Hales

1727

Plants took their nourishment in part from the atmosphere.
Light energy participated in this process.

Priestley

1771

Green plants "renewed" air that was made "bad" by breathing animals or burning candles
"Renewed" air = oxygen
"Bad" air = carbon dioxide

At this time the composition of the atmosphere was being revealed and the gases in it were being discovered.

Ingenhousz

1779

Light is necessary for the production of O₂ by plants
("Plants should be removed from the house at night")

Senebier

1782

Plants breathe like animals in the dark

De Saussure

1804

Quantitative measurements of photosynthesis.

Mass gain by plants through growth > Mass of CO₂ absorbed – Mass of O₂ released

The difference was attributed to water.

Sachs

1864

Observed the growth of starch grains in illuminated chloroplasts.
Classic experiment: Leaf exposed to light with certain parts covered.
In the exposed parts starch was produced
In the covered parts starch was absent.

Therefore:

Engelmann

1894

Blackman

1920s