a journal or Proceelngs. Since change8 may be made before pubhcstron, thla preprint 18 made avzullble w t h the underStandingthat 11 wll not be u t e d or rcprcduced

The measurement of transpiration of water by trees in the field with tritiated water as a tracer depends upon a new application of established theory of radionuclide dynamics in steady-state systems. The techniques required are non-destructive to the trees and probably have negligible disturbing effects on transpiration. Average transpiration rates ranging from 1.75 to 372 liters per day per tree were measured by the proposed method on tropical forest trees which ranged from the understory to the canopy in size. Statistical errors range from 12.1% on the largest tree to 6.2% on the smallest for one standard deviation. Non-random sources of error in the method may include ( 1) loss of tritium from leaves due to rainfall; and (2) possible enrichment of tritiated water in leaves due to differences in vapor pressure and molecular diffusion coefficients between tritiated water and ordinary water. These require further experimental evaluation. The method may be generally applicable to field measurements of transpiration in trees.

The accumulation of radionuclides by plants acting as a monitoring system in the environment may occur by two modes; foliar absorption by the leaves and shoot of the plant, or by root uptake from the soil. Data on plant accumulation of radionuclides may be obtained from studies of fission product radionuclides deposited as worldwide fallout, and from tracer studies of plant physiology. The epidermal features of plant foliage may exert an effect upon particle retention by leaves, and subsequent uptake of radionuclides from the surface. The transport of radionuclides across the cuticle and epidermis of plant leaves is determined in part by the anatomy of the leaf, and by physiological factors. The foliar uptake of fallout radionuclides, 99Sr, 131I, and 137Cs, is described with examples from the scientific literature. The environmental half-life of 131I, for example, is considerably shorter than its physical half-life because of physical and biological factors which may produce a half-life as short as 0.23/day. 99Sr and 137Cs are readily taken up by the leaf, but 137Cs undergoes more translocation into fruit and seeds than 99Sr which tends to remain in the plant part in which it was initially absorbed. Soil-root uptake is conditioned primarily by soil chemical and physical factors which may selectively retain a radionuclide, such as 137Cs. The presence of organic matter, inorganic colloids (clay), and competing elements will strongly affect the uptake of 99Sr and 137Cs by plants from the soil. The role of plants as monitors of radionuclides is twofold: as monitors of recent atmospheric releases of radionuclides; and as indicators of the long-term behavior of aged deposits of radionuclides in the soil.

Measurements at Point Barrow, Alaska, indicate that rate and depth of thaw in Arctic soils are highly related to soil type. Arctic brown soil thawed earlier in the spring, thawed to a greater depth, and froze earlier in the fall than did the adjacent poorly drained areas. During 1956, arctic brown soil thawed to a maximum depth of 40 inches while the depth of thaw in the nearby tundra soil approximated 18 inches. Diurnal near‐surface temperature amplitude was greater in the arctic brown than the adjacent poorly drained areas. The willow‐lichen‐moss complex was the dominant vegetation with the arctic brown soil, while in the tundra soil, it consisted mainly of Dupontia‐Carex‐Eriophorum scheuchzeri. Other vegetation units in relation to soils are given.

The accumulation of radionuclides by plants acting as a monitoring system in the environment may occur by two modes; foliar absorption by the leaves and shoot ofthe plant, or by root uptake from the soil. Data on plant accumulation of radionuclides may be obtained from studies of fission product radionuclides deposited as worldwide fallout, and from tracer studies of plant physiology.The epidermal features of plant foliage may exert an effect upon particle retention by leaves, and subsequent uptake of radionuclHdes from the surface. The transport of radionuclides across the cuticle and epidermis of plant leaves is determined in part by the anatomy of the leaf, and by physiological factors.The foliar uptake of fallout radionuclides, "'Sr, 131I, and 137Cs, is described with examples from the scientific literature. The environmental half-life of 131I, for example, is considerably shorter than its physical half-life because of physical and biological factors which may produce a half-life as short as 0.23/day. 9'Sr and 137Cs are readily taken up by the leaf, but '37Cs undergoes more translocation into fruit and seeds than "°Sr which tends to remain in the plant part in which it was initially absorbed.Soil-root uptake is conditioned primarily by soil chemical and physical factors which may selectively retain a radionuclide, such as '37Cs. The presence of organic matter, inorganic colloids (clay), and competing elements will strongly affect the uptake of 9°Sr and 137Cs by plants from the soil.The role of plants as monitors of radionuclides is twofold: as monitors of recent atmospheric releases of radionuclides; and as indicators of the long-term behavior of aged deposits of radionuclides in the soil.