Transpiration and stomata

Follow one water molecule

Step through the journey from root to air.

1. 1Root hair cellWater enters the plant at the roots, by osmosis into the root hair cells.
2. 2XylemThe water is pulled up through xylem tubes. It has mineral ions dissolved in it, including nitrate ions.+
3. 3Inside the leafIn the leaf, water evaporates out of the cells in the central part and forms water vapour in the air spaces.
4. 4Open stomataThe water vapour diffuses out through the open stomata into the atmosphere. That loss of water from the leaves is transpiration.+

Predict, then check

Think about loss from the leaves against uptake by the roots.

On a hot, sunny day a plant's leaves lose water faster than its roots can absorb it. What happens?

What you need to know

• Leaves have tiny pores called stomata (one stoma, many stomata). Water vapour, oxygen and carbon dioxide can diffuse through them.
• Each stoma is surrounded by two guard cells. They change shape to open or close the pore, so they control its size.
• When it is light and sunny the stomata open. Carbon dioxide diffuses in, waste oxygen diffuses out, and photosynthesis can take place.
• Have a goThink of the cost. A leaf opens its stomata to take in carbon dioxide. What else can slip out through those same open pores?

  Water vapour (and waste oxygen).

  Stomata are open pores that water vapour, oxygen and carbon dioxide can all diffuse through, so letting carbon dioxide in also lets water vapour out.

• Transpiration is the loss of water from a plant's leaves. Water evaporates into air spaces in the leaf, then diffuses out through open stomata.
• Most water is lost through the open stomata, because the waxy cuticle greatly reduces evaporation from the general surface of the leaf.
• Have a goYour friend Jas is sure that water just seeps out through the whole surface of a leaf, like a wet sponge. Using what you just read, what would you tell Jas?

  The waxy cuticle greatly reduces evaporation from the general surface, so most water goes out through the open stomata.

  Seeping everywhere sounds sensible for a wet leaf, but the cuticle seals most of the surface, so open stomata are where most of the water is lost.

• Water enters at the roots by osmosis into root hair cells, then is pulled up the xylem. That continuous flow is the transpiration stream.
• If a plant loses water faster than the roots absorb it, water may leave the cells by osmosis. They go flaccid and the plant wilts.
• Many leaves have fewer stomata on the top surface. Sunlight warms it, warmth increases transpiration, so fewer stomata there reduces water loss.
• To see stomata, make a nail-varnish imprint: paint, let it dry, peel with tape, stick it on a slide, view with a light microscope.
• Stomatal density = stomata counted in the field of view ÷ its area, where area = π × radius². The unit is stomata per mm².
• Have a goYou count 12 stomata in a field of view that has an area of 0.5 mm². What do you do with 12 and 0.5, and what is the unit of the answer?

  12 ÷ 0.5 = 24 stomata per mm².

  Density is the count divided by the area. Dividing 0.5 by 12 would turn it upside down.

The big picture

Stomata are pores in the leaf, opened and closed by guard cells, that let gases and water vapour through. Water lost from the leaves is transpiration, and replacing it pulls water up from the roots in the transpiration stream.

⚠ Watch out

Dividing by the radius (or the diameter) instead of by the area when estimating stomatal density. The count is divided by the whole area of the field of view, π × radius².

Memory hook

Open the doors to breathe in, and the water slips out of the same doors.

Check yourself

Shut the page and follow one water molecule from the root to the air, naming each place in order. Why can't it take a shortcut through the leaf surface?