Video clip – Van Helmont’s experiments on plant growth

This video clip from BBC Four Botany: A Blooming History demonstrates Van Helmont’s classic experiment to investigate how plants grow.

In 1634 Jean Baptist Van Helmont was arrested for the crime of studying plants and other phenomena. He considered the question “how do plants grow?”. The contempory theory was that plants grew by eating soil. He devised an investigation to test the 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 stones. 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. The importance of the use of scientific evidence to support ideas is discussed, even though the conclusion he made was wrong.

An good introduction to the puzzle of how plants grow. If asked, many younger pupils will believe that plants grow by taking material from the soil. The idea can be tested using rapid cycling brassicas or other fast growing species. It is also an opportunity to discuss the importance of measuring dry mass when doing experiments with living material. This can be used to develop the idea of controlling variables in experiments. The clip also 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.