How plants make their own food

Ambika Nag

Everybody loves stories, especially children. A science classroom may become interesting for students if the teacher tells them stories about discoveries, about scientists and how gradually our understanding of a concept evolved in the past. During my work with teachers, I realized that they hardly know about historical accounts of discoveries of process and the periodical changes in definitions.

We all know that we need green plants to survive. We show our concern towards the environment. Plants are venerated in all societies across the globe and there are many proverbs that establish their significance and benefits. We find people campaigning to plant more trees. Let’s explore the reasons that they give:

We know that plants are important for us as a source of food, fibre, timber, medicine, oils, rubber, gum, etc., but what is more important is that they are responsible for the production of free oxygen in the atmosphere creating the conditions that allow for more complex forms of life to evolve. Plants check global warming by using carbon dioxide during the process of food-making and oxygen is a waste product of this process.

So the three important benefits of growing plants are – they give out oxygen, they reduce carbon dioxide levels in the atmosphere and they are the primary producers of food. And what makes this possible is the process of food-making called “photosynthesis” that is so central for the survival of not only plants but other organisms on earth as well.

We eat food to survive and grow, but where do the plants get their food from? We know that plants do not grow unless you plant them in soil and water them. It seems, therefore, that either soil or water (or both) have to turn into plant material for plants to grow.

In 1643, a Belgian scientist, Jan Baptista van Hel-mont (1577-1644), thought he would decide the matter by experiment.

This is an extract from van Helmont’s diary…

“I took an earthenware pot in which I put 200 pounds of earth that had dried in a furnace.

I moistened it with rain water and implanted in it a trunk of a willow tree weighing 5 pounds. I planted it in the garden and covered the earth with an iron lid punched with many holes to allow rain water in.

At length, after 5 years, the tree did weigh 169 pounds and 3 ounces. I again dried the earth in the vessel and found it weighed almost 200 pounds (less about 2 ounces). Therefore 164 pounds of wood, bark and roots arose out of water only.”

Let’s ponder:

  • What was the change in the mass of the tree?
  • What was the change in the mass of the soil? Could this have contributed to the growth of the plant? How much?
  • Can plants grow without soil?
  • What did van Helmont conclude from his experiment?
  • Do you agree with his conclusion?
  • What other explanations can there be for the results he found?

If you agree with van Helmont that water alone accounted for the growth of the willow tree, then can you survive on water alone? Did Helmont measure how much water was added to the pot over the five years? Helmont didn’t take air into account. Hardly anyone did in those days. You can’t see or feel air, so people usually ignore it. A British scientist, Stephen Hales (1677-1761), studied gases in detail, and, in 1727, he wondered if a gas might be involved in plant growth.

A Dutch scientist, Jan Ingenhousz (1730-1799), kept studying the way in which plants formed oxygen and, in 1779, noticed that this only happened when there was light. Plants did not produce oxygen in the dark. (Asimov, 1989)

Let’s ponder:
• Does sunlight have mass?
• Living things are made of atoms. Are there any atoms in sunlight?
• Can sunlight contribute to the increase in the mass of the plant?
• Is sunlight needed for plants to grow? What role do you think it might have?

Sunlight contains energy and this energy makes it possible for plants to grow and manufacture within themselves the complicated substances that animals use as food.

The author has done her doctorate in Botany from Mohanlal Sukhadia University, Udaipur. She has over 20 years’ experience of teaching Botany and Biotechnology. She is currently a Resource Person at Azim Premji Foundation and is based at Jaipur. She can be reached at ambika.nag@azimpremjifoundation.org.

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