The Q5 for the respiration of Avena coleoptiles is higher than the Q5 of their protoplasmic movement. The rate of the protoplasmic streaming therefore is not determined directly by the rate of the respiration. The Q5 is higher in 96 hours old coleoptiles than in 72 hours old ones. Under the experimental conditions the diffusion of oxygen towards the cells is not limiting the respiration, in contrast with what has been found for the protoplasmic movement. Addition of glucose increases the respiration and decreases the Q5. Narcosis with ethylurethane to such an extent that glucose is no longer limiting the respiration also causes a decrease of the Qs. After inhibition of the respiration with KCN to the same extent as with urethane or more, addition of glucose still increases the respiration. Besides, the respiration has become sensitive to changes in the oxygen tension. The phenomena appearing after inhibition with KCN can be explained by assuming, according to the views of T h e o r e 11, that in normal respiration the lactoflavin system is reoxidized by the cytochrome-c only, but after inhibition with KCN the oxygen tension in the cell is increased to such an extent, that the lactoflavin system can partly be oxidized directly by free oxygen. In the Avena coleoptile relatively large quantities of lactoflavin must be present, since in 96 hours old coleoptiles after total inhibition with KCN 43% of the initial respiration remains. The author wishes to express his sincerest thanks to Prof. Dr. V. J. Koningsberger, director of the botanical laboratory, for his interest in the work and the liberty he has given him in the performance of these investigations. To Dr. E. C. Wassink the author is indebted for valuable suggestions concerning the explanation of the experiments with KCN.