The Basil That Stood Back Up
Maya's basil plant droops over a hot weekend. An hour after she waters it, every stem stands straight again.
What holds a soft green plant up, when it has no skeleton?
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Science · Grade 7 Science
Chapter 1: Cells: The Unit of Life
The differences follow from the way of life.
Maya's basil plant droops over a hot weekend. An hour after she waters it, every stem stands straight again.
What holds a soft green plant up, when it has no skeleton?
Make your guess to turn the page.
Still to come: 10 pages · Words to Know · What We Learned · 4 questions · Explain It in Writing · Ask and Wonder
The idea this lesson is working on — Children reason that plants are green, so every plant cell must be green. The book first shows that plant cells carry what a plant’s way of life needs (a wall and vacuole for standing, chloroplasts for food), then shows onion and root cells with none. Ask: what would a chloroplast in a root cell have to do? The idea to reach is that a part is present where its job is.
Turgor pressure — The pressure of a plant cell’s contents against its wall is called turgor pressure; a firm cell is turgid. Water moves in or out by osmosis, toward the saltier side, which is why salty water drew water out of the celery and why salted lettuce goes limp. Woody stems are also held up by lignin in their walls, but soft green stems depend on turgor.
About the numbers — A limp celery stick soaked in water for an hour can gain 10–20% of its mass, and salty water makes it lose mass; the book’s averages are a realistic illustration, not a published study. Red blood cells in pure water swell and burst within seconds, which is why hospital drips are made with salt at the same strength as blood.