, or it may leach radionuclides out of surface soils and thus remove them from the level in which plants are rooted. The leaching may carry radionuclides elsewhere, however, possibly causing mild contamination of the water table. [Illustration: _Trench dug on Rongelap Island to expose soil strata and root systems to determine penetration of radionuclides in coral-sand “topsoil”._] Plants take up radionuclides through their roots or through their foliage. But the role of soils is significant. Some radionuclides are bound as ions to clays and thus are withheld in large measure from entry into the plant system. Cesium-137, for example, is held so tightly by soils that uptake through plant roots is slight, and thus a more significant mode of entry of cesium-137 into food chains is by direct deposit on plant leaves. Variables are introduced by the physical configuration of the plant itself, by seasonal differences in plant metabolism, and by the effects of rain and snow. In the case of iodine-131, a short half-life—8 days—virtually precludes the possibility of extensive uptake through plant roots. But the half-life is not too short to prevent grazing cattle from ingesting radioiodine deposited in fallout and thus allow the appearance of radioiodine in milk. [Illustration: _Survey of pasture grasses to determine whether radioactive materials are present. If they are, they could be passed from the grasses to cows and then from the cows’ milk to humans._]