For plants sensitive to photoperiodism light is generally essential for flowering, and the requirement for C02 with the light indicates a photosynthetic function for the light period (21, 10). There appears to be an additional role of light in which it apparently alters a photoreceptor pigment controlling floral initiation (22). Evidence has been advanced recently that C02 is also essential during the dark period for floral initiation in some short day plants (13) as also in the allied phenomenon of vernalization of Alaska peas (15). Gregory et al (8) and Langston and Leopold (13) have suggested that the C02 metabolism in darkness is closely related to the photoperiodic response of short day plants. It would be interesting to know more concerning the products of C02 fixation in this connection. Calvin and co-workers (4) have demonstrated that products of dark and light fixation of C02 by green plants are dependent upon preceding light and dark conditions. It has long been accepted that some materials synthesized in a high intensity light process are utilized in the long dark period essential for flowering of short day plants (9). This suggests that a study of C*02 fixation in light and darkness during the period of photoperiodic induction with the aid of Chromatographie and autoradiographic techniques may yield useful information regarding the biochemistry of the photoperiodic reactions.