Photosynthesis in Higher Plants - Early experiments

Joseph Priestley (1770) performed experiments with a bell jar, a candle, and a mouse to show that plants restore the air that breathing animals and burning candles remove. He discovered Oxygen ($O_2$) in 1774.

Jan Ingenhousz (1730–1799) demonstrated that sunlight is essential for the plant process that purifies air. Using aquatic plants, he showed that in bright sunlight, small bubbles were formed around the green parts, which were identified as $O_2$.

Julius von Sachs (1854) provided evidence that green parts in plants produce glucose (usually stored as starch). He showed that the green substance (chlorophyll) is located in special bodies (chloroplasts) within plant cells.

T.W. Engelmann (1843–1909) used a prism to split light into spectral components and illuminated a green alga, $Cladophora$, placed in a suspension of aerobic bacteria. The bacteria accumulated mainly in the regions of blue and red light, representing the first action spectrum of photosynthesis.

Cornelius van Niel (1897–1985) demonstrated that photosynthesis is essentially a light-dependent reaction in which hydrogen from a suitable oxidizable compound reduces carbon dioxide to carbohydrates. He inferred that the $O_2$ evolved by the green plant comes from $H_2O$, not from $CO_2$.

Van Niel's work with purple and green sulfur bacteria showed that when $H_2S$ is the hydrogen donor, the oxidation product is sulfur or sulfate rather than $O_2$.