TL;DR:
Water evaporates from leaves through stomata (transpiration). This pulls a continuous column of water up the xylem from the roots.
Water molecules stick to each other (cohesion) and to the xylem walls (adhesion), so the column does not break.
Hot, dry, windy and bright conditions raise the transpiration rate. A potometer measures it.
A tall tree lifts water many metres from its roots to its highest leaf, and it has no heart. No pump pushes the water. Instead, the leaves pull it. The pull comes from something that looks like waste: water evaporating from the leaf surface.
The idea in plain words
Transpiration is the loss of water vapour from a plant, mostly through tiny pores called stomata on the underside of leaves. Each stoma sits between two guard cells that open and close it. Inside the leaf, water evaporates from damp cell surfaces into air spaces, then diffuses out through the open stomata.
Xylem is the tissue that carries water and dissolved minerals upward. It is made of long dead cells joined end to end into hollow tubes. Because the cells are dead, they cannot pump anything. The water moves because of physical forces.
Why the column holds together
Water molecules are cohesive: they attract one another through hydrogen bonds. They are also adhesive: they stick to the walls of the narrow xylem tubes. As water leaves the leaf, the water behind it is pulled along, like a chain, because the molecules hold on to each other. Adhesion helps stop the column from slipping back down.
The result is a continuous stream from soil through root, stem and leaf to the air. This is the cohesion-tension idea: evaporation creates tension (a pull), and cohesion transmits it down the column. Roots take in water from the soil to replace what is lost, so the stream keeps going.
Worked example: transpiration on a hot day
In a lab you can measure the rate with a potometer: a cut shoot attached to a thin tube with a bubble that moves as the shoot takes up water. The numbers below are made up for practice. The tube has an inside radius of 0.5 mm. In 10 minutes in the shade, the bubble moves 30 mm. In 10 minutes in hot sun, it moves 60 mm.
Volume of water taken up equals the tube's cross-section area times the distance the bubble moved:
π r² × distance.Shade:
π × 0.5² × 30 = π × 0.25 × 30 ≈ 23.6 mm³. Per minute:23.6 ÷ 10 ≈ 2.36 mm³/min.Hot sun:
π × 0.25 × 60 ≈ 47.1 mm³. Per minute:47.1 ÷ 10 ≈ 4.71 mm³/min.Compare:
4.71 ÷ 2.36 = 2, so the shoot takes up twice as much water in the sun.
Why? Heat speeds up evaporation, and light opens the stomata. A steeper gradient between the wet leaf interior and the drier air pulls water out faster. Wind makes the gradient steeper by sweeping away the damp air next to the leaf. Humid air does the opposite, because it is closer to the leaf's own moisture.
Condition | Effect on transpiration | Reason |
|---|---|---|
Higher temperature | Increases | Faster evaporation and diffusion |
Brighter light | Increases | Stomata open for photosynthesis |
More wind | Increases | Removes humid air from the leaf surface |
Higher humidity | Decreases | Smaller gradient between leaf and air |
What the potometer does and does not measure
A potometer measures water taken up, which is close to, but not exactly, the water lost. A little water is used in photosynthesis or stays in the plant. For school work, uptake is a good stand-in for transpiration, as long as the shoot is cut under water so no air bubble blocks the xylem and the joints are sealed.
To compare conditions fairly, change one thing at a time. If you move the shoot into the sun and also turn on a fan, you cannot tell which factor caused the change. Use the same shoot, the same tube and the same time interval, and repeat the reading to see how consistent it is.
The trade-off of open stomata
Stomata let CO₂ in for photosynthesis, but they also let water out. On a hot, dry day a plant that loses water faster than its roots can supply it will begin to wilt. Guard cells respond by closing the stomata, which saves water but also stops CO₂ from entering. The plant balances the two needs all the time.
How to check your answer
Units:
mm² × mm = mm³, and dividing by minutes givesmm³/min.Check the radius, not the diameter. If the problem gives a diameter of 1.0 mm, the radius is 0.5 mm.
Compare rates, not just distances, if the times differ.
Doubling the distance in the same time and the same tube must double the volume.
Common mistakes
Saying roots pump water up the plant. Root uptake is part of the story, but the main force is the pull from the leaves.
Mixing up xylem and phloem. Xylem carries water up. Phloem carries sugars around the plant.
Saying transpiration happens only in light. Stomata are mostly closed at night, but some loss continues.
Forgetting that water is also a reactant in photosynthesis, though the amount is tiny compared with what is lost.
Where you will use it next
The same ideas explain wilting, why irrigation timing matters, and how adaptations such as thick waxy leaves help desert plants. Plants also keep an internal balance, as animals do, which you can compare in homeostasis and feedback loops.
Frequently asked questions
Does a plant need transpiration?
It is largely a side effect of having open stomata for gas exchange, but the flow it creates also carries minerals up and helps cool the leaf.
Why can water travel up very tall trees?
Cohesion and adhesion keep the water column unbroken while evaporation pulls from the top.
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