Possible variation in the probability that absorbed quanta of ultra-violet energy will produce observable inhibitory and lethal effects in the yeast cell, due to non-uniformity in sensitivity of the different regions of the cell, may be further modified by the reproductive stage of the cell at the time of irradiation. Tests of the survival of yeast cells of 15 day and 24 hour cultures indicate that the older resting cells are more resistant to ultra-violet irradiation effects than cells undergoing rapid cell division.

The effects of temperature changes within the range of normal growth are evidently small, as judged from the temperature coefficient (1.10).

Possible inhibitory effects due to the action of ultra-violet radiation on the malt agar medium and to toxic substances diffused from cells killed by irradiation were not found under the conditions of the experiments.

Tests of the validity of the Bunsen-Roscoe reciprocity law for variation in the intensity of the incident ultra-violet radiation up to 30 per cent indicate that for this range the rate of absorption of quanta by the cell does not produce any marked change in the lethal effects observed.

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