A substance called loculin is found in most thylakoids of tansy leaf chloroplasts during flower bud formation. It is retained in thylakoids when leaves are fixed with glutaraldehyde before chloroplast isolation. After 24 hours, loculin leaves thylakoids and gathers on the chloroplast envelope. Loculin remains unchanged when moving between thylakoid membrane and chloroplast. This substance is believed to be important for active growth.