Transpiration in Plants | O-Level Biology

A tall plant has no heart-like pump pushing water from its roots to its leaves. Yet water can travel from the soil, through the roots and stem, and all the way to leaves high above the ground. The key is that water is constantly being lost from the leaf surface.

That loss may seem wasteful, but it creates the conditions for a continuous movement of water through the plant. To understand transpiration, it helps to follow one water molecule step by step: from the soil into a root hair cell, into the xylem, towards a leaf, onto a moist mesophyll cell surface, into a leaf air space and finally out through a stoma as water vapour.

Diagnostic Quiz

What Do You Already Know About Transpiration?

Try these five questions before reading the chapter. They test the route of water through a plant, the meaning of transpiration and the main environmental factors that affect it.

5 diagnostic questions
Core ideas before you begin
Instant explanation for every answer option

Question 1 of 5

Which statement best defines transpiration?

The loss of liquid water from the roots of a plant

Water uptake occurs at the roots; transpiration refers to water loss from the aerial parts of the plant.

The loss of water vapour from the aerial parts of a plant, mainly through the stomata of the leaves

Transpiration is the loss of water vapour from aerial parts, especially leaves, with most water leaving through stomata.

The movement of mineral ions from the soil into root hair cells

Mineral ion uptake is a separate process and may involve active transport.

The movement of sugars through the phloem

Movement of sugars in phloem is translocation, not transpiration.

A precise definition should include loss of water vapour, aerial parts of the plant and, in most leaves, loss mainly through the stomata.

What happens to water just before it leaves a leaf through a stoma?

It evaporates from moist mesophyll cell surfaces into the leaf air spaces

Water changes from liquid to vapour at moist mesophyll surfaces before the vapour diffuses out through the stomata.

It is pumped from the xylem directly into the atmosphere

Xylem carries liquid water. Water first moves to mesophyll cells, evaporates and then diffuses out as vapour.

It is converted into glucose by respiration

Respiration does not convert water into glucose.

It moves through the phloem and then becomes vapour

Phloem transports assimilates such as sucrose; the transpiration stream is associated mainly with xylem.

Inside the leaf, liquid water reaches mesophyll cells, evaporates from their moist surfaces into the air spaces, and the water vapour then diffuses out through the stomata.

When the soil has a higher water potential than a root hair cell, how does water enter the root hair cell?

By osmosis

Water moves through the partially permeable cell surface membrane from higher water potential to lower water potential.

By active transport

Active transport uses energy to move substances against a concentration gradient; it is not the usual mechanism for water entry into root hair cells.

By translocation

Translocation is the movement of assimilates in phloem.

By evaporation

Evaporation is a change from liquid water to water vapour.

Water enters root hair cells by osmosis, moving from a region of higher water potential in the soil to a region of lower water potential in the cell sap through a partially permeable membrane.

Why does moving air usually increase the rate of transpiration?

It removes water vapour from around the leaf, maintaining a steep water vapour concentration gradient

Moving air carries away moist air near the stomata, so water vapour can continue diffusing out rapidly.

It forces liquid water through the stomata

Water normally leaves stomata as vapour, not as liquid forced out by wind.

It closes the stomata in every situation

Wind does not automatically close stomata; moderate air movement usually increases transpiration by removing humid air.

It increases the concentration of water vapour outside the leaf

Moving air generally removes accumulated water vapour rather than increasing it.

Wind speeds transpiration mainly by removing the humid boundary layer next to the leaf and maintaining a steep diffusion gradient for water vapour.

How does high humidity usually affect transpiration?

It increases transpiration because humid air contains more water

High humidity reduces the difference in water vapour concentration between the leaf and the surrounding air.

It decreases transpiration because the water vapour concentration gradient from leaf to air is smaller

A smaller gradient means slower net diffusion of water vapour out of the leaf.

It has no effect because transpiration depends only on temperature

Humidity is one of the important environmental factors affecting transpiration.

It stops water entering the roots by osmosis in every plant

Humidity mainly affects water loss from leaves; it does not automatically stop root water uptake.

High humidity usually reduces transpiration because the surrounding air already contains more water vapour, so the gradient from the moist leaf air spaces to the atmosphere is smaller.

What is transpiration?

Transpiration is the loss of water vapour from the aerial parts of a plant, especially from the leaves. In most leaves, the greatest loss occurs through the stomata.

Water does not usually move straight from the xylem vessel through a stoma as liquid water. Instead, two different processes occur inside the leaf:

Step 1 — evaporation: liquid water evaporates from the moist surfaces of mesophyll cells into the leaf air spaces.

Step 2 — diffusion: water vapour diffuses from the humid air spaces inside the leaf through the stomata to the drier air outside.

This distinction is important. Evaporation is the change of liquid water into water vapour. Diffusion is the net movement of that water vapour down its concentration gradient.

Why leaves lose water when stomata open

Stomata are pores in the epidermis of a leaf. They allow gases to move between the leaf and the atmosphere. Carbon dioxide must enter the leaf for photosynthesis, while oxygen can move in or out depending on the conditions.

The same open pore also provides a route for water vapour to escape. The air spaces inside a healthy leaf are usually very humid because they are next to moist mesophyll surfaces. If the surrounding air is less humid, there is a water vapour concentration gradient from inside the leaf to the atmosphere. Water vapour therefore diffuses out.

Inside the leaf: air spaces are moist and contain a relatively high concentration of water vapour.

Outside the leaf: the air usually contains a lower concentration of water vapour.

Result: water vapour diffuses out through open stomata.

How water reaches the leaf

Water lost from a leaf has to be replaced. The replacement water ultimately comes from the soil. The pathway begins at the root.

1. Water enters root hair cells by osmosis

Root hair cells have long extensions that provide a large surface area for absorption. When the water potential of the soil is higher than the water potential of the root hair cell sap, water moves into the root hair cell by osmosis through the partially permeable cell surface membrane.

A useful exam phrase is: water moves from the soil, where water potential is higher, into the root hair cell, where water potential is lower, by osmosis through a partially permeable membrane.

2. Water moves across the root into the xylem

From the root hair cells, water passes through the root cortex towards the xylem. Water moves from regions of higher water potential to regions of lower water potential. Once it reaches the xylem vessels, it becomes part of the transpiration stream.

3. Xylem carries water towards the leaves

Xylem vessels form long, continuous tubes through the roots, stems and leaves. They transport water and dissolved mineral ions. Because mature xylem vessel elements are dead and hollow, water can move through their lumen with little obstruction.

Water in the xylem is not being pumped upward by living xylem cells. The major driving force is linked to water loss from the leaves.

From xylem to mesophyll to atmosphere

When water evaporates from mesophyll cell surfaces, those cells lose water and their water potential falls. Water then moves towards them from neighbouring cells and from the xylem. This replaces the water that has evaporated.

Pathway of transpiration in a leaf showing liquid water moving from the xylem to mesophyll cells, evaporation from moist mesophyll cell surfaces, water vapour entering the leaf air spaces and diffusion through a stoma into the atmosphere. — Pathway of transpiration in a leaf. Blue arrows show the movement of liquid water through the xylem and mesophyll; red dashed arrows show the movement of water vapour through the leaf air spaces and out through a stoma.

The diagram shows the leaf stage of transpiration. The complete pathway from the soil to the atmosphere is:

Soil → root hair cells → root cortex → xylem → leaf mesophyll → leaf air spaces → stomata → atmosphere

At different parts of this route, different processes are involved. Water enters root hair cells by osmosis. Water is transported through the plant in the xylem. Water evaporates from mesophyll cell surfaces. Water vapour then diffuses through stomata into the surrounding air.

What is transpiration pull?

The loss of water from the leaf helps create an upward pull on water in the xylem. This is called transpiration pull.

When water evaporates from mesophyll cells, replacement water is drawn from the xylem. This creates tension in the water column inside the xylem. Water molecules attract one another, so the column of water tends to remain continuous. As water is pulled towards the leaves, more water is drawn upward from the stem and roots.

Water loss at the leaf does not merely remove water. It helps maintain a chain of water movement from the soil, through the xylem, to the leaf.

This continuous movement of water from roots to leaves is the transpiration stream.

Why transpiration is useful to a plant

Although excessive water loss can be harmful, the transpiration stream has several important consequences for the plant.

Effect	Why it matters
Moves water through the plant	Water absorbed by the roots is carried through the xylem to the leaves and other aerial tissues.
Transports mineral ions	Mineral ions dissolved in xylem sap are carried upwards with the water.
Supplies water to leaf cells	Water is needed for photosynthesis and helps cells remain turgid.
Helps cool leaves	Evaporation requires energy, so evaporation from leaf surfaces can have a cooling effect.

Factors affecting the rate of transpiration

The rate of transpiration depends mainly on how quickly water evaporates from mesophyll surfaces, how wide the stomata are open and how steep the water vapour concentration gradient is between the leaf and the surrounding air.

Factor	Change	Usual effect on transpiration	Reason
Temperature	Temperature increases	Rate usually increases	Water evaporates more rapidly from mesophyll surfaces, and water vapour molecules have more kinetic energy.
Humidity	Humidity increases	Rate decreases	The difference in water vapour concentration between the leaf air spaces and the surrounding air becomes smaller.
Air movement	Wind speed increases	Rate usually increases	Moving air removes the humid layer of air next to the leaf and maintains a steep water vapour concentration gradient.
Light intensity	Light intensity increases	Rate usually increases in a well-watered plant	Stomata usually open more for carbon dioxide uptake during photosynthesis, increasing the route for water vapour loss.

Temperature: why warmer conditions usually increase transpiration

At a higher temperature, water molecules have more kinetic energy. Evaporation from the moist mesophyll surfaces therefore occurs more rapidly. Water vapour also moves faster by random motion, so diffusion out of the leaf can be faster.

This does not mean that transpiration must keep increasing indefinitely as temperature rises. If a plant becomes water-stressed, its stomata may close and reduce water loss.

Humidity: why moist air reduces transpiration

The air spaces inside the leaf are already humid. When the surrounding air is also very humid, the difference in water vapour concentration between the inside and outside of the leaf is small. The diffusion gradient is therefore less steep, so water vapour leaves more slowly.

In dry air, the opposite is true. The concentration gradient is steeper, so transpiration is usually faster.

Air movement: why wind usually increases transpiration

In still air, water vapour that leaves the stomata can accumulate close to the leaf surface, forming a humid boundary layer. This reduces the water vapour concentration gradient.

Moving air carries this humid air away and replaces it with drier air. The gradient remains steep, so water vapour continues to diffuse out more rapidly.

Light intensity: why brighter conditions usually increase transpiration

In many plants, stomata open in the light so that carbon dioxide can enter for photosynthesis. Wider stomatal opening also makes it easier for water vapour to escape. Therefore, increasing light intensity usually increases transpiration in a well-watered plant.

Again, this is not an absolute rule under severe water shortage. A plant may close its stomata to conserve water even when light intensity is high.

How environmental factors work together

In real conditions, environmental factors do not change one at a time. A hot, dry and windy day usually produces a much higher transpiration rate than a cool, humid and still day because several factors are acting in the same direction.

Conditions favouring faster transpiration: warmer air, lower humidity, greater air movement and open stomata.

Conditions favouring slower transpiration: cooler air, higher humidity, still air and closed stomata.

Using a potometer to estimate transpiration rate

A potometer is commonly used to investigate factors affecting transpiration. It measures the rate at which a cut shoot takes up water. In a bubble potometer, the movement of an air bubble along a capillary tube can be used to estimate how quickly water is being taken up.

The important exam point is that a potometer does not directly measure water vapour leaving the leaf. It measures water uptake. Water uptake is used as an estimate of transpiration because most of the water taken up by a shoot is eventually lost by transpiration.

Potometer result: faster bubble movement → faster water uptake → usually a higher estimated transpiration rate.

For a fair test, only the chosen variable should be changed. Other factors such as temperature, light intensity, air movement, humidity and leaf area should be kept as constant as possible. The apparatus should also be airtight so that changes in water level are due to uptake by the shoot rather than leakage.

What causes wilting?

A plant wilts when it loses water faster than its roots can replace it for long enough that cells begin to lose turgor.

As water leaves cells, their vacuoles become smaller and the cell contents press less strongly against the cell wall. The cells become less turgid and may become flaccid. When many cells in leaves and young stems lose turgor, these tissues droop.

Water loss exceeds water uptake → cells lose water → turgor pressure falls → cells become flaccid → leaves and young stems droop.

Stomatal closure can help reduce further water loss because it decreases the pore area through which water vapour can diffuse. However, there is a trade-off: closed stomata also reduce carbon dioxide entry, which can lower the rate of photosynthesis.

It is therefore better to think of wilting as a consequence of water deficit, while stomatal closure is one way the plant can reduce further water loss.

How to explain transpiration in an examination

Strong answers link a factor to the physical process that changes. Avoid writing only “wind increases transpiration” or “humidity decreases transpiration”. Explain why.

For wind, write: moving air removes water vapour from around the leaf, maintaining a steep water vapour concentration gradient between the leaf air spaces and the atmosphere, so water vapour diffuses out faster.

For humidity, write: higher humidity reduces the water vapour concentration gradient between the leaf air spaces and the atmosphere, so the rate of diffusion of water vapour out of the leaf decreases.

Common Mistakes

Common mistake	Why it is wrong	Better approach
Defining transpiration as simply “water loss from a plant”	This is too vague and does not identify the form of water or the main site of loss.	State that transpiration is the loss of water vapour from aerial parts, mainly through stomata.
Saying water diffuses from the xylem directly into the atmosphere	Water first moves to mesophyll cells and evaporates into the leaf air spaces.	Separate evaporation from mesophyll surfaces from diffusion of water vapour through stomata.
Saying xylem cells actively pump water upwards	Mature xylem vessels are dead and do not pump water using ATP.	Explain that water loss from leaves creates transpiration pull that draws the water column upwards.
Saying high humidity increases transpiration	High humidity reduces the difference in water vapour concentration between the leaf and the air.	Higher humidity usually means a smaller gradient and therefore a lower transpiration rate.
Saying wind “sucks water out” of stomata	The mechanism is diffusion, not suction by the wind.	State that wind removes humid air near the leaf and maintains a steep diffusion gradient.
Saying brighter light directly heats all leaves and therefore always increases transpiration	The main syllabus explanation is usually the effect of light on stomatal opening, and water stress can cause stomata to close.	Explain that stomata usually open more in light for carbon dioxide uptake, which also increases water vapour loss.
Saying a potometer directly measures transpiration	A potometer directly measures water uptake by the shoot.	Say that water uptake is used to estimate transpiration rate.
Saying wilting means the plant is immediately dead	Wilting can be temporary if water is restored before tissues are badly damaged.	Explain wilting in terms of loss of water, reduced turgor and flaccid cells.

What you should be able to do

Define transpiration precisely

State that it is the loss of water vapour from aerial parts of a plant, mainly through the stomata of leaves.

Trace the pathway of water

Follow water from soil to root hair cells, across the root, into xylem, to mesophyll cells, leaf air spaces, stomata and the atmosphere.

Use water potential correctly

Explain water movement from higher water potential to lower water potential, including osmosis into root hair cells.

Explain evaporation and diffusion

Distinguish evaporation from mesophyll cell surfaces from diffusion of water vapour out through stomata.

Explain transpiration pull

Relate water loss from leaves to the upward pull on the continuous water column in xylem.

Explain environmental factors

Relate temperature, humidity, air movement and light intensity to changes in transpiration rate.

Interpret potometer results

Recognise that a potometer measures water uptake and is used to estimate transpiration rate.

Explain wilting

Link excessive water loss to reduced turgor, flaccid cells and drooping leaves or young stems.

Mastery Quiz

Can You Explain Transpiration in a Plant?

These ten questions test whether you can connect water potential, xylem transport, leaf evaporation, environmental factors, potometers and wilting.

10 mastery questions
Application and examination reasoning
Instant explanation for every answer option

Question 1 of 10

Which sequence correctly traces the overall pathway of water from the soil to the atmosphere?

Soil → root hair cells → cortex → xylem → mesophyll → leaf air spaces → stomata → atmosphere

This sequence follows water uptake at the root, transport in xylem, evaporation in the leaf and diffusion out through stomata.

Soil → phloem → root hair cells → mesophyll → xylem → atmosphere

Phloem does not carry the main transpiration stream, and the order is incorrect.

Soil → stomata → xylem → cortex → mesophyll → atmosphere

Stomata are in the leaf and are near the end of the pathway, not the beginning.

Soil → xylem → root hair cells → phloem → stomata → atmosphere

Water enters root hair cells before reaching the xylem, and phloem is not the main route for the transpiration stream.

Think of the pathway in three stages: uptake by roots, transport in xylem, then evaporation and diffusion from the leaf.

A plant is moved from still air into a steady breeze while temperature, light intensity and humidity are kept constant. What is the most likely immediate effect?

Transpiration rate increases because humid air near the leaf is removed

Air movement reduces the humid boundary layer around the leaf and maintains a steep water vapour gradient.

Transpiration rate decreases because wind adds water to the leaf

Wind does not add water to the leaf; it usually removes water vapour from around it.

Transpiration stops because stomata cannot open in moving air

Stomata can remain open in moving air; wind does not automatically stop transpiration.

Transpiration is unchanged because diffusion is independent of concentration gradients

Diffusion rate depends strongly on the size of the concentration gradient.

Moving air usually increases transpiration by continually replacing moist air near the leaf with less humid air.

Why does a rise in temperature usually increase the rate of transpiration?

Water evaporates more rapidly from mesophyll cell surfaces and water vapour molecules move faster

Higher temperature supplies more kinetic energy, increasing evaporation and generally speeding diffusion.

The xylem turns water into mineral ions

Xylem transports water and dissolved mineral ions; it does not convert water into ions.

The stomata become permanently wider at every higher temperature

Stomatal opening is not guaranteed to increase at all temperatures, especially during water stress.

Humidity inside the leaf falls to zero

The leaf air spaces remain moist; the effect is not that internal humidity becomes zero.

Within the normal range for a healthy plant, higher temperature generally increases evaporation from moist mesophyll surfaces and increases the kinetic energy of water vapour molecules.

Two identical plants are kept at the same temperature and light intensity. Plant P is in dry air and plant Q is in very humid air. Which statement is correct?

Plant P usually has a higher transpiration rate because the water vapour gradient from leaf to air is steeper

Dry air contains less water vapour, giving a larger gradient for diffusion out of the leaf.

Plant Q usually has a higher transpiration rate because humid air absorbs more water

High humidity reduces the gradient for diffusion and therefore usually lowers transpiration.

Both plants must have exactly the same transpiration rate

Humidity is an important factor affecting the rate of transpiration.

Plant P cannot absorb water through its roots

Dry air affects water loss from leaves; it does not prevent root water uptake.

Lower humidity outside the leaf creates a larger difference in water vapour concentration between the leaf air spaces and the atmosphere, so transpiration is usually faster.

Why does brighter light usually increase transpiration in a well-watered plant?

Stomata usually open wider for gas exchange during photosynthesis, allowing more water vapour to diffuse out

In light, stomata commonly open to allow carbon dioxide entry, which also provides a route for water vapour loss.

Light converts xylem vessels into stomata

Xylem and stomata are different structures and one does not change into the other.

Light forces liquid water out through the cuticle

Most transpired water leaves as vapour through stomata, not as liquid forced through the cuticle.

Photosynthesis uses so much water that transpiration stops

Photosynthesis uses some water, but opening stomata in light usually increases water vapour loss.

Light has an indirect effect: stomata generally open for carbon dioxide uptake during photosynthesis, and this also increases the route available for water vapour to escape.

Water evaporating from mesophyll cell surfaces lowers the water potential of those cells. What happens next?

Water moves from nearby cells and xylem towards the mesophyll, helping maintain the transpiration stream

The water potential gradient created by evaporation draws replacement water towards the evaporating surfaces.

Water moves from the mesophyll into the xylem because the mesophyll now has the higher water potential

Evaporation lowers mesophyll water potential, so water tends to move towards the mesophyll, not away from it.

Sucrose moves from xylem into the air spaces

Sucrose transport is associated mainly with phloem, not the transpiration stream.

The roots immediately stop absorbing water

Continued water loss tends to maintain, not stop, the movement of water through the plant.

Evaporation from mesophyll surfaces lowers their water potential. Replacement water is drawn from neighbouring cells and the xylem, contributing to transpiration pull.

What is meant by transpiration pull?

The upward pull on the continuous column of water in xylem produced by water loss from the leaves

Evaporation and transpiration at the leaf create tension that pulls water upward through xylem.

The active pumping of water by xylem vessel cells using ATP

Mature xylem vessels are dead and do not actively pump water using ATP.

The movement of sucrose from leaves to roots in phloem

That is translocation, not transpiration pull.

The pressure created when stomata force water vapour back into the leaf

Stomata allow water vapour to diffuse out; they do not force it back in to create the upward pull.

Transpiration pull is a pull, not a pump. Water loss from the leaves creates tension in the xylem and draws the water column upward.

A bubble potometer shows that a shoot takes up water more rapidly after a fan is switched on. What can you conclude most safely?

The rate of water uptake by the shoot has increased, and this is commonly used as an estimate of an increased transpiration rate

A potometer directly measures water uptake. Because most absorbed water is lost by transpiration, uptake rate is often used to estimate transpiration rate.

The apparatus has directly measured the exact mass of water vapour lost from every leaf

A potometer does not directly measure water vapour loss; it measures water uptake.

The plant has stopped transpiring

Faster water uptake after increased air movement is consistent with faster transpiration, not with transpiration stopping.

The phloem is carrying water faster than the xylem

The main water pathway measured in this context is through xylem.

A potometer is best described as measuring water uptake. It provides an estimate of transpiration rate because most of the water taken up by a shoot is eventually lost by transpiration.

A plant loses water faster than its roots can replace it. Which sequence best explains wilting?

Cells lose water → turgor pressure falls → cells become flaccid → leaves and young stems droop

Wilting occurs when loss of water reduces cell turgor, so soft tissues lose their normal support.

Cells gain water → turgor pressure rises → cells burst → leaves droop

Wilting is associated with loss of water and loss of turgor, not increased turgor.

Xylem fills with sucrose → stomata enlarge → leaves droop

Sucrose is mainly transported in phloem, and this does not explain wilting.

Guard cells gain water → stomata open wider → all cells become turgid

Water deficit tends to reduce guard-cell turgor and promote stomatal closure.

The immediate cellular basis of wilting is loss of turgor. When many support cells become flaccid, leaves and young stems droop.

Why can stomatal closure help a plant during water shortage?

It reduces the route through which water vapour diffuses out, lowering transpiration

Closing stomata decreases the effective pore area available for water vapour loss.

It increases evaporation from mesophyll surfaces

Stomatal closure reduces water vapour loss rather than increasing it.

It pumps water from the air directly into the xylem

Stomata do not pump atmospheric water into xylem.

It makes the outside air more humid everywhere around the plant

Stomatal closure acts by reducing vapour loss from the leaf, not by controlling the humidity of the entire surroundings.

Stomatal closure conserves water by reducing the pathway for diffusion of water vapour to the atmosphere. The trade-off is that carbon dioxide entry for photosynthesis is also reduced.

Frequently asked questions

Transpiration: questions students often ask

Most mistakes in transpiration come from mixing up evaporation, diffusion, osmosis and xylem transport. These questions connect the processes so that the whole pathway makes sense.

Is transpiration the same as evaporation?

No. Evaporation is one part of transpiration. Liquid water evaporates from moist mesophyll cell surfaces into the leaf air spaces. The resulting water vapour then diffuses out through the stomata. Transpiration describes the overall loss of water vapour from the aerial parts of the plant.

Why does water vapour diffuse out of a leaf?

The air spaces inside a healthy leaf are very humid because they are next to moist mesophyll surfaces. The surrounding air usually has a lower water vapour concentration, so water vapour diffuses down its concentration gradient through open stomata.

Does water enter root hair cells by diffusion or osmosis?

The expected process is osmosis. Water moves through a partially permeable cell surface membrane from a region of higher water potential in the soil to a region of lower water potential in the root hair cell sap.

What is the difference between the transpiration stream and transpiration pull?

The transpiration stream is the continuous movement of water through the plant, mainly in the xylem, from roots towards leaves. Transpiration pull is the pulling force created when water is lost from the leaves and replacement water is drawn from the xylem.

Why does high humidity reduce transpiration?

High humidity means the surrounding air already contains a large amount of water vapour. The difference in water vapour concentration between the leaf air spaces and the atmosphere is therefore smaller, so water vapour diffuses out more slowly.

Why does wind increase transpiration if diffusion is the process involved?

In still air, water vapour can build up around the leaf and reduce the concentration gradient. Wind removes this humid air and replaces it with drier air. The steeper concentration gradient increases the rate of diffusion of water vapour out of the leaf.

Does brighter light always increase transpiration?

In a well-watered plant, brighter light usually increases transpiration because stomata tend to open for carbon dioxide uptake during photosynthesis. Under severe water stress, however, stomata may close to conserve water, so the simple trend may no longer continue.

Why is xylem suitable for carrying the transpiration stream?

Xylem vessels form long, hollow, continuous tubes with no cytoplasm blocking the lumen. Their walls are strengthened with lignin, helping them withstand the tension produced as water is pulled upwards through the plant.

Does a potometer measure transpiration directly?

No. A potometer directly measures the rate of water uptake by a shoot. This is used to estimate transpiration rate because most of the water taken up is later lost from the aerial parts of the plant.

Why does a plant wilt?

Wilting occurs when water loss exceeds water uptake and cells lose water. Their turgor pressure falls, many cells become flaccid, and leaves or young stems lose support and droop.

Can closing stomata help a wilting plant?

Yes. Closing stomata reduces the pore area available for water vapour to diffuse out, lowering transpiration. The disadvantage is that carbon dioxide entry is also reduced, so photosynthesis may slow down.

What is the best way to answer a factor-of-transpiration question?

Do not stop at stating whether the rate increases or decreases. Link the factor to evaporation, stomatal opening or the water vapour concentration gradient, and then state how this changes the rate of diffusion of water vapour from the leaf.

Transpiration is easiest to understand as one connected chain. Water enters roots by osmosis, travels through xylem, replaces water lost from mesophyll cells, evaporates into leaf air spaces and then diffuses out through stomata. That loss helps create transpiration pull, which keeps more water moving upwards through the plant. Once this pathway is clear, the effects of wind, humidity, temperature, light and water shortage become consequences of the same underlying movement of water.