Significant discoveries that contributed towards understanding photosynthesis
Scientists	Contribution
Aristotle and Theophrastus	Plants obtain organic and inorganic nutrients from the soil - Humus Theory
Von Helmont	Plants obtain nutrients from water and soil - Willow tree experiment
Stephan Hales	Air and light are important for the nourishment of plants
Joseph Priestley	Green plants purify the air. The vital air is released from the plants.
Ingenhousz	Sunlight is required for the purification of air by the plants. Provided equation for photosynthesis: $H_{2}CO_{3}$ $+ Light \rightarrow Organic matter + O_{2}$
T. Desaussure	Plants absorb CO2 from the atmosphere. Plants need light and water for photosynthesis. The amount of O2 released is equal to the amount of CO2 absorbed
Pelletier and Caventon	Proposed the term ‘chlorophyll’
Mayer	Proposed that plants store energy from sunlight in the form of chemical energy
Leibig	Organic carbon comes from CO2
Engelmann	Plotted action spectrum of photosynthesis
Blackman	Discovered dark reaction of photosynthesis
Von Neil	Indicated that photosynthetic bacteria fix CO2 in the presence of H2S and S is precipitated. Similarly, green plants use H2O and O2 is released from water.
Robert Hill	Hill’s reaction to explain the light reaction of photosynthesis
Reuben, Hasid and Kamen	Confirmed that oxygen is released due to dissociation of water (by using $H_{2}O_{18}$ )

Early Experiments

Exp. 1

Joseph Priestley in 1770 performed a series of experiments that revealed the essential role of air in the growth of green plants.
Priestley observed that a candle burning in a closed space – a bell jar, soon gets extinguished. Similarly, a mouse would soon suffocate in a closed space.
He concluded that a burning candle or an animal that breathes the air, both somehow, damage the air.
But when he placed a mint plant in the same bell jar, he found that the mouse stayed alive and the candle continued to burn.
Priestley hypothesized as follows: Plants restore the air whatever breathing animals and burning candles remove.

Exp. 2

Using a similar setup as the one used by Priestley, but by placing it once in the dark and once in the sunlight, Jan Ingenhousz (1730-1799) showed that sunlight is essential to the plant process that somehow purifies the air fouled by burning candles or breathing animals.
Ingenhousz in an elegant experiment with an aquatic plant showed that in bright sunlight, small bubbles were formed around the green parts while in the dark they did not.
Later he identified these bubbles to be of oxygen.
Hence he showed that it is only the green part of the plants that could release oxygen.

Exp. 3

In 1854, Julius von Sachs provided evidence for the production of glucose when plants grow. Glucose is usually stored as starch.
His later studies showed that the green substance in plants (chlorophyll as we know it now) is located in special bodies (later called chloroplasts) within plant cells.
He found that the green parts in plants are where glucose is made and that the glucose is usually stored as starch.

Exp. 4

T.W Engelmann used a prism and he split light into its spectral components and then illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria.
The bacteria were used to detect the sites of O2 evolution.
He observed that the bacteria accumulated mainly in the region of blue and red light of the split spectrum.
A first action spectrum of photosynthesis was thus described.
It resembles roughly the absorption spectra of chlorophyll a and b.