What makes trees grow?

Back in the 17th century, when the modern scientific method was still being worked out, a Flemish scientist / philosopher / alchemist named Jan van Helmont set out an experiment to determine what was the principle agent in tree growth - water or soil?

Up to that point, nobody had really bothered to try and figure out what caused a tree to grow (or any plant for that matter). People might discuss it. They might have there pet ideas, but none of those ideas were put to the test - they were just common sense, or good arguer might make a compelling case, or it was clearly documented in some ancient text (people have this really annoying habit of assigning far greater authority to something that has been written down, or ascribed to some dead person than to their own thoughts and ideas).

Anyway, nobody really knew what was the principle agent in a tree growing - water or the soil.

So Jan performed an experiment. He got himself 200lbs of soil and put it in a box. He planted a 5lb tree in the soil and covered the soil so that nothing would be admitted, save for the water he watered it with.

For 5 years he watered that tree and it grew and grew and grew. At the end, he removed the tree, carefully disentangled earth and roots and weighed both the tree and the soil. The tree weighed 169lbs; the soil had lost only 2 ounces of mass.

So were did the tree gain its additional mass if not from the soil? Clearly it was the water with which Jan has watered his tree.

This was actually a very good experiment. It carefully controlled two variables: soil and water. It came to a solid and sound conclusion, but it was wrong. It was wrong, not because of any problem with the experiment, but because of a lack of wider general knowledge. It would have been incredibly insightful of Jan to go beyond his obvious conclusion, notwithstanding it was he who brought the word gas to us (by way of the Greek kaos) and he also discovered carbon dioxide. Nevertheless, he believed that everything arose from the waters mentioned in Genesis. This was what his experiment was about - proving that water that is the elemental constituent of everything.

Four hundred years later, we understand thing s a lot better and realize that photosynthesis is responsible for the growth of the tree.

While present knowledge is better and more complete than in the past, I am always skeptical whenever I hear scientists' carefully laid out experiments and obvious conclusions because I always wonder if they missed something. Maybe they asked the wrong question or got an answer that is meaningless because that answer needs to be trimmed down further. What if the obvious answer is not really so obvious - like Jan van Helmont and his water.

Image copyright of me.