Learning target: I can explain, with evidence from a passage, how a series of experiments over 160 years showed that a plant's matter comes mostly from air and water and that light is required.
Read the passage once straight through. Read it again with a pencil, numbering the paragraphs 1 to 7 and circling every number and every scientist's name. Answer in complete sentences. When a question asks for evidence, quote or closely paraphrase the passage and give the paragraph number.
You will need: This worksheet, Pencil, Scrap paper for the evidence ladder
[1]A tree can have a mass of many tons, yet it begins as a seed you can hold in your palm. Where does all that new material come from? For centuries the obvious answer was the soil. The first person to test the obvious answer with a balance was a Flemish physician, Jan Baptist van Helmont. His experiment was published in 1648, a few years after his death.
[2]Van Helmont dried a batch of soil in an oven, weighed out 200 pounds of it, and planted a willow sapling with a mass of 5 pounds. He covered the pot with a pierced metal plate so that dust could not fall in, and for five years he added nothing but rainwater. After five years he weighed everything again. The willow had a mass of 169 pounds and 3 ounces, about 76 kilograms. The soil, dried again, had lost only about 2 ounces, roughly 57 grams. Van Helmont concluded that the wood had been made from water alone. He was half right: he had ruled out the soil, but he had not thought to weigh the air.
[3]The air entered the story in 1772, when an English chemist, Joseph Priestley, reported that a candle sealed in a glass jar soon went out, and a mouse sealed in a jar could not survive. But if a sprig of mint grew in the jar for several days, the candle would burn again and a mouse could breathe. The plant had somehow 'restored' the spoiled air. Priestley did not know what the plant was adding; today we call that gas oxygen. His was the first evidence that plants change the air around them, not just the soil beneath them.
[4]Priestley's results were hard to repeat, and a Dutch physician, Jan Ingenhousz, wanted to know why. In 1779 he published hundreds of trials. Green leaves held under water in bright sunlight released a stream of tiny bubbles, and the collected gas made a glowing splint burst into flame. The same leaves in the shade or in the dark released almost nothing. Roots, flowers and the non-green parts of plants never produced the bubbles. Ingenhousz had added two conditions to Priestley's discovery: the gas came only from green parts and only in light. Priestley's mint had sometimes failed because his jars had sometimes sat in the shade.
[5]That left van Helmont's question open. The willow had gained 164 pounds, and water could not be the whole answer if plants were also taking something from the air. The Swiss chemist Nicolas-Théodore de Saussure took up the problem in 1804. He grew plants in sealed containers holding known amounts of carbon dioxide and measured what disappeared. Plants supplied with carbon dioxide grew; plants in air with the carbon dioxide removed barely grew at all. More telling, the plants gained more mass than the mass of the carbon dioxide they took in. The extra mass, he argued, had to come from water joining the carbon dioxide inside the plant.
[6]Put the four experiments together and a picture appears. A plant's new matter comes mostly from carbon dioxide in the air, with water contributing a smaller share and the soil only a few percent of minerals. Making that matter requires light, and the process gives off oxygen. Modern measurements agree: dry a plant completely and about 45 percent of its mass is carbon and about 45 percent is oxygen, both traceable to carbon dioxide; about 6 percent is hydrogen from water; and only around 4 percent is minerals from the soil.
[7]No single experiment settled the question. Van Helmont ruled out the soil but drew the wrong conclusion. Priestley found that plants change the air but could not explain his own failures. Ingenhousz fixed the conditions. Saussure weighed the gas. Each scientist kept what held up in the earlier work and corrected what did not. That is how a scientific explanation is built: not in one stroke, but by measurement after measurement, each one narrowing the places the answer could hide.
Before writing, draw four rungs on scrap paper labeled 1648, 1772, 1779 and 1804. On each rung write the scientist's name, what he measured or observed in five words or fewer, and what he concluded in five words or fewer. Most questions can be answered by finding the right rung and rereading that paragraph. For q8, draw arrows between rungs and label each arrow 'kept' or 'corrected'.
Van Helmont never weighed the air, and it is easy to see why: how do you put air on a balance? Design a modern version of his experiment that keeps track of the gas as well as the soil and water. Describe the container, what you would measure at the start and the end, and how you would tell carbon dioxide taken in from water taken in. Then explain one way your design could still fool you, and how you would guard against it.