to leaf surfaces, particularly in the case of sugar beet. The retention behavior of the smaller hydrated oxide particles (count mode ~ 0.019 yim) is slightly different than for the fresh oxide. The synthetic rain water was about as effective in removal of particles from both bushbean and sugar beet, while the acid leach was slightly more effective with bushbean. Although the gross surface structure of bushbean and sugar beet leaves is obviously different, the microtopography of the surface itself may not be as different with respect to retention of very small particles (0.02 um). This, in fact, may explain similarities in the retention behavior of plutonium deposited onto foliage of sugar beet and bushbean. Obviously, with limited data, it is impossible to generalize as to mechanisms controlling the fate of particles associated with foliar surfaces. However, in the case of the small hydrated oxide particles, leachability and retention is not only dependent on particle size with respect to leaf topography and physical attraction such as charge, but also the ability of water droplet to contact the particle, thus wettability and contact angle become important (Gregory, 1971), along with a host of environmental factors (Hull et al., 1975). Aside from our lack of understanding of mechanism, it is important to note that the behavior of small particles, such as plutonium on leaf surfaces with respect to retention half-time, differ markedly from that commonly reported for fission products and larger particles (> 10 um). The tenacity of plutonium attachment observed by Romney et al., (1975) and Iranzo (1968) tend to reinforce the above laboratory studies with plutonium. AVAILABILITY OF FOLIAR DEPOSITS FOR UPTAKE AND TRANSPORT TO OTHER PLANT TISSUES Foliar structures are a source of many organic and inorganic substances which either migrate to the surface by diffusion and mass flow or are actively exuded by secretory structures. This provides a chemical environment on the leaf surface enabling readily hydrolyzable species to be complexed or chemically stabilized and therefore more available for foliar absorption. Since foliar surfaces also represent an efficient absorptive structure (Wittwer et al., 1963; Franke, 1967 and 1971), foliar applicaton of micro-nutrients to correct nutrient deficiencies is found to be effective, especially in those situations where a specific nutrient tends to be immobile and not as available for plant uptake on soil amendment (Krantz et al., 1962; Franke, 1967). The actual mechanisms involved in foliar absorption are not totally understood. Available information would indicate that although the cuticle of the leaf is hydrophobic in nature, penetration is facilitated via intermolecular spaces (Fisher and Boyer, 1972), modification in cutin composition at anticlinal epidermal walls and the presence of ectodesmata (Franke, 1967, 1971), and trichomes (Benzing and Burt, 1970). The role of stomates as a route of foliar penetration under normal conditions is in question and is currently held to be of negligible importance (Greene and Bukovac, 1974). 40 The relative importance of foliar absorption as compared to root absorption as a route of entry into plant tissues depends on several factors, For soluble species which remain relatively available in soil solution, root absorption processes are as, or more effective than, foliar absorption processes. This does not imply that foliar surfaces are not effective sites of absorption, Elements reported to be absorbed and transported from foliar surfaces include inorganic and organic N, Rb, K, Na, Cs, P, C1, S, Zn, Cu, B, Mn, Fe, and Mo (Wittwer et al., 1963). In the case of specific nutrient deficiencies, foliar applicaton is the method of choice (Bradford, 1966; Labanauskas, 1966). This is especially true for those nutrilites which tend to readily hydrolyze in soil solution or are rapidly adsorbed to soil particles, thereby limiting avallatility for root adsorption. The fate, with respect to foliar absorption, of relatively insoluble elements such as plutonium making up or carried on discrete particles is in some way analogous to the behavior of micronutrients such as Fe, which tend to form relatively insoluble products in aqueous environments. I£ we can assume that small particles containing plutonium (<1.0 um) can be retained in foliar surfaces over an extended period of time, the question arises as to the absorptive capacity of foliar surfaces for available plutonium, Since absorption of a particular element is a function of the concentration of the available or “soluble” component, an extended residence time on plant foliage may provide time necessary for "soluble" components to be chemically modified and/or absorbed by internal tissues. This may represent a more efficient route of entry, as compared to the same finite amount of plutonium deposited into soil where insolubilization and adsorption to soil particles are more likely to occur, thus reducing ever further the concentration available for root absorption. Absorption data from laboratory studies with bushbean plants contaminated with aerosolized plutonium are given in Tables 3 and 4 (Cataldo, unpublished); the protocol for this study has been previously reported by Cataldo et al. (1976). The objective of this study was to evaluate the extent of absorption and translocation of foliarly applied plutonium based on chemical form supplied, and the presence or absence of a solution vector (simulated rainfall), All plants were exposed to plutonium at 20 days from planting (preflowering) and held for an additional four weeks to allow time for both absorption of plutonium and seed filling. Seed and root tissues were analyzed to determine the extent of translocation of plutonium; these were protected from aerosol contamination, The pots containing soil and root were double-bagged with polyethylene and sealed at the lower stem; the seed tissue was contained in pods formed after exposure. Transport ratios (TR values) for root and seed tissues from plants not subjected to leaching (solution vector) ranged from <4.5x10-® to 3.3x10-5, Applicaton of a simulated rainfall to provide a solution vector for diffusion and absorption of soluble components on the leaf surface resulted in an increased uptake and transport of Pu to seed and root tissues for all compounds of Pu studied. Apparent differences between the various Pu forms may result from the relative size of the "soluble" fraction. The fresh plutonium dioxide was truly particulate at the time of contamination, the aged oxide 341