Lesson 1
Tiny doors on every leaf
If you looked at a leaf under a microscope, you'd see it's covered in tiny holes. Each hole is called a stoma (STOH-muh), which means "mouth" in Greek. More than one are called stomata.
- Every stoma has two guard cells around it, shaped like beans or sausages.
- Most stomata are on the underside of leaves, where it's shadier and cooler.
- Guard cells have chloroplasts, even though most of the leaf's outer skin cells don't.
Wow!
A single leaf can have thousands of stomata in a space as small as your fingernail!
Lesson 2
How the doors open and close
Guard cells work like two balloons:
- When guard cells fill up with water, they swell and bend outward. The hole between them opens.
- When they lose water, they go limp and press together. The hole closes.
Stomata usually open in sunlight, so carbon dioxide can come in for making food (photosynthesis), and oxygen can go out. They close at night, or when the plant is thirsty, to save water.
Try this
Scroll up to the 3D model. Press Close the door, then Show air moving.
Lesson 3
Do plants sweat?
Kind of! When stomata are open, water vapor escapes from the leaf into the air. This is called transpiration (tran-spuh-RAY-shun).
As water leaves through the stomata, it pulls more water up from the roots through the plant's pipes, like sipping through a straw. Learn more on the water pipe cells page!
Try this: catch plant water
With a grown-up, put a clear plastic bag over a few leaves of a plant outside on a sunny day, and tie it gently. After a few hours, look inside. See the water drops? That water came out of the stomata!
Wow!
A big tree can let out hundreds of liters of water into the air on a hot day!