1. Though the quantum yield remains constant for different samples of the same culture despite great changes in respiration due to dark adaptation, the quantum requirement for different cultures varies from 6.1 to 13.5 quanta per molecule of oxygen evolved (q/m).
2. This variation from one culture to another appears to depend upon chlorophyll concentration, though other paralleling factors cannot be ruled out.
3. Both chlorophyll concentration and quantum requirement show a random distribution. A statistical median for 50 cultures and 99 determinations gives q/m = 8.5 with a systematic uncertainty of perhaps 10 per cent. Since the variations are real, the median is regarded as less important than the lower limit approached (about q/m = 6).
4. Dark adaptation under aerobic conditions produces an initial photosynthetic rate of nearly zero. The immediate rise to steady state is somewhat logarithmic in character and may require over 3 minutes.
5. In intermittent light (of periods from 1 to 6 minutes) the induction observed in subsequent light periods starts from a finite initial rate and occupies a shorter time, often as little as 30 seconds.
6. The theoretical importance of aerobic induction is discussed. A chlorophyll cycle of two photochemical steps is found to satisfy most of the observed characteristics and to be compatible with an efficiency independent of intensity.