Jan van Helmont (1580-1644) was a Belgian scientist, often referred to as the "father of photosynthesis". He conducted a simple experiment:

Using a potted plant, he carefully measured the mass of the soil of the plant in the beginning of the experiment. When the plant had grown, he weighed the plant's mass and the mass of the soil which remained almost unchanged thus determining that the plant's mass was not obtained from the soil which he had assumed. He thus attributed the plant's growth to the only substance he had added to the plant, the water.