Jan Ingenhousz demonstrated in 1779 that plants produce oxygen only in sunlight. His experiments showed that submerged green parts of plants released bubbles in sunlight, while bubble production stopped in shade or darkness.
Ingenhousz built on Joseph Priestley’s earlier observation that plants could restore air made unsuitable for animal respiration. Priestley had not clearly identified the importance of light, however. Ingenhousz carried out extensive experiments at a rented country house in Southall Green, England, and identified the bubbles as oxygen.
His work also revealed that the green parts of plants were especially important. He observed that plants release carbon dioxide in darkness, helping establish the distinction between photosynthesis and respiration. Modern biology explains that photosynthesis requires light-driven reactions, while cellular respiration continues day and night.
Ingenhousz published his findings in 1779 in “Experiments upon Vegetables.” The result did not yet provide the modern chemical equation for photosynthesis, but it identified sunlight as an essential condition for oxygen release and became a major step in the history of plant physiology.