Carbon dioxide exchanges in the dark and in the light indicate that photosynthetic activity occurs m young leaves of barley seedlings after about 3 hours illumination under favourable conditions. Activity increases rapidly as chlorophyll is formed in the expanding leaf blade; a compensation point is reached after 30–40 hours and thereafter carbon dioxide assimilation exceeds respiration.
Changes in the fine structure of plastids in the young leaves were examined by electron microscopy; some of the chief stages in the development of the lamella system and the differentiation of grana are described. The presence of ribosome- and nucleoplasm-like structures in the chloroplast stroma was indicated.
The relationship between the development of chloroplasts and their photosynthetic activity is discussed. It is suggested that the formation of distinctive double membranes between the thylakoids of the grana plays an important part in the structural organization of the pigment system. The action of light on chloroplast development indicates that a primary step in photomorphogenesis is the conversion of protochlorophyllide to chlorophyll. The further extension and development of the lamellar system probably depends upon the biosynthesis of proteins and lipids, closely linked with photosynthesis and operating under the control of nucleic acids.