The ancient Greeks and Romans believed that plants derived their food from the soil. The earliest experiment to test this hypothesis was performed by the Belgian scientist Jan Baptista van Helmont (1577–1644), who grew a willow tree in a container of soil and fed it only water. At the end of five years, the weight of the willow tree had increased by 164 pounds (74.4 kilograms), while the weight of the soil had decreased by 2 ounces (57 grams). Van Helmont concluded the plant had received all its nourishment from the water and none from the soil. Joseph Priestley (1733–1804) demonstrated that air was “restored” by plants.

In 1771, Priestly conducted an experiment in which he placed a lighted candle in a glass container and allowed it to burn until extinguished by lack of oxygen. He then put a plant into the same chamber and allowed it to grow for a month. Repeating the candle experiment a month later, he found that the candle would now burn. Priestley’s experiments showed that plants release oxygen (O₂) and take in carbon dioxide (CO₂) produced by combustion. The Dutch physician Jan Ingenhousz (1730–1799) confirmed Priestley’s ideas, emphasizing that air is “restored” only by green plants in the presence of sunlight.

Evidence of photosynthesis’s two-stage process was first presented by F.F. Blackman (1866–1947) in 1905. Blackman had identified that both a light-dependent stage and a light-independent stage occur during photosynthesis. In 1930 C.B. van Niel (1923–1977) became the first person to propose that water, rather than carbon dioxide, was the source of the oxygen that resulted from photosynthesis. In 1937 Robert Hill (1899–1991) discovered that chloroplasts are capable of producing oxygen in the absence of carbon dioxide only when the chloroplasts are illuminated and provided with an artificial electron acceptor.