Transpiration: Why Plants Lose Almost All Their Water

A plant drinks water from the soil all day. It uses only a tiny part of it. Almost all of the rest leaves the plant as vapor through its leaves. That loss is called transpiration.

This post explains what transpiration is, how water climbs a tree without a pump, what controls the rate, and how some plants have solved the problem.

Droplets on a leaf after a hot day. Much of this water left the plant as vapor, cooled the night air, and came back as liquid — Image: Doggo19292, CC BY 2.5, via Wikimedia Commons

What Transpiration Actually Is

Transpiration has two parts. First, water moves through the inside of the plant. Second, some of that water turns into vapor and leaves. Both together are transpiration.

The name tells you this. It joins two Latin words. Trans means across. Spirare means to breathe. So the name means breathing across, and the vapor really is water that the plant has breathed out.

The plant spends no energy on this. It burns no sugar and makes no ATP. The heat of the sun does the work instead. This is the one large piece of plant life that runs on physics alone.

Transpiration also cools the plant, in the same way sweat cools you. Water soaking up heat at the leaf surface keeps leaves from getting too hot in full sun.

Where almost all the water goes

Roots take in water and minerals by osmosis. A growing plant builds new tissue with only a small share of that water. Between 97 and 99.5 percent is lost again through transpiration and guttation. That is why a thirsty field needs so much irrigation.

The amounts look strange until you see them written out:

Plant or process	Amount of water lost
One acre of corn in one growing day	3,000–4,000 US gallons (11,000–15,000 L)
One large oak in one year	about 40,000 US gallons (150,000 L)
Growing a crop	200–1,000 kg of water for every 1 kg of dry plant matter
A leaf over one growing season	many times the leaf’s own weight

Where Water Leaves the Plant

Roots are covered in tiny hairs. Water crosses into them by osmosis, and from there it climbs a pipe network. That network is the xylem. The xylem carries water up to the leaves.

In a leaf, the water has one main exit. The leaf surface is dotted with pores called stomata. Each stoma is a hole in the leaf skin with a pair of guard cells around it. The two guard cells can swell and shrink like a pair of hands. Swollen, they curve apart and open the pore. Flaccid, they meet and shut it.

How guard cells work. Water enters them, they swell, and the pore between them opens — Image: June Kwak and Pascal Mäser, public domain, via Wikimedia Commons

One open stoma on a tomato leaf, seen down a microscope — Image: Photohound, public domain, via Wikimedia Commons

A stoma, its guard cells, and the accessory cells around them are together called the stomatal complex. A leaf can hold tens of thousands of stomata, and a single square millimeter of leaf can carry a few hundred.

One Correction Worth Making

The first version of this post said stomata are the only way water leaves a plant. That is not true. Water also evaporates through the waxy layer that covers the whole leaf. This is called cuticular transpiration. Young stems lose water too, through little breathing holes called lenticels.

Stomata still handle most of the loss, which is why the plant keeps such fine control over them. Leaves are built with two different sides. The top side carries the thick cuticle, and the underside carries most of the stomata and all of the leaf hairs.

A leaf in cross section. The two sides are built differently, and only the lower one carries most of the stomata — Image: Mnolf, CC BY-SA 3.0, via Wikimedia Commons

Some plants push water out of their leaf tips instead. The water collects as clear drops, and this is called guttation. It happens at night or early morning, from pores at the leaf edge called hydathodes. Unlike dew, which comes from the air, guttation water comes from inside the plant.

How Water Climbs Against Gravity

Water climbs to the top of a tall tree with no pump at all. Two sticking properties of water do the work.

The first is cohesion. Water molecules hold on to each other, so a chain of them can be pulled without the chain breaking. The second is adhesion. Water molecules also stick to the inside walls of the xylem, which helps hold the column in place. Both come from the way water molecules form hydrogen bonds with each other.

A cut celery stalk. The dots are xylem vessels, and they are the pipes a plant uses to carry water upward — Image: fir0002, GFDL 1.2, via Wikimedia Commons

The cohesion-tension theory

Here is the part that surprises most people. Evaporation does not do the pulling. It creates the tension, and the tension does the pulling.

Inside a leaf, water soaks the cell walls. When a molecule evaporates from a wet wall, the curve of water left behind pulls gently on the molecule next to it. That one pulls on the next. The pull travels all the way down the xylem to the roots.

The leaf then has a lower water potential than the soil. Because of that difference, water flows inward on its own. The root is not pushing and the plant is not pumping. The gap between wet soil and dry air is doing all of the work.

Xylem vessels are long tubes. Water has to travel a long way inside them, so they need wall strength and a low friction lining — Image: Zaoui58, CC BY-SA 3.0, via Wikimedia Commons

What Controls the Rate

A plant does not transpire at a fixed speed. The rate changes through the day, and the plant changes it on purpose by opening and closing stomata. The main controls are:

Factor	What happens	Why
Light	Rate goes up during the day	Stomata open to take in carbon dioxide
Humidity	Rate goes down as the air gets damp	Damp air slows evaporation
Wind	Rate goes up, up to a limit	Moving air removes the humid layer at the leaf
Temperature	Rate goes up as it gets warmer	More heat means faster evaporation
Soil moisture	Rate falls as the soil dries	Less water reaches the roots

Two of these work in opposite ways, and that is worth understanding. Temperature speeds evaporation, but it also lowers the humidity of the air, and drier air speeds evaporation again. So a hot dry day gives the highest losses of the year.

Light usually opens stomata, because the plant needs to take in carbon dioxide to make sugars. There is one big exception, and it is covered below.

How scientists measure it

A potometer is the simple classroom tool. It counts the water droplets pulled up a tube by a leafy shoot. A lysimeter weighs a block of soil and measures how much water the soil loses. A porometer measures how fast a leaf is losing vapor. Bigger tools, such as photosynthesis systems and sap flow sensors, measure whole plants or whole trees.

When water and heat both leave a field together, the total is called evapotranspiration. Isotope studies show that transpiration is the larger share of it. This matters for forecasting and for river water supply, because plants decide how much water a landscape returns to the sky instead of to the stream.

Plants That Have Solved the Problem

Plants in dry places have to lose far less water, or lose it at times when the air is already damp. There are two main strategies.

The first is to shrink the loss. Plants called xerophytes live in deserts and dry scrub. They use small leaves, thick waxy cuticles, sunken stomata set in little pits, and dense hairs on the leaf surface. The hairs trap a layer of humid air, so the air right at the surface is damper than the air above it, and evaporation slows down.

Leaf hairs under an electron microscope. They hold still air at the leaf surface, which slows water loss — Image: Dartmouth Electron Microscope Facility, public domain, via Wikimedia Commons

Many cacti go further and drop their leaves altogether. A green, fleshy stem does the photosynthesis instead, and a round or upright shape keeps the surface area small for the amount of tissue inside.

Leaf shapes of dry-country plants. Small, thick, and waxy beats large and thin — Image: Rickproser, CC BY-SA 3.0, via Wikimedia Commons

A cactus that has lost its leaves. All the photosynthesis happens in the stem, and the spines shade it — Image: Kalebzhan, CC0, via Wikimedia Commons

The second strategy is to change the timing. Many succulents, including cacti and pineapples, use CAM photosynthesis. CAM stands for crassulacean acid metabolism. In these plants the stomata open at night and close in the day. At night the air is cooler and much damper, so far less water is lost. The carbon dioxide taken in at night is stored as an acid and used in daylight, when the pores are shut.

Eucalyptus leaves hang edge-on to the sun, so they lose less water at the hottest part of the day — Image: JonRichfield, CC BY-SA 4.0, via Wikimedia Commons

When the System Breaks

The whole setup has a weakness. The water in the xylem is under tension, and a stretched column can snap. If the plant cannot replace the water fast enough, the tension gets too strong and the column breaks. The break is a bubble of water vapor inside the pipe. Air can then enter and the xylem stops working.

This is called cavitation. It happens in real plants in hot dry weather, in freezing weather, and when roots rot or are damaged. Some plants can refill the pipe afterwards, but refilling costs energy and takes time. That is part of why stomata close during a heat wave, even when the light is good for growth. Closing them slows transpiration and protects the pipes.

Key Takeaways

• Transpiration is water moving through a plant and evaporating from its surfaces, mostly its leaves.
• It is passive. The plant spends no energy on it.
• Between 97 and 99.5 percent of the water a plant takes up is lost this way.
• Stomata are the main exit. Guard cells open and close them to set the rate.
• Water also escapes through the leaf cuticle and through lenticels on stems.
• Evaporation creates tension in the xylem, and that tension is what pulls water up. The plant has no pump.
• Light, humidity, wind, temperature, and soil moisture all change the rate.
• Desert plants cut the loss with hairs, sunken stomata, small leaves, or green stems. CAM plants simply do it at night instead.

Frequently Asked Questions

Is transpiration the same as evaporation?

No. Evaporation is water turning into vapor anywhere. Transpiration is that evaporation happening inside a living plant, after the water has climbed from the roots. A lake evaporates. A tree transpires.

Why is transpiration bad for a plant?

It costs nothing to run, but the water has to be replaced. A plant has to grow in a place with enough water to refill what it loses. That is the whole reason desert plants are shaped the way they are.

Why is transpiration not a waste after all?

For the plant it is a necessary cost. For the whole landscape it does a lot of good. It shades and cools the ground, so it helps cities and forests stay cooler than the bare ground and roofs around them. That is the opposite of the urban heat island effect, which happens when plants are replaced with hard surfaces.

Do plants close their stomata at night?

Most do, because there is no need for carbon dioxide once the sun is down. Some plants open their stomata at night instead, and use the carbon dioxide the next morning. That is the CAM method.

What is guttation, and is it dew?

Guttation is water pushed out of the leaf tip by root pressure, usually at night. Dew is vapor from the air condensing on a cold surface. They look the same but come from opposite directions.

What is cavitation, and can a plant recover from it?

Cavitation is the breaking of the water column in a xylem vessel, usually by a bubble of vapor. The vessel stops carrying water. Many plants can push the bubble back out, but it costs them energy and growth.

Sources: Wikipedia articles on transpiration, stoma, the cohesion–tension theory, xylem, hydathode, cavitation, xerophyte, CAM photosynthesis and evapotranspiration. Images: Wikimedia Commons, with authors and licenses noted in each caption.