A sunflower can lose more than a litre of water on a hot day, almost all of it as water vapour from its leaves. This loss is transpiration. It depends on the weather, it can be measured with a potometer, and (Extended) it is the engine that pulls water up the xylem. When the loss outstrips the supply, the plant wilts.
Transpiration is the loss of water vapour from leaves. Water evaporates from the surfaces of the mesophyll cells into the air spaces and then diffuses out of the leaves through the stomata as water vapour. Higher temperature and higher wind speed increase the rate. Extended: the loss of water pulls a continuous column of water molecules up the xylem (the transpiration pull), because water molecules are attracted to each other; high humidity slows transpiration; and when water is lost faster than it is taken up, cells become flaccid and the plant wilts.
Fig. 1: water evaporates from the wet surfaces of mesophyll cells into the air spaces, then diffuses out of the leaf through an open stoma as water vapour
Section 01
Evaporation, then diffusion.
The sim shows a block of leaf: palisade and spongy mesophyll cells with air spaces between them and stomata in the lower surface. Watch the water vapour molecules, then change the temperature, wind and humidity and read the Water vapour lost counter. Untick Stomata open to see what happens in the dark.
Step 1, evaporation: the mesophyll cells have a thin film of water on their surfaces. Water evaporates from these surfaces into the air spaces, so the air inside the leaf is almost saturated with water vapour.
Step 2, diffusion: the air outside usually holds less water vapour, so water vapour diffuses out of the leaf through the open stomata, down its concentration gradient.
Stomata closed: in the dark (or when the plant is short of water) the guard cells close the stomata, so transpiration almost stops.
Extended.Surface and stomata: the interconnecting air spaces give a large internal surface area of wet mesophyll cell walls for evaporation. More stomata, and larger (wider open) stomata, give more and wider exits for the vapour, so the rate is higher.
Factor increased
Effect on rate
Explanation (Extended)
temperature
increases
water molecules have more kinetic energy, so they evaporate and diffuse faster
wind speed
increases
moving air carries water vapour away from around the leaf, keeping the concentration gradient steep
humidity
decreases
the air outside already holds more water vapour, so the concentration gradient is shallower
Examiner trap
Transpiration is the loss of water vapour, and the vapour leaves by diffusion through the stomata. Liquid water does not flow out of the stomata, and the stomata are not where the water evaporates: it evaporates inside the leaf from the mesophyll cell surfaces.
Section 02
Measuring it: the potometer.
A potometer measures how fast a leafy shoot takes up water. Almost all the water taken up is lost by transpiration, so the uptake rate is a good estimate of the transpiration rate. In the sim, set a temperature and fan speed, press Time the bubble over 50 mm and read the results table.
Setting up: cut the shoot under water and fit it into the potometer under water, so no air enters the xylem. Seal the joints with petroleum jelly so the apparatus is airtight.
Taking a reading: let one air bubble into the capillary tube. As the shoot draws up water the bubble moves along the scale. Time how long it takes to move a fixed distance (here 50 mm).
Calculating: rate = distance ÷ time. For 50 mm in 375 s, rate = 50 ÷ 375 × 60 = 8.0 mm per minute.
Resetting: open the reservoir tap to push the bubble back to the start before each new reading.
Fair test: change one factor at a time (temperature, or fan speed for wind), keep the same shoot, the same light intensity and the same distance, and repeat each reading to calculate a mean.
Examiner trap
A potometer measures water uptake, not water loss. A small amount of the water taken up is used in the plant, for example in photosynthesis, so uptake is only approximately equal to transpiration.
Section 03
The transpiration pull, and wilting.
Extended. Why does water rise tens of metres up a tree with no pump? In the sim, watch the chain of water molecules in the xylem vessel, then cut the stem, let the soil dry out and water the plant again.
Transpiration pull: as water evaporates from the mesophyll cells and leaves the leaf, more water moves out of the xylem in the leaf to replace it. This creates a pull on the water in the xylem.
A continuous column: water molecules are held together by forces of attraction between them, so the whole column of water in the xylem is drawn up the stem from the roots, like a chain.
Breaking the column: if air gets into the xylem (for example when a stem is cut in air), the column breaks and water above the break is no longer pulled up. Cut flowers are re-cut under water for this reason.
Wilting: when water is lost by transpiration faster than the roots can take it up (hot, dry or windy weather, or dry soil), the cells lose water. They become flaccid: they no longer press outwards on their cell walls, so they cannot support the stem and leaves, and the plant droops.
Why it helps: wilting leaves fold and droop, exposing less surface to the sun, and the stomata close, which reduces further water loss. Watering restores turgor and the plant recovers.
Examiner trap
The transpiration pull is a pull from the top, caused by water loss from the leaves. Do not describe it as the roots pushing water up, and do not say the molecules are "sucked" up: the key idea is the attraction between water molecules holding the column together.
Stage 1 · Learn
Check what the sim just showed you
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In the transpiration sim, what happens to the water vapour lost when you untick Stomata open?
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Set the humidity slider in the transpiration sim to 75% or more. Why does the explanation say transpiration slows?
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In the potometer sim, you raise the temperature from 20 °C to 30 °C. What happens to the bubble?
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In the transpiration pull sim, what happens to the water column when the stem is cut?
Stage 2 · Exam
Exam-style questions
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Which statement describes transpiration?
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A potometer bubble moves 36 mm in 3 minutes. What is the rate of water uptake?
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Extended: how does wind increase the rate of transpiration?
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Extended: what holds the column of water together as it is pulled up the xylem?
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Transpiration, mastered.
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