Transpiration is the evaporation of water from the leaves (and other aerial parts) of a plant.
Water is absorbed by roots → travels up the stem through xylem → reaches the leaves → evaporates through stomata as water vapour into the atmosphere.
This continuous movement of water from roots to leaves through the xylem is called the TRANSPIRATION STREAM.
How it works: as water evaporates from leaf cells into the air spaces and out through stomata, the cells become slightly drier → they absorb water from neighbouring cells by osmosis → this creates a pulling force that draws water up through the xylem all the way from the roots.
This pull is called TRANSPIRATION PULL — it works because water molecules are attracted to each other (cohesion) and to the xylem walls (adhesion), forming a continuous column of water.
Why Transpiration Matters
Transpiration serves several important functions:
WATER SUPPLY — transports water from roots to leaves where it is needed for photosynthesis.
MINERAL TRANSPORT — mineral ions dissolved in water are carried up from the roots to the leaves.
COOLING — evaporation of water from leaf surfaces has a cooling effect — similar to sweating in humans.
However, too much transpiration is a problem — excessive water loss can cause wilting and ultimately death. Plants have several adaptations to reduce water loss (waxy cuticle, closing stomata, reduced leaf surface area in dry environments).
Factors That Increase the Rate of Transpiration
Four main factors affect how fast water evaporates from leaves:
1. TEMPERATURE:
Higher temperature → more kinetic energy → water molecules evaporate faster from leaf surface → steeper concentration gradient between leaf and air → faster transpiration.
2. LIGHT INTENSITY:
Brighter light → stomata open WIDER to allow more CO₂ in for photosynthesis → more water vapour can escape → faster transpiration.
In the dark, stomata close → transpiration almost stops.
3. HUMIDITY:
Low humidity (dry air) → large difference in water vapour concentration between inside the leaf and outside → steep diffusion gradient → faster transpiration.
High humidity (moist air) → gradient is smaller → slower transpiration.
4. AIR MOVEMENT / WIND:
Wind blows away water vapour that has accumulated near the stomata → prevents the air near the leaf becoming saturated → maintains a steep diffusion gradient → faster transpiration.
In still air, a layer of moist air builds up around the leaf → reduces the gradient → slows transpiration.
Measuring Transpiration — The Potometer
A POTOMETER is an apparatus used to measure the rate of water uptake by a plant shoot — used as an indicator of transpiration rate.
How it works:
A leafy shoot is placed in a sealed tube of water with a capillary tube attached.
As the plant transpires and takes up water, an air bubble in the capillary tube moves towards the plant.
The DISTANCE the bubble moves per unit time = rate of water uptake.
The potometer measures water UPTAKE, not transpiration directly — but the two are closely related.
You can investigate the effect of each environmental factor by:
Changing temperature (place near heater or in cold room).
Changing humidity (blow dry air at the plant or enclose in moist environment).
Changing air movement (use a fan).
⚠️ Common Mistake
Transpiration is fastest in HOT, BRIGHT, DRY and WINDY conditions. Students often get humidity wrong — LOW humidity means DRY air which INCREASES transpiration rate (bigger concentration gradient for water vapour). HIGH humidity SLOWS transpiration. Think of it like this: dry air 'pulls' the water out of the leaf faster.
📌 Key Note
Transpiration: water evaporates from stomata → transpiration pull draws water up xylem from roots. Fastest in: high temperature, high light intensity, low humidity, high wind. Measured with a potometer.
🎯 Matching Activity — Does this Speed Up or Slow Down Transpiration?
Sort each condition — does it increase or decrease the rate of transpiration? — drag the symbols on the right to match the component names on the left.
Increases transpiration
Drop here
Increases transpiration
Drop here
Increases transpiration
Drop here
Increases transpiration
Drop here
Decreases transpiration
Drop here
Decreases transpiration
Drop here
Bright light — stomata open wider, more water vapour escapes
High humidity — gradient between leaf and air is reduced
Darkness — stomata close, water vapour cannot escape
Wind — blows away moist air near stomata, maintains steep gradient
Low humidity (dry air) — steep concentration gradient for water vapour
High temperature — more kinetic energy, faster evaporation
⭐ Higher Tier Only
Investigating transpiration: use a potometer to measure water uptake (indicative of transpiration). Vary one factor (temperature, humidity, light intensity, wind speed) while controlling others. Record distance air bubble moves per unit time. Calculate rate and plot a graph. Students should be able to design and evaluate such an investigation.
🎯 Test Yourself
Question 1 of 3
1. What is transpiration?
2. A plant is moved from a humid greenhouse to a dry, windy environment. What happens to its transpiration rate?
3. Why do plants transpire faster in brighter light?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
Don't get itGetting thereNailed it!
🤖 Ask Mr Badmus AI
Stuck? Just ask! 💬
I'll use FIFA for calculations and flag Higher/Triple content clearly.