Jan Baptista van Helmont (1580-1644) partially discovered the process of photosynthesis. He grew a willow tree in a weighed amount of soil. After five years, he discovered that the willow tree weighed about 74 kg more than it did at the start. As the weight of the soil had hardly changed, van Helmont concluded that plant growth cannot only be due to minerals from the soil. He thought that the extra plant material had come from the water alone.
For the first time this disproved the previous theory of the Ancient Greeks which was the plants gained mass by taking in minerals from the soil. Van Helmont was the first person to test this idea and find it incorrect.
Several centuries later, Joseph Priestley (1733 - 1804) carried out an experiment that showed that plants produce oxygen. He put a mint plant in a closed container with a burning candle. The candle flame used up the oxygen and went out. After 27 days, Priestley was able to re-light the candle. This showed that plants produce a gas that allows fuels to burn. This gas is oxygen.
Jan Ingenhousz (1730-1799) was a Dutch scientist who is best known for showing that light is essential for photosynthesis. As a consequence he is remembered as the person who discovered photosynthesis.
Ingenhousz met with and built upon the work of the English Joseph Priestly. In 1779 Ingenhousz found out that in the presence of light, plants give off bubbles from their green areas, while in the shade these bubbles stop. He determined this gas to be oxygen.
Ingenhousz continued his work and discovered that plants produce carbon dioxide in the dark. This led him to determine that plants undergo cellular respiration like animals. He realised that more oxygen is given off in the light than carbon dioxide in the dark and so proved that some of the mass of plants comes from the soil.