was about 30 days; and it increased, for the tnterval from 30 to 60 days postdetonation, to about 130 days. The D+5-day concentrations can be compared with assumed D+0 concentrations which would reconcile the difference between Martin's estimate of the plant interception factor and Miller and Lee's estimate of the foliage contamination factor. This procedure suggests an average weathering half-time of approximately 1.4 days during the interval from 0 to 5 days after fallout. These observations lead us to the hypothesis that the decay-corrected concentration of a radionuclide in fallout-contaminated plant material is a very rapidly declining exponential fumction of time at times soon after the contaminating event but approaches a lower asymptote. As this hypothesis appears to be correct, the effective rate at which a radionuclide is removed from surfaces following external deposition cannot be expressed precisely by a single coefficient because the weathering half-time increases as a function of time after contamination. If the initial deposition is a heavy one, a significant fraction of it (perhaps as much as 90 percent) may be removed by weathering in a matter of hours, or a few days at most. A portion of what remains after this initial period of fast weathering (something in the range of 10 to 60 percent) is so tightly trapped that it cannot be removed even by vigorous washing (Romney et al., 1963). Presumably, this nonremovable fraction is composed predominately of particles which are small and mechanically trapped on plant surfaces. Plant Growth Rates As indicated earlier, the growth of new plant tissue may dilute both the external and the internal concentrations of plutonium or other transuranium elements in plant materials. As different plant parts may grow at different rates, it is obvious that the growth rate of interest with respect to external contamination is the growth rate of leaves (and other edible parts formed above ground), If we assume that internal plutonium due to root uptake is uniformly distributed to all parts of the plant, the growth rate of interest with respect to dilution of root uptake is the overall growth rate, i.e., the growth rate of leaves plus the growth rate of all other plant parts. Plant growth is not a continuous process, nor is it the same for all species in a given area or for all the parts of a given plant. In the temperate zone, at least, plant growth is confined to the warm season, and the rate of growth 1s not uniform throughout the growing season because different plant organs develop at different times. Ignoring the morphogenic aspects of plant growth (1.e., the differentiation and development of structure), growth is most simply conceived as an increase in biomass (i.e., dry weight of tissue per unit area). For annuals, the biomass at the beginning of the growing season consists of seeds; for herbaceous perennials in which the aboveground parts dite back during the winter, it consists mostly of roots and other belowground parts; for woody perennials, it consists of roots and stems (mostly dead tissue) plus, in the case of evergreens, leaves, and vestiges of fruits produced the previous growing seasons. In addition to these seasonal and spectes variations, it is reasonable to suppose that the roots, stems, leaves, fruits, and other organs of a given species grow at different times and at different rates and that additional variations can be expected in response to environmental factors such as temperature, soil moisture, and availability of nutrients. To attempt a mathematical description of vegetation growth which includes all factors mentioned above (and others not mentioned above) would be a monumental undertaking. What is needed at present ts a simple expression of growth rate, as a continuous function, which will provide a reasonable but conservative estimate of the potential, overall concentration of plutonium in plant materials which have been contaminated externally by airborne deposits and/or internally by root uptake from soil, To obtain a rough estimate of growth rate, we can define s as follows: ».” In (1 + P,/B,)/365 where: and * is the growth rate coefficient averaged over the year (day7})y, n is the net gain in biomass during a growing season (g/m*), By is the biomass at the beginning of the growing season (g/m2). Odum (1971) has estimated that the average gross primary productivity (GPP) of deserts and tundras is about 200 kcal/m? per year. As the fraction of GPP (0.2) used up in respiration does not appear as new tissue, the dilution growth rate is proportional to 0.8 GPP = 160 kcal/m? per year. At 4.5 kcal/g dry weight (Odum, 1971), this amounts to a net gain of P_ = 36 g/m?, approximately. The biomass of desert vegetation varies from plaice to place. The mean biomass for Area 13 is B_ = 289 g/m? (Wallace and Romney, 1972). Substi- tuting these values of P_ and°B_ in Equation (9), 4 = 3x 107" day-!. Assuming internally deposited Pu Yo be uniformly distributed@above- and belowground, this would be the value to use in Equation (8). Assuming two-thirds of P_ to be above ground and one-third below ground, the dilution growth rate for the external (aboveground) component, Equation (7), would be he = 2x 107% day-!, Root Uptake and Plant/Soil Concentration Factor In order for plutonium to enter plants via root uptake, it must first reach the roots. Plowing, of course, accomplishes this "transport" quite rapidly by mixing the soil, but the downward movement of plutonium in an undisturbed soil profile is such a slow process that much of the plutonium deposited on the surface may stay near the surface for many years. To circumvent the variability inherent in these and other soil processes affecting the behavior of plutonium in soils (factors reviewed by Price, 1973a; Francis, 1973), we have made the simplifying and conservative assumption that plutonium deposited on soil is diluted by only the top 5 cm of soil and that root uptake is related to the resulting concentration in surface soil, i.e., the probable concentration of Plutonium in the root-zone soil is deliberately overestimated. Most of the available data (Price, 1973a; Francis, 1973) on plutonium uptake by plants has been derived from short-term greenhouse experiments. Typical values thus derived for the plant/soil concentration factor range from 1073 to 10-®, Uptake has been shown to be enhanced by reduction of pH or addition of chelating agents. There is some evidence that plutonium uptake by plants may increase with time (Romney et al., 1970) and that the mobility of plutonium, i.e., its ability to move in the soil and its availability for root uptake, 637 636 (9)