. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. PHOTOREACTIVATION IN COLPIDIUM 341 tivation. Figure 5 shows the relative efficiency of a dosage of 1000 ergs/mm-, of each of the various UV wave-lengths used in retarding the division of Colpidium. The effectiveness is compared in two ways: 1) on the basis of the energy incidence per/mm2., and 2) on the basis of the number of quanta per mm'-. The results give a crude action spectrum for the UV retardation of division. The most effective wave-lengths are 2537, 2654, and 2804 A. Wave-lengths on the short end: 2450 and 2383 A,
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. PHOTOREACTIVATION IN COLPIDIUM 341 tivation. Figure 5 shows the relative efficiency of a dosage of 1000 ergs/mm-, of each of the various UV wave-lengths used in retarding the division of Colpidium. The effectiveness is compared in two ways: 1) on the basis of the energy incidence per/mm2., and 2) on the basis of the number of quanta per mm'-. The results give a crude action spectrum for the UV retardation of division. The most effective wave-lengths are 2537, 2654, and 2804 A. Wave-lengths on the short end: 2450 and 2383 A, and on the long end: 3025 and 3130 A, are less effective in retarding division than are the intermediate wave-lengths. In Figure 6 is shown the degree of photoreactivation produced by exposing to a dosage of ergs/mm-, at A4350A colpidia previously treated with 1000 ergs/mm2, at each of the UV wave- Illumination in all cases was performed shortly after irradiation. Pretreatment of colpidia with blue light (dosage: 46,000 ergs/mm2.) does not protect them significantly from subsequent exposure to UV (2654 A, 1000 100. 2383A 2450A 2537A 2654A 2804A 3025A WAVELENGTH of ULTRAVIOLET 3I30A 6. Degree of photoreactivation by blue light (56,900 ergs/mm2.) after a dosage of 1000 ergs/mm2, at each of the UV wave-lengths specified. ergs/mm2.). Colpidia so treated showed almost the same degree of retardation as those exposed to UV alone. Pretreatment with four hours of daylight before ex- posure to UV, however, produced some reduced sensitivity to UV applied thereafter. This phenomenon has been called photodesensitization by Weatherwax and Dobson (personal communication, 1951) who first observed it in Eschcricliia coli strain B. The decrease in sensitivity in Colpidium was only a small fraction of the photore- versal achieved by exposure to visible light after UV treatment. DISCUSSION UV affects division of Colpidium in two ways : a lag appears before division oc- curs, and,
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