1. Photoreversal of ultraviolet (UV) injury was studied in the ciliate protozoan Tetrahymena pyriformis (geleii) strain W, cultured in the absence of other living organisms. The division pattern of progeny of single animals was followed in hanging drop preparations.

2. A sublethal dose of 450 ergs/mm.2 of monochromatic UV of wave length 2654 A produces a lag before the first division followed by a period of cessation of fission after the second division. This cessation sometimes lasts as long as 6 weeks, during which time the animals become smaller and rounder and more opaque. Organisms about to resume division increase in size and transparency; after a few divisions the animals regain their normal division rate.

3. The effect of UV ranging in intensity from 5 to 15 ergs/mm.2/sec. was found to obey the reciprocity law quite well for the UV effect on the division pattern of T. pyriformis. However, the same dose at lower and at higher intensities was less effective.

4. The effect of a dose of UV delivered at high intensity (19 ergs/mm.2/sec.) could be increased by flashing the light, indicating that the system became saturated in the continuous light.

5. A photoreversing dose of monochromatic blue light of wave length 4350 A was found to be more effective when delivered as continuous light at a low intensity, or as intermittent light at a high intensity, rather than as continuous light at the high intensity—indicating that a dark mechanism participates in photoreversal.

6. The time for the dark reaction was determined to be of the order of a few hundredths of a second in experiments in which different lengths of dark period were used while maintaining a constant light period of 0.0025 second.

7. For Colpidium colpoda the efficiency of a given dose of photoreversing light was increased by flashing the light.

8. The present experiments are interpreted in terms of data available in the literature.

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