LETHALITY AND THE BIOLOGICAL EFFECTS OF X-RAYS IN PARAMECIUM: RADIATION RESISTANCE AND ITS VARIABILITY

Abstract

1. Irradiation with x-rays markedly increases the viscosity of the protoplasm of Paramecium caudatum; greater dosages lead to irreversible coagulation. With increased irradiation, paramecia become immobilized, become broadly ellipsoidal and settle on the bottom of the irradiation chambers. Contractile vacuoles function more slowly and occasionally become abnormally large. Prior to death, cyclosis ceases and the protoplasm becomes darker and vacuolated. Clear, blister-like Swellings appear at the pellicle. Before death, waves of trichocysts are extruded suggesting that their function may represent an injury-reaction. Finally, paramecia frequently become sub-spherical before their disintegration. 2. It was found that one of the most important factors influencing the lethal effects of x-radiation was the degree and extent of exposure of the fluid containing paramecia to air. Paramecia in hanging drops were killed by dosages (170 kr.) that exhibited no lethality for paramecia in larger volumes of culture fluid. This difference in lethality occurred even though the numbers of paramecia per unit volume were kept uniform in both drops and larger volumes. 3. A new method using Nylon syringes was devised to minimize the variability of x-radiation effects. 4. Survival curves were established for Paramecium caudatum using this new method. It was found that the LD 50, 24 hours was approximately 340 kr.