Retardation of division of three ciliates by intermittent and continuous ultraviolet radiations at different temperatures.

The same dosage of ultraviolet (UV) radiation retards division of several protozoans more effectively when the light is intermittent than when it is continuous, and especially at temperatures of 25-35 degrees C. At lower temperatures the difference between the effects of intermittent and continuous radiations is less marked. Somewhat similar results were obtained with the ciliates Paramecium caudatum, Blepharisma japonicum, and Colpidium colpoda, the disparity between intermittent and continuous light decreasing in the order given. The data are taken to indicate that thermochemical dark reactions succeed the absorption of UV radiations by the cells. In Blepharisma, besides initial delay in division, the cells stop dividing after one or two divisions, a "stasis" ensuing. Stasis is marked when the cells are irradiated at higher temperatures but is slight when they are irradiated at low temperatures, as if the temperature-sensitive reaction involved stasis (in all cases cultures are grown at 25 degrees C). The data are related to findings in the literature.