A Tall Tree Lifts Water Higher Than Any Pump Could Suck It
Learn the four forces that raise sap to the top of a tree, how to demonstrate and measure transpiration with a bell jar and a potometer, what speeds transpiration up, and how it differs from guttation.
How does water reach the top of a tree with nothing pushing it there?
Mostly by being pulled from above rather than pushed from below.
The root pressure of the previous part can lift water only a few metres, which is nowhere near enough for a tall tree. What does the real work is evaporation from the leaves. As water vapour escapes through the stomata, it leaves behind a suction that tugs on the water in the leaf, which tugs on the water below it, and so on down an unbroken column all the way to the roots.
The column holds together because water molecules cling to one another remarkably strongly. This page covers the second part of the ICSE Class 8 Biology chapter on transportation in plants: the ascent of sap, transpiration and guttation.
The root pressure of the previous part can lift water only a few metres, which is nowhere near enough for a tall tree. What does the real work is evaporation from the leaves. As water vapour escapes through the stomata, it leaves behind a suction that tugs on the water in the leaf, which tugs on the water below it, and so on down an unbroken column all the way to the roots.
The column holds together because water molecules cling to one another remarkably strongly. This page covers the second part of the ICSE Class 8 Biology chapter on transportation in plants: the ascent of sap, transpiration and guttation.
What forces are responsible for the ascent of sap?
The ascent of sap is the upward movement of water and dissolved minerals from the roots to the leaves through the xylem. Four forces contribute, and they are not equal partners.
Root pressure — a push from below. Water entering the root by osmosis builds up pressure in the root xylem and pushes water upwards. It is real but weak, effective only for a few metres, and it operates mainly at night when the leaves are losing little water.
Capillarity — a rise in narrow tubes. Water rises on its own in a very fine tube, and xylem vessels are extremely narrow. This helps, but on its own capillarity in tubes of that size can raise water only a modest distance — far short of the height of a tree.
Transpiration pull — the main force. Water evaporating from the mesophyll cells of the leaf leaves those cells short of water. They draw water from the cells next to them, which draw from the xylem, and the result is a tension, or suction, transmitted down the entire water column from leaf to root. This is the force that does most of the lifting, and it costs the plant no energy — the Sun provides it.
Cohesion and adhesion — what makes the pull possible. Cohesion is the strong attraction between water molecules themselves, which holds the column together as one continuous thread instead of letting it snap under tension. Adhesion is the attraction between water molecules and the xylem walls, which keeps the column in place against the pipe.
Together with transpiration pull these make up the cohesion-tension theory, which is the accepted explanation for the ascent of sap.
Why the column not breaking is the crux. A suction pump at the top of a tall pipe cannot lift water beyond a limited height, because the water column breaks and air comes out of solution. A tree escapes this because its water is in very narrow, water-wetted tubes where cohesion and adhesion together hold the thread intact under tension that would tear an open column apart.
And the dead xylem now makes sense. Vessels and tracheids are dead and hollow with stiff lignified walls precisely because a tube under suction must not collapse and must not obstruct the flow. A living cell full of cytoplasm would do both.
Root pressure — a push from below. Water entering the root by osmosis builds up pressure in the root xylem and pushes water upwards. It is real but weak, effective only for a few metres, and it operates mainly at night when the leaves are losing little water.
Capillarity — a rise in narrow tubes. Water rises on its own in a very fine tube, and xylem vessels are extremely narrow. This helps, but on its own capillarity in tubes of that size can raise water only a modest distance — far short of the height of a tree.
Transpiration pull — the main force. Water evaporating from the mesophyll cells of the leaf leaves those cells short of water. They draw water from the cells next to them, which draw from the xylem, and the result is a tension, or suction, transmitted down the entire water column from leaf to root. This is the force that does most of the lifting, and it costs the plant no energy — the Sun provides it.
Cohesion and adhesion — what makes the pull possible. Cohesion is the strong attraction between water molecules themselves, which holds the column together as one continuous thread instead of letting it snap under tension. Adhesion is the attraction between water molecules and the xylem walls, which keeps the column in place against the pipe.
Together with transpiration pull these make up the cohesion-tension theory, which is the accepted explanation for the ascent of sap.
Why the column not breaking is the crux. A suction pump at the top of a tall pipe cannot lift water beyond a limited height, because the water column breaks and air comes out of solution. A tree escapes this because its water is in very narrow, water-wetted tubes where cohesion and adhesion together hold the thread intact under tension that would tear an open column apart.
And the dead xylem now makes sense. Vessels and tracheids are dead and hollow with stiff lignified walls precisely because a tube under suction must not collapse and must not obstruct the flow. A living cell full of cytoplasm would do both.
How do you demonstrate and measure transpiration?
Transpiration is the loss of water in the form of water vapour from the aerial parts of a plant, mainly through the stomata of the leaves.
It also occurs to a small extent through lenticels on woody stems and through the cuticle of the leaf surface, but stomatal transpiration accounts for nearly all of it.
The bell jar experiment — to demonstrate it.
- Take a well-watered potted plant.
- Wrap the pot and the soil in polythene, tied at the stem, so that no water can evaporate from the soil.
- Place the plant under a dry bell jar standing on a glass plate, and seal the rim with grease.
- Set up an identical control with the pot and soil wrapped but no plant in it.
- Leave both in sunlight for a few hours.
Observation. Water droplets collect on the inner walls of the bell jar containing the plant. The control jar stays dry.
Conclusion. The water came from the plant, not from the soil — which is exactly what wrapping the pot and running the control were for. Without those two precautions the experiment would prove nothing, and that is where marks are usually lost.
The potometer — to measure the rate. A potometer measures the rate of water uptake by a cut leafy twig, which is taken as a measure of the rate of transpiration.
- A freshly cut leafy twig is fitted air-tight into a tube completely filled with water.
- The tube leads to a horizontal capillary tube carrying a graduated scale.
- An air bubble is introduced into the capillary tube.
- As the twig transpires, it draws water up, and the bubble moves along the scale. Timing its movement gives the rate.
A worked measurement. Suppose the bubble travels in minutes, and the capillary tube has a cross-sectional area of . The volume of water taken up is
so the rate is
Changing one condition — moving the apparatus into a breeze, say — and repeating the timing lets you compare rates, which is how the factors in the next section are investigated.
The assumption built into a potometer. It measures water taken in, not water given out, and a small amount of the uptake is used by the plant rather than transpired. So the reading is a close approximation rather than an exact measure of transpiration — a limitation worth stating if asked.
It also occurs to a small extent through lenticels on woody stems and through the cuticle of the leaf surface, but stomatal transpiration accounts for nearly all of it.
The bell jar experiment — to demonstrate it.
- Take a well-watered potted plant.
- Wrap the pot and the soil in polythene, tied at the stem, so that no water can evaporate from the soil.
- Place the plant under a dry bell jar standing on a glass plate, and seal the rim with grease.
- Set up an identical control with the pot and soil wrapped but no plant in it.
- Leave both in sunlight for a few hours.
Observation. Water droplets collect on the inner walls of the bell jar containing the plant. The control jar stays dry.
Conclusion. The water came from the plant, not from the soil — which is exactly what wrapping the pot and running the control were for. Without those two precautions the experiment would prove nothing, and that is where marks are usually lost.
The potometer — to measure the rate. A potometer measures the rate of water uptake by a cut leafy twig, which is taken as a measure of the rate of transpiration.
- A freshly cut leafy twig is fitted air-tight into a tube completely filled with water.
- The tube leads to a horizontal capillary tube carrying a graduated scale.
- An air bubble is introduced into the capillary tube.
- As the twig transpires, it draws water up, and the bubble moves along the scale. Timing its movement gives the rate.
A worked measurement. Suppose the bubble travels in minutes, and the capillary tube has a cross-sectional area of . The volume of water taken up is
so the rate is
Changing one condition — moving the apparatus into a breeze, say — and repeating the timing lets you compare rates, which is how the factors in the next section are investigated.
The assumption built into a potometer. It measures water taken in, not water given out, and a small amount of the uptake is used by the plant rather than transpired. So the reading is a close approximation rather than an exact measure of transpiration — a limitation worth stating if asked.
What affects the rate of transpiration?
Four main factors, and each has a clear reason.
Light — increases transpiration. Stomata open in light and close in darkness, so the main escape route is available by day and shut by night. Light also warms the leaf. This is why transpiration is high at midday and nearly zero at night.
Temperature — increases transpiration. A higher temperature gives water molecules more kinetic energy, so more of them evaporate from the leaf's inner surfaces. It also makes the surrounding air able to hold more vapour. A plant on a hot afternoon transpires far faster than the same plant on a cool morning.
Humidity — decreases transpiration. Humidity is the water vapour already in the air. When the air is nearly saturated there is little difference between the vapour inside the leaf and outside it, so the net loss is small. This is why plants transpire slowly during the monsoon and quickly in dry weather, even at the same temperature.
Wind speed — increases transpiration. Moving air carries the escaped vapour away from the leaf surface, keeping the air just outside the stomata dry and the outward gradient steep. In still air a humid layer builds up around the leaf and slows further loss.
Very strong wind, however, can cause the stomata to close, which reduces transpiration — a boundary case worth mentioning.
Three further factors:
- Available soil water — a plant that cannot absorb enough will close its stomata and transpire less.
- Leaf surface area — more exposed leaf means more loss, which is why a plant transpires less after pruning.
- Number and distribution of stomata — desert plants have fewer stomata, often sunken and open at night, precisely to reduce loss.
The common thread. Every one of these acts on the same two things: whether the stomata are open, and how steep the vapour gradient is between the inside of the leaf and the air outside. Naming which of those two a factor affects turns a memorised list into a reasoned answer.
And notice the overlap with evaporation. Temperature, humidity and wind affect transpiration exactly as they affect the drying of washed clothes, for the same physical reasons. Light is the one factor unique to transpiration, because only a living leaf has stomata that open and close.
Light — increases transpiration. Stomata open in light and close in darkness, so the main escape route is available by day and shut by night. Light also warms the leaf. This is why transpiration is high at midday and nearly zero at night.
Temperature — increases transpiration. A higher temperature gives water molecules more kinetic energy, so more of them evaporate from the leaf's inner surfaces. It also makes the surrounding air able to hold more vapour. A plant on a hot afternoon transpires far faster than the same plant on a cool morning.
Humidity — decreases transpiration. Humidity is the water vapour already in the air. When the air is nearly saturated there is little difference between the vapour inside the leaf and outside it, so the net loss is small. This is why plants transpire slowly during the monsoon and quickly in dry weather, even at the same temperature.
Wind speed — increases transpiration. Moving air carries the escaped vapour away from the leaf surface, keeping the air just outside the stomata dry and the outward gradient steep. In still air a humid layer builds up around the leaf and slows further loss.
Very strong wind, however, can cause the stomata to close, which reduces transpiration — a boundary case worth mentioning.
Three further factors:
- Available soil water — a plant that cannot absorb enough will close its stomata and transpire less.
- Leaf surface area — more exposed leaf means more loss, which is why a plant transpires less after pruning.
- Number and distribution of stomata — desert plants have fewer stomata, often sunken and open at night, precisely to reduce loss.
The common thread. Every one of these acts on the same two things: whether the stomata are open, and how steep the vapour gradient is between the inside of the leaf and the air outside. Naming which of those two a factor affects turns a memorised list into a reasoned answer.
And notice the overlap with evaporation. Temperature, humidity and wind affect transpiration exactly as they affect the drying of washed clothes, for the same physical reasons. Light is the one factor unique to transpiration, because only a living leaf has stomata that open and close.
How is guttation different from transpiration, and why does the plant bother?
Guttation is the loss of water as liquid droplets from the margins or tips of leaves, through special pores called hydathodes.
It is what you see on grass and on the leaves of tomato, colocasia and nasturtium in the early morning — drops of water at the leaf edges, often mistaken for dew. Dew condenses out of the air onto a cold surface; guttation comes from inside the plant.
Setting the two side by side:
- Form of water. Transpiration: vapour. Guttation: liquid droplets.
- Pores used. Transpiration: stomata. Guttation: hydathodes.
- Time. Transpiration: mainly during the day. Guttation: at night and early morning.
- Position on the leaf. Transpiration: over the whole surface. Guttation: at the margins and tips.
- Purity. Transpiration gives off pure water vapour. Guttation liquid contains dissolved salts and sugars, which is why it can leave a crust when it dries.
- Quantity. Transpiration: large. Guttation: small.
- Control. Transpiration is regulated by the opening and closing of stomata. Guttation is not regulated.
Why guttation happens at night. At night the stomata are closed so transpiration nearly stops, while the roots keep absorbing water by osmosis and root pressure builds. With no vapour route available, the excess water is forced out as liquid through the hydathodes. So guttation is a direct consequence of the root pressure of the previous part.
The significance of transpiration to the plant:
- It creates the transpiration pull that raises sap to the top of the tallest tree.
- It therefore helps the absorption and distribution of minerals throughout the plant.
- It cools the leaves by evaporation, protecting them from the midday sun — the same cooling mechanism as sweating.
- It removes excess water and helps maintain the turgidity of cells.
- It concentrates the sap in the leaf, keeping minerals available where they are needed.
The cost, and why it is worth paying. Most of the water a plant absorbs is lost by transpiration rather than used, and in drought it causes wilting and can kill the plant. For this reason transpiration is sometimes described as a necessary evil — wasteful on the face of it, but the price of having a water-transport system that needs no pump and no energy of the plant's own.
It is what you see on grass and on the leaves of tomato, colocasia and nasturtium in the early morning — drops of water at the leaf edges, often mistaken for dew. Dew condenses out of the air onto a cold surface; guttation comes from inside the plant.
Setting the two side by side:
- Form of water. Transpiration: vapour. Guttation: liquid droplets.
- Pores used. Transpiration: stomata. Guttation: hydathodes.
- Time. Transpiration: mainly during the day. Guttation: at night and early morning.
- Position on the leaf. Transpiration: over the whole surface. Guttation: at the margins and tips.
- Purity. Transpiration gives off pure water vapour. Guttation liquid contains dissolved salts and sugars, which is why it can leave a crust when it dries.
- Quantity. Transpiration: large. Guttation: small.
- Control. Transpiration is regulated by the opening and closing of stomata. Guttation is not regulated.
Why guttation happens at night. At night the stomata are closed so transpiration nearly stops, while the roots keep absorbing water by osmosis and root pressure builds. With no vapour route available, the excess water is forced out as liquid through the hydathodes. So guttation is a direct consequence of the root pressure of the previous part.
The significance of transpiration to the plant:
- It creates the transpiration pull that raises sap to the top of the tallest tree.
- It therefore helps the absorption and distribution of minerals throughout the plant.
- It cools the leaves by evaporation, protecting them from the midday sun — the same cooling mechanism as sweating.
- It removes excess water and helps maintain the turgidity of cells.
- It concentrates the sap in the leaf, keeping minerals available where they are needed.
The cost, and why it is worth paying. Most of the water a plant absorbs is lost by transpiration rather than used, and in drought it causes wilting and can kill the plant. For this reason transpiration is sometimes described as a necessary evil — wasteful on the face of it, but the price of having a water-transport system that needs no pump and no energy of the plant's own.
Exam tip
Exam tip: name transpiration pull as the main force
When asked what raises sap, list all four forces but say plainly that transpiration pull is the main one, and that root pressure works only for a few metres. Giving them as equal partners loses the point of the question.
Always pair cohesion and adhesion with what they achieve — cohesion holds the water column unbroken, adhesion holds it to the xylem walls.
In the bell jar experiment, state both precautions and the reason for each: wrap the pot so soil water cannot evaporate, and run a control without a plant. The conclusion depends on them.
For the potometer, say it measures water uptake as an approximation of transpiration, and describe the air bubble moving along a scale.
For the factors, give the direction and the reason: humidity decreases transpiration because the vapour gradient between leaf and air is smaller. Naming the factor alone is half an answer.
Say stomata open in light and close in darkness — this single fact explains the day-night pattern.
Compare transpiration and guttation in pairs: vapour against liquid, stomata against hydathodes, day against night, pure water against water with dissolved salts.
And distinguish guttation from dew — dew condenses from the air, guttation is forced out from inside by root pressure.
Always pair cohesion and adhesion with what they achieve — cohesion holds the water column unbroken, adhesion holds it to the xylem walls.
In the bell jar experiment, state both precautions and the reason for each: wrap the pot so soil water cannot evaporate, and run a control without a plant. The conclusion depends on them.
For the potometer, say it measures water uptake as an approximation of transpiration, and describe the air bubble moving along a scale.
For the factors, give the direction and the reason: humidity decreases transpiration because the vapour gradient between leaf and air is smaller. Naming the factor alone is half an answer.
Say stomata open in light and close in darkness — this single fact explains the day-night pattern.
Compare transpiration and guttation in pairs: vapour against liquid, stomata against hydathodes, day against night, pure water against water with dissolved salts.
And distinguish guttation from dew — dew condenses from the air, guttation is forced out from inside by root pressure.
Did you know
Why does the shade under a tree feel cooler than shade under a roof?
Stand under a concrete slab at noon and you are out of the sunlight but the air is still hot. Stand under a large tree and the difference is unmistakable.
Part of it is simply that leaves block the sunlight. But a tree is also transpiring, and every gram of water that evaporates from its leaves carries away a substantial quantity of heat — the same latent heat that makes sweating and an earthen pot work. A large tree releases a great deal of water on a hot day, and all of it takes heat from the leaves and the air around them.
So a tree is not a passive umbrella. It is running an evaporative cooler over its whole crown, powered by sunlight, and the cool air it produces sinks around you.
This is also why a row of trees makes a street measurably more comfortable than the same street with awnings, and why cutting mature trees in a city raises local temperatures in a way that replacing them with young saplings does not immediately undo — the cooling scales with the leaf area doing the transpiring.
Part of it is simply that leaves block the sunlight. But a tree is also transpiring, and every gram of water that evaporates from its leaves carries away a substantial quantity of heat — the same latent heat that makes sweating and an earthen pot work. A large tree releases a great deal of water on a hot day, and all of it takes heat from the leaves and the air around them.
So a tree is not a passive umbrella. It is running an evaporative cooler over its whole crown, powered by sunlight, and the cool air it produces sinks around you.
This is also why a row of trees makes a street measurably more comfortable than the same street with awnings, and why cutting mature trees in a city raises local temperatures in a way that replacing them with young saplings does not immediately undo — the cooling scales with the leaf area doing the transpiring.
Key takeaways
Ascent of sap and transpiration: quick revision
- Ascent of sap is the upward movement of water and minerals from root to leaves through the xylem.
- Four forces: root pressure (a weak push from below, only a few metres, mainly at night), capillarity (rise in narrow tubes, a modest contribution), transpiration pull (the main force, a suction created by evaporation from the leaves), and cohesion and adhesion — cohesion holding the water column unbroken, adhesion holding it to the xylem walls. Together these are the cohesion-tension theory.
- Xylem conducting cells are dead, hollow and lignified because a tube under suction must not collapse or obstruct flow.
- Transpiration is the loss of water as vapour from aerial parts, mainly through stomata, with small amounts through lenticels and the cuticle.
- Bell jar experiment: wrap the pot in polythene, seal a dry bell jar over the plant, and run a control without a plant. Droplets form only in the jar with the plant, so the water came from the plant.
- Potometer: a leafy twig draws water and an air bubble moves along a scale. A bubble travelling in a tube of area over minutes gives , so per minute. It measures uptake, so it approximates transpiration.
- Factors — light increases it (stomata open in light), temperature increases it, humidity decreases it (smaller vapour gradient), wind increases it (vapour carried away). Also soil water, leaf area and stomatal number. Very strong wind can close stomata.
- Every factor acts on either whether the stomata are open or how steep the vapour gradient is.
- Guttation loses liquid water from leaf margins through hydathodes, at night, containing dissolved salts, in small amounts and unregulated — driven by root pressure when stomata are closed. It is not dew.
- Significance of transpiration: creates the transpiration pull, aids mineral distribution, cools the leaves, removes excess water and maintains turgidity. Excessive loss causes wilting, so it is called a necessary evil.
Try listing the four forces with their relative importance and then the four factors with their reasons — those two lists between them answer most of what this chapter asks.
- Four forces: root pressure (a weak push from below, only a few metres, mainly at night), capillarity (rise in narrow tubes, a modest contribution), transpiration pull (the main force, a suction created by evaporation from the leaves), and cohesion and adhesion — cohesion holding the water column unbroken, adhesion holding it to the xylem walls. Together these are the cohesion-tension theory.
- Xylem conducting cells are dead, hollow and lignified because a tube under suction must not collapse or obstruct flow.
- Transpiration is the loss of water as vapour from aerial parts, mainly through stomata, with small amounts through lenticels and the cuticle.
- Bell jar experiment: wrap the pot in polythene, seal a dry bell jar over the plant, and run a control without a plant. Droplets form only in the jar with the plant, so the water came from the plant.
- Potometer: a leafy twig draws water and an air bubble moves along a scale. A bubble travelling in a tube of area over minutes gives , so per minute. It measures uptake, so it approximates transpiration.
- Factors — light increases it (stomata open in light), temperature increases it, humidity decreases it (smaller vapour gradient), wind increases it (vapour carried away). Also soil water, leaf area and stomatal number. Very strong wind can close stomata.
- Every factor acts on either whether the stomata are open or how steep the vapour gradient is.
- Guttation loses liquid water from leaf margins through hydathodes, at night, containing dissolved salts, in small amounts and unregulated — driven by root pressure when stomata are closed. It is not dew.
- Significance of transpiration: creates the transpiration pull, aids mineral distribution, cools the leaves, removes excess water and maintains turgidity. Excessive loss causes wilting, so it is called a necessary evil.
Try listing the four forces with their relative importance and then the four factors with their reasons — those two lists between them answer most of what this chapter asks.