284 BULLETIN OF THE TORREY BOTANICAL CLUB [Vou. 91 mental radiation on organisms began as early as 1896 (Miller 1896). The possibility that ionizing radiation has played an important role in evolution was recognized as early at 1928 (Olson & Lewis 1928). During the past two deeades, the ‘‘natural’’ sources of ionizing radiation in the biosphere have been augmented by the release of considerable quantities of ‘‘man-made’’ radioactive materials. The spread of radio- nuclides in fallout has been world-wide, and probably there is no place in the biosphere where the level of environmental radiation has not been in- creased to some extent. Ecological studies of the effects of nuclear detona- tions were made as early as 1948 (Bellamy 1949). Since then, the subject has been discussed at a variety of Congressional Hearings (Special Sub- committee 1957, 1959a, 1959b) and scientific meetings all over the world (A.LB.S. 1962, Caldecott and Snyder 1960, Comar 1957, Dunning and Wilkin 1956, Health and Safety Laboratory 1958, Singleton 1958, Sparrow et al. 1958, United Nations 1955, and United Nations 1958). This paper is a brief review of some of the currently available information concerning: (a) sources of environmental radiation, (b) the formation and dispersal of fallout, (¢) the influence of organisms and of environmental processes on the redistribution of radioactive materials in the biosphere, and (d) the possible influence of increased environmental radiation on organisms. Its purpose is to outline a few of the ecological problems involved in evaluating the potential hazard, if any, of increased environmental radiation to populations of plants, animals, and men. Sources of ionizing radiation in the biosphere. ‘‘NATURAL’’ SOURCES. The natural or ‘‘background’’ radiation to which organisms are exposed in their natural environments is derived either from cosmic radiation or from radionuclides. Radionuclides in environmental media are referred to as ‘‘external emitters’’ while the radionuclides assimilated by plants and animals are called ‘‘internal emitters.’’ 1. Cosmic Radiation. Cosmic rays consist chiefly of protons and alpha particles which enter the earth’s atmosphere from outer space. Nuclear reactions between cosmic rays and the atmosphere produce a variety of | secondary radiations. At the lower altitudes, the ionization associated with cosmic radiation is caused primarily by protons, mesons, high energy electrons and photons. The average intensity of cosmic radiation at sea level (Burch 1954) is about 0.1 mr/day (mr = milliroentgens). This inereases with altitude to a maximum of about 15 mr/day at 70,000 feet. In the thinner air above 70,000 feet, ionization due to cosmic rays is less intense. At sea level the intensity of cosmie radiation increases slightly with increasing latitude. 2. External emitters. In addition to radium, thorium, uranium, and