The carbon dioxide assimilation is a chain reaction, consisting of at least three consecutive processes viz. a diffusion process, a photochemical process and a dark chemical process. This may be derived from Blackman’s principle of limiting factors and from a number of other phenomena. Each of the processes in question may individually determine the velocity of the assimilation. According to several investigators considerable deviations from Blackman’s scheme are to be observed. This must be ascribed almost entirely to the fact, that the different chloroplasts in a leaf or a plant are subject to different conditions in regard to light and carbon dioxide. A method was elaborated, in which this objection was evaded. A single cell layer of the filamentous terrestrial alga Hormidium was used as the subject in these studies. The carbon dioxide concentration of the air introduced into the assimilation vessel was regulated by means of a buffer solution of carbonate and bicarbonate. The consumption of C02 was determined by gas analysis. In a number of experiments the influence of light intensity, temperature and C02 concentration was studied. The results agree fairly well with Blackman's formulation. The deviations from the „ideal Blackman curve” are very slight. At temperatures between 12° C. and 20° C. a Q10 = 1.87 was found under conditions, in which temperature was the limiting factor, while a Q10 = ± 1 was found when light was the limiting factor. When the concentration of carbon dioxide is the limiting factor, the assimilation velocity, even in very small, unicellular organisms, is always determined by the diffusion process. The process, subsequent to the diffusion process, is caused by an agent (probably chlorophyll), which has an extremely high affinity for carbon dioxide. Even at a carbon dioxide pressure of ——■—— atm. it is already almost en- K 100.000 3 tirely saturated. .... gm C02 Per hour The assimilation number —— – was detergm chlorophyll mined for Hormidium. For this purpose the chlorophyll content of the algae was measured spectrophotometrically. The assimilation number at 20° C. proved to be 6.75. O, The assimilation quotient —— in Hormidium is pro- L/O2 bably greater than 1, which may be related to the fact, that the first visible assimilation product in this organism is an oil. The investigations here reported were carried out in the Botanical Laboratory at Utrecht. I wish to express my sincere thanks to Prof. Dr. F. A. F. C. Went, not only for his aid and helpful criticism, but also for his encouragement throughout the course of these studies.