Q.Joseph Priestley's bell-jar experiments of the 1770s, using a burning candle or a mouse together with a sprig of mint, first demonstrated that a green plant kept in a sealed container:

The modern understanding of photosynthesis was built up through a sequence of classic experiments spanning nearly two centuries. Priestley (1770s) first showed a green plant restores something to sealed air that a burning candle or living mouse had used up. Ingenhousz refined this, showing the effect requires both sunlight and the plant's green parts specifically. Sachs (1860s) demonstrated that illuminated leaf tissue accumulates starch (tested with iodine) specifically within chloroplasts, localising CO2 fixation to that organelle. Engelmann (1882) split light through a prism across a filamentous alga in a suspension of aerobic bacteria, showing the bacteria clustered densest in the blue and red regions -- establishing that the action spectrum of photosynthesis parallels chlorophyll's absorption spectrum. Blackman (1905) showed photosynthesis must comprise at least two mechanistically distinct phases, based on differing light- and temperature-sensitivity. Hill (1937) proved, using isolated chloroplasts supplied with an artificial electron acceptor instead of CO2, that O2 evolution and CO2 reduction are separable events (the Hill reaction). Van Niel, comparing green-plant and sulfur-bacterial photosynthesis, correctly predicted that photosynthetic O2 originates from water, not CO2 -- later confirmed using isotopically labelled water (H2^18O).