Final answer:
Jan van Helmont's experiments led to the insight that plants gain mass mainly through water uptake rather than from soil, setting the stage for the later discovery of photosynthesis where plants convert carbon dioxide and water into sugars.
Step-by-step explanation:
Based on Jan van Helmont's experiments, he could determine that plants gain mass through the water they take in, not the soil. These experiments led to the understanding that plant growth does not consume significant amounts of soil and that the mass of a grown plant cannot solely be attributed to the uptake of soil. Van Helmont's work indicated that water contributes to the mass of the plant, but the full understanding of photosynthesis came later when scientists discovered that plants absorb carbon dioxide from the air and convert it into organic mass, primarily through the carbon atoms becoming part of sugar molecules.