Abstract
Broad bean roots were irradiated with alpha particles in 02-free and oxygenated water, and their lengths were measured 10 days after irradiation. The sensitivity of the bean roots to alpha particles was increased by dissolved 02 by a factor of about 1.4 as compared to a factor of 2.8 found with X-rays. The O2 is considered to augment, principally, the effectiveness of the delta-rays which account for about half the ionization due to the alpha particles.