While investigating Joseph Priestley's discovery made in 1771 that plants could "restore" air made unfit for respiration through combusion or putrefaction, Ingen-Housz became the first to observe and elucidate the processes of photosynthesis and plant respiration. In his Experiments upon Vegetables, Ingen-Housz established that only the green parts of a plant give off the "restoring" gas (oxygen), and only when exposed to visible sunlight. He also found that plants, "like animals, exhibit respiration, that respiration continues day and night, and that all parts of the plant—green as well as nongreen, flowers and fruit as well as roots—take part in the process.
Hook & Norman, The Haskell F. Norman Library of Science and Medicine (1991) no. 1141.