الأسبوع 10 · الوحدة الثالثة «تدفق الطاقة في عمليتي البناء الضوئي والتنفس الخلوي» (افتتاحية) والدرس الأول «البناء الضوئي» (١): فان هلمونت والبلاستيدة الخضراء والكلوروفيل والضوء والتفاعلات الضوئية · 3 دروس
1
Van Helmont’s willow: where does a tree’s mass come from?
Science Prep 2 Term 1, Unit 3: pp. 64–69
GoalUse Van Helmont’s experiment to show that a tree does not get its mass from the soil, and say what a plant really needs.
New words
photosynthesis · the process in which green plants use light, water and carbon dioxide to make fooddry mass · the mass of a plant with the water taken outhypothesis · an idea we can test
Explanation
About 400 years ago, a scientist from what is now Belgium, Jan Baptist van Helmont, asked a question: where does a growing tree get its mass? He planted a willow of 2.2 kg in 90 kg of dry soil, and watered it with rain water for five years. At the end the tree weighed 75 kg, but the soil had lost only about 55 g. So a tree does not get its mass from soil: plants do not “eat soil”. Van Helmont concluded that the mass came from the water. Later work showed that this was only part of the story: carbon dioxide from the air and light energy from the Sun are also needed, and most of the dry mass of a plant is carbon that came from carbon dioxide. The fresh mass of a tree also includes a lot of water. This is a good example of science: an experiment gives a result, a conclusion is drawn, and later work improves the conclusion. Weighing carefully was the key skill here.
Start
After five years
willow tree
2.2 kg
75 kg
dry soil
90 kg
about 55 g less
Worked examples
Van Helmont’s tree gained about 72.8 kg (75 − 2.2). The soil lost only 0.055 kg. 72.8 is far greater than 0.055, so the soil cannot be the main source of the tree’s mass.
A bean seed grows into a plant in a cup of damp cotton wool, with no soil at all. The plant gets bigger. Soil is not the thing that makes it grow; water, air and light matter.
Why might a scientist repeat Van Helmont’s experiment with a covered pot? A cover keeps dust from falling into the soil, so the mass of the soil is measured fairly. It is a way to control a variable.
Class activity
Light or dark? With a grown-up, sow bean or lentil seeds in two cups of damp cotton wool. Put one cup in a bright place and one in a dark cupboard. After several days describe the colour and size of the seedlings in each. Do not predict before you look. Write the independent, dependent and controlled variables.
Practice
1
What did Van Helmont measure at the start and the end?
The mass of the tree and the mass of the dry soil.
2
The tree gained about 72.8 kg and the soil lost 55 g. What does this show?
The tree’s mass did not come mainly from the soil.
3
What did later work add to Van Helmont’s conclusion?
Carbon dioxide from the air and light are needed too; most of the dry mass is carbon from carbon dioxide.
4
Do plants “eat soil”? Explain.
No. They make their own food from carbon dioxide and water using light.
