296 BULLETIN OF THE TORREY BOTANICAL CLUB (Vou. 91 are ecologically interesting because they illustrate the operation of environmental processes which may result in the concentration of radioactivity in a particular area of the biosphere. A more recent case of long distance, apparently tropospheric fallout mayillustrate this point even better. On September 10, 1961 two nuclear devices were detonated at Novya Zemlya in northern Russia. The air masses containing debris from these detonations traveled southward over Labrador and along the Atlantic Coast of the United States, then westward, and finally northwardinto the Mississippi drainage basin where it was deposited from cold, descending air. The radioiodine content of milk collected in St. Louis, Missouri on September 4.5 3.5 4 MC/MI2/MONTH 3.0 4 2.5 4 1.5 4 1.0 5 T duly 1957 t Nov 1957 T Mar 1958 T July 1958 T Nov 1958 TT Mar 1959 tT July 1959 T Nov 1959 T Mar 1960 T July 1960 T Nov 1960 v Mar 1961 July 1961 Fig. 4. Rate of Sr-90 deposition on the United States based on average monthly deposition at 10 HASL collection stations. After Kuroda (1960). 20 was 80 pye/1 (micro-mierocuries per liter) ; on September 27 it was 500 pye/1; on October 6, 250 puc/1; and October 31, 80 pyc/1 (Dunning 1962). 3. Stratospherie fallout. Except at extremely high latitudes, surface or near surface detonations of less than 200 kilotons do not produce sufficient thermal energy to carry significant quantities of radioactive debris into the stratosphere. Detonations of larger yields usually produce fireballs which penetrate the tropopause, and most of the fine debris produced by a nuclear detonation in the megaton rangeis readily carried into the stratosphere. The latitudinal distribution of Sr-90 (Fig. 3) indicates that stratospheric debris may be deposited in either hemisphere, but the greatest deposition has been in the northern hemisphere between 30° and 60° latitude (Alexander et al. 1960). Also there is evidence (Fry and Kuroda 1959, Health and Safety Laboratory 1958, and Kurodaet al. 1960) that the rate of fallout