302 BULLETIN OF THE TORREY BOTANICAL CLUB [Vou 91 radioactive particles per unit area of leaf surface have indicated that most of the particles retained by foliage are < 44. in diameter and have a mean diameter of about 20 ». Leaves having sticky or hairy surfaces retain more particles per unit area than do leaves having smooth or waxysurfaces. As indicated by Figure 6 and Table 7, the best correlations between fallout and plant activity have been obtained by comparing the activity of fallout < 44, with the gross activity of plants. Both the total and the percentage of fallout intercepted by vegetation tend to varyin relation to the mechanical distribution of fallout particles < 44» in diameter. The percentage of fallout intercepted by a unit area of vegetation, e.g., by the clover or wheat growing on a square foot of soil surface tends to increase with increasing distance from ground zero and increasing time of deposition. In a given fallout pattern, the maximum contamination of vegatation usually occurs in areas where the total fallout activity per unit area is moderately high, and a relatively large percentage of the total activity is associated with particles < 44. or < 88% in diameter. Depending on the conditions of detonation and the particle size composition of fallout, maxi- mum plant contamination may occur in areas close to ground zero orat distances of from 50 to several hundred miles from ground zero. 3, Animals, The external contamination on the skin and pelts of small mammals collected from 24 to 48 hours after fallout has been observed to account for 22% to 62% of the total beta activity associated with those animals (Lindberg et al. 1959). Experimental studies (Taplin et al. 1957) have indicated that some animal contamination may result from the inhalation of radioactive particles and aerosols. Field studies at the Nevada Test Site (Lindberg et al. 1954, 1959) have indicated that the average activity of lung tissues following fallout is rarely more than two or three times normal. 4. Acute vs. chrome exposure. For both the plants and the animals living in a fallout-contaminated environment, maximum exposure to external emitters occurs at or about the time of fallout deposition and then diminishes in accordance with the decay rate of mixed fission products. In a general way, the acute phase of plant contamination coincides with the period that fallout materials are retained on foliage and/or assimilated byfoliar absorption. In desert ecosystems, this period is probably less than a year and may be said to end with the loss of contaminated foliage. In the case of animals, the acute phase of exposure to increased ionizing radiation may be said to last as long as the exposure to external emitters and/or the total body content of mixedfission products is more than twice the pre-contamination levels of exposure. Admittedly, these criteria are arbitrary, but they should be useful in distinguishing between: (a) the short-term biological hazards associated with relatively high levels of exposure to external emitters or to internal