“…And the wellknown depressing effect of the oxygen of the air on photosynthesis could hardly exist if the oxygen partial pressure inside the illuminated algal cell were much higher than outside. (See also Harvey, 1928. ) THE ADAPTATION OF GREEN ALGAE TO THE USE OF HYDROGEN.-In a preceding paper (Gaffron, 1940) evidence has been brought forward for the existence of a photoxidation or light respiration inside the photosynthetic mechanism.…”