biology question help

In the mid-seventeenth century, Dutch scientist Jan Baptista van Helmont struggled to understand where plants get the material that allows them to grow larger. He planted a willow tree of a given weight in a fixed quantity of soil and then, five years later, weighed both the willow and the soil. He found that the tree had gained 164 pounds during the period but the soil had lost very little weight. His conclusion was that the willow's weight gain came solely from the water it absorbed through its roots. If you could be transported back to van Helmont's time, what would you tell him about the source of the willow's weight gain?