Effects of measured ultra-violet light on the yeast S. cerevisiae have been studied. Methods of culturing and irradiating the yeast, of estimating the nature and extent of the changes produced, and the means used in producing and measuring the radiation are given.

No evidence of a stimulative action was observed. The absorbed energy did not produce an all or none effect; arbitrary criteria must be used for judging the various inhibitory and lethal effects. With increase in the incident energy diverse effects were produced until abnormal cell growth and "death" of the cells resulted. Changes in the rate of oxygen consumption did not occur until after a high proportion of the irradiated cells were so damaged that they produced abnormal cells.

The shape of the curves relating effect to energy are similar for various wave-lengths but different energies are required to produce the same effect at each of the wave-lengths studied. The similarity of the curves to that for a first order reaction is noted, but attention is called to the modifying influence of accessory factors such as the age of the cells. A comparison is made of the morphological and metabolic changes on the basis of energy requirements, and their relative value as criteria in judging the effects of ultra-violet light on yeast and physiologically similar microorganisms.

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