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In 1634, Jan Baptist van Helmont was arrested by agents of the Spanish Inquisition for the crime of studying plants and other phenomena. While under house arrest, he started to consider how plants grew. The prevailing theory at the time was that plants grew by eating soil, and van Helmont devised a clever investigation to test this idea. He weighed a willow tree and weighed dry soil. He planted the tree, watered it and then left it for 5 years. He then re-weighed the tree, which had increased in mass by over 12 stone. He dried the soil and weighed it, showing that the soil was almost the same mass. He concluded that the tree grew by drinking water.

Students could replicate the experiment using cress, but instead of focusing on the soil uptake, give the plants different quantities of fertiliser in their soil to test its effect on growth. Use to develop the idea of controlling variables in experiments. The clip raises issues about the use of scientific evidence to support conclusions. It can be used to emphasise that firm conclusions can only be drawn when clear evidence is found. Students can also discuss the gaps in Van Helmont’s knowledge, which they can fill now with current evidence.