1. Respiration changes as a result of illumination.
2. In the absence of glucose or other supply of substrate, respiration decays in the dark showing at least two types—a fast decay in a few minutes and a slow decay lasting hours.
3. Respiratory response to illumination is delayed.
4. Intermittent illumination (in the absence of glucose, etc.) produces a periodic variation in respiration with a delay or phase lag.
5. Periodic variation of respiration may produce a higher average value in the dark than in the light due to the lag and depending upon the period of intermittent illumination.
6. Based upon average respiration values our data confirm the Kok effect.
7. Interpolated values of respiration, however, result in photosynthetic rates which are linearly dependent upon intensity of illumination.
8. Thus the quantum efficiency is found to be independent of intensity, over the wide range of intensities investigated.
