He grew a tree for five years to prove plants are made of water, and was half right

Around 1640, Jan Baptist van Helmont planted a 5-pound willow shoot in a weighed tub of dried soil, watered it with nothing but rain, and five years later found the tree had gained about 164 pounds while the soil had lost only about two ounces. He concluded the extra mass came purely from water. He was wrong about the mechanism, most of a plant's bulk is actually fixed from carbon dioxide, but his careful weighing is considered one of biology's first quantitative experiments.

— Jan Baptist van Helmont, Ortus Medicinae — Published posthumously, 1648