A Leafy Shoot Can Pull an Air Bubble Along a Glass Tube as It Loses Water
See how light, temperature, humidity and wind speed up or slow down transpiration, learn how Ganong's potometer measures water uptake and where it falls short, work through the weight-loss and cobalt chloride experiments, and tell transpiration apart from guttation and bleeding.
Why do plants lose water faster on some days than others?
Hang wet clothes on a terrace on a hot, dry, breezy afternoon and they are dry within an hour. Hang the same clothes on a still, humid monsoon morning and they stay damp all day. Leaves lose water in a very similar way, and the same conditions that dry clothes speed up transpiration.
Part 1 explained what transpiration is and how stomata control it. This part asks three practical questions:
- What changes the rate of transpiration? Light, temperature, humidity and wind all do, for clear reasons
- How can the rate be measured? A simple instrument, Ganong's potometer, lets a leafy shoot pull an air bubble along a graduated tube as it takes up water
- How can transpiration be demonstrated? By weighing a potted plant as it loses water, and by using cobalt chloride paper, which changes colour when it meets water vapour
Finally, it separates transpiration from two other ways plants lose water — guttation, the liquid drops seen at leaf tips at dawn, and bleeding, the sap that flows from cut or injured stems.
Why measuring transpiration matters beyond the laboratory. Farmers decide how often to irrigate partly from how fast crops lose water, which depends on exactly the factors in this lesson. A field of wheat on a hot, windy day in April needs far more water than the same field on a cool, cloudy day.
The link to Part 1. Most transpiration happens through stomata, so anything that opens the stomata or speeds evaporation from the leaf increases the rate. Keep that single idea in mind, and every factor and every experiment in this part follows from it.
One caution before starting. A potometer measures water taken in, not water lost. For a leafy shoot the two are very nearly equal, which is why the instrument works — but understanding that small difference is the key to its limitations.
This page covers the second part of the ICSE Class 10 Biology chapter on transpiration: factors affecting its rate, Ganong's potometer, demonstration experiments, and the difference between transpiration, guttation and bleeding.
Part 1 explained what transpiration is and how stomata control it. This part asks three practical questions:
- What changes the rate of transpiration? Light, temperature, humidity and wind all do, for clear reasons
- How can the rate be measured? A simple instrument, Ganong's potometer, lets a leafy shoot pull an air bubble along a graduated tube as it takes up water
- How can transpiration be demonstrated? By weighing a potted plant as it loses water, and by using cobalt chloride paper, which changes colour when it meets water vapour
Finally, it separates transpiration from two other ways plants lose water — guttation, the liquid drops seen at leaf tips at dawn, and bleeding, the sap that flows from cut or injured stems.
Why measuring transpiration matters beyond the laboratory. Farmers decide how often to irrigate partly from how fast crops lose water, which depends on exactly the factors in this lesson. A field of wheat on a hot, windy day in April needs far more water than the same field on a cool, cloudy day.
The link to Part 1. Most transpiration happens through stomata, so anything that opens the stomata or speeds evaporation from the leaf increases the rate. Keep that single idea in mind, and every factor and every experiment in this part follows from it.
One caution before starting. A potometer measures water taken in, not water lost. For a leafy shoot the two are very nearly equal, which is why the instrument works — but understanding that small difference is the key to its limitations.
This page covers the second part of the ICSE Class 10 Biology chapter on transpiration: factors affecting its rate, Ganong's potometer, demonstration experiments, and the difference between transpiration, guttation and bleeding.
How do light, temperature, humidity and wind affect the rate of transpiration?
Bright light, high temperature, low humidity and moderate wind all increase transpiration, because they open the stomata or speed the escape of water vapour from the leaf; the opposite conditions reduce it.
1. Light.
- In light, stomata open, so water vapour can escape
- Light also warms the leaf, speeding evaporation
- So transpiration is faster in bright light and very slow in darkness, when most stomata close
2. Temperature.
- A higher temperature makes water evaporate faster from the leaf cells
- Warm air can also hold more water vapour, so it takes up moisture more readily
- So transpiration increases with temperature
3. Humidity.
- Humidity is the amount of water vapour in the air
- When the air is already humid, the difference between the moist air inside the leaf and the air outside is small, so water vapour diffuses out slowly
- So transpiration decreases as humidity increases, and is fastest in dry air
4. Wind.
- Still air around a leaf becomes a humid layer, which slows further loss
- Wind sweeps that humid layer away, replacing it with drier air
- So a moderate wind increases transpiration
- A very strong wind can cause the stomata to close, which then reduces the rate
Other factors. Leaf area, the number and position of stomata, thickness of the cuticle and water available in the soil also affect the rate — a plant in dry soil closes its stomata and transpires less.
Worked example — ranking conditions. Arrange these conditions from fastest to slowest transpiration for the same plant.
- A: bright sun, hot, dry air, gentle breeze
- B: cloudy, cool, humid, still air
- C: bright sun, hot, humid, still air
Fastest: A — every factor increases the rate. Middle: C — light and heat increase it, but humidity and still air slow it. Slowest: B — every factor reduces it.
Worked example — reading potometer data. A bubble moves in minutes in sunlight, and in minutes in shade. How many times faster is uptake in sunlight?
An everyday example. Potted plants on a sunny, windy balcony need watering far more often in summer than the same plants in a sheltered, shaded corner. More light, heat and wind mean more transpiration, and the soil dries out faster to replace the lost water.
The boundary case. The effects of the factors combine, and one can override another. On a very hot, dry afternoon many plants close their stomata to save water, so transpiration can actually fall at the time of day when the weather seems to favour it most.
1. Light.
- In light, stomata open, so water vapour can escape
- Light also warms the leaf, speeding evaporation
- So transpiration is faster in bright light and very slow in darkness, when most stomata close
2. Temperature.
- A higher temperature makes water evaporate faster from the leaf cells
- Warm air can also hold more water vapour, so it takes up moisture more readily
- So transpiration increases with temperature
3. Humidity.
- Humidity is the amount of water vapour in the air
- When the air is already humid, the difference between the moist air inside the leaf and the air outside is small, so water vapour diffuses out slowly
- So transpiration decreases as humidity increases, and is fastest in dry air
4. Wind.
- Still air around a leaf becomes a humid layer, which slows further loss
- Wind sweeps that humid layer away, replacing it with drier air
- So a moderate wind increases transpiration
- A very strong wind can cause the stomata to close, which then reduces the rate
Other factors. Leaf area, the number and position of stomata, thickness of the cuticle and water available in the soil also affect the rate — a plant in dry soil closes its stomata and transpires less.
Worked example — ranking conditions. Arrange these conditions from fastest to slowest transpiration for the same plant.
- A: bright sun, hot, dry air, gentle breeze
- B: cloudy, cool, humid, still air
- C: bright sun, hot, humid, still air
Fastest: A — every factor increases the rate. Middle: C — light and heat increase it, but humidity and still air slow it. Slowest: B — every factor reduces it.
Worked example — reading potometer data. A bubble moves in minutes in sunlight, and in minutes in shade. How many times faster is uptake in sunlight?
An everyday example. Potted plants on a sunny, windy balcony need watering far more often in summer than the same plants in a sheltered, shaded corner. More light, heat and wind mean more transpiration, and the soil dries out faster to replace the lost water.
The boundary case. The effects of the factors combine, and one can override another. On a very hot, dry afternoon many plants close their stomata to save water, so transpiration can actually fall at the time of day when the weather seems to favour it most.
How does Ganong's potometer work, and what are its limitations?
A potometer holds a freshly cut leafy shoot in a sealed, water-filled apparatus connected to a graduated capillary tube; as the shoot takes up water, an air bubble moves along the tube, and the distance moved in a given time measures the rate of water uptake, which is taken as roughly equal to the rate of transpiration.
Principle. A leafy shoot absorbs water at almost the same rate as it loses water by transpiration. Measuring uptake therefore gives an estimate of transpiration.
Construction of Ganong's potometer:
- A wide glass vessel filled with water, with one opening into which a freshly cut leafy shoot is fixed through a split cork
- A reservoir with a stopcock, used to let in water and reset the bubble
- A long, horizontal, graduated capillary tube, whose free end is bent down and dips into a beaker of water
- All joints sealed with grease or vaseline so that no air enters
Working:
- The whole apparatus is filled with water, and the shoot is cut under water and fixed in place so no air enters its xylem
- An air bubble is introduced into the capillary tube by lifting its end out of the water for a moment
- As the shoot transpires, it absorbs water from the apparatus, and the bubble moves along the capillary tube towards the shoot
- The distance the bubble travels in a measured time gives the rate of water uptake
- Opening the reservoir stopcock pushes the bubble back to the start for a repeat reading
Worked example — converting distance to volume. A bubble moves in minutes along a capillary tube of cross-sectional area . Find the volume of water taken up per minute.
Limitations of the potometer:
- It measures water absorbed, not water transpired — a small amount of absorbed water is used by the shoot in photosynthesis and growth
- A cut shoot may not behave like a whole, rooted plant
- The shoot may begin to wilt over a long experiment, changing its rate
- Air leaking in through a poor joint breaks the water column and spoils readings
- Changes in temperature can make the bubble expand or contract, moving it without any uptake
Precautions:
- Cut the shoot under water, so no air blocks the xylem
- Make every joint airtight with vaseline
- Keep the apparatus under steady conditions while comparing one factor at a time
An everyday example. Science laboratories in many schools use a potometer beside a table fan and a lamp to show how wind and light change the rate. Switching on the fan visibly speeds up the bubble — the effect of wind from the previous section, measured directly.
The boundary case. The potometer is best at comparing rates, not at giving an exact figure for transpiration. Because the same shoot and apparatus are used for every condition, its small errors largely cancel, so its readings are reliable for saying which condition makes transpiration faster.
Principle. A leafy shoot absorbs water at almost the same rate as it loses water by transpiration. Measuring uptake therefore gives an estimate of transpiration.
Construction of Ganong's potometer:
- A wide glass vessel filled with water, with one opening into which a freshly cut leafy shoot is fixed through a split cork
- A reservoir with a stopcock, used to let in water and reset the bubble
- A long, horizontal, graduated capillary tube, whose free end is bent down and dips into a beaker of water
- All joints sealed with grease or vaseline so that no air enters
Working:
- The whole apparatus is filled with water, and the shoot is cut under water and fixed in place so no air enters its xylem
- An air bubble is introduced into the capillary tube by lifting its end out of the water for a moment
- As the shoot transpires, it absorbs water from the apparatus, and the bubble moves along the capillary tube towards the shoot
- The distance the bubble travels in a measured time gives the rate of water uptake
- Opening the reservoir stopcock pushes the bubble back to the start for a repeat reading
Worked example — converting distance to volume. A bubble moves in minutes along a capillary tube of cross-sectional area . Find the volume of water taken up per minute.
Limitations of the potometer:
- It measures water absorbed, not water transpired — a small amount of absorbed water is used by the shoot in photosynthesis and growth
- A cut shoot may not behave like a whole, rooted plant
- The shoot may begin to wilt over a long experiment, changing its rate
- Air leaking in through a poor joint breaks the water column and spoils readings
- Changes in temperature can make the bubble expand or contract, moving it without any uptake
Precautions:
- Cut the shoot under water, so no air blocks the xylem
- Make every joint airtight with vaseline
- Keep the apparatus under steady conditions while comparing one factor at a time
An everyday example. Science laboratories in many schools use a potometer beside a table fan and a lamp to show how wind and light change the rate. Switching on the fan visibly speeds up the bubble — the effect of wind from the previous section, measured directly.
The boundary case. The potometer is best at comparing rates, not at giving an exact figure for transpiration. Because the same shoot and apparatus are used for every condition, its small errors largely cancel, so its readings are reliable for saying which condition makes transpiration faster.
How do the weight-loss and cobalt chloride experiments demonstrate transpiration?
A potted plant with its soil sealed off loses weight as water vapour escapes from its leaves, while dry blue cobalt chloride paper turns pink faster on the lower surface of a dorsiventral leaf, showing that leaves lose water vapour and that the lower surface loses more.
Experiment 1 — loss in weight of a potted plant.
- Setup: water a potted plant well, then cover the pot and soil completely with a polythene sheet tied round the base of the stem
- Why the cover: it prevents water evaporating from the soil and pot, so any weight lost comes from the plant
- Weigh the whole set-up, then leave it in sunlight for a few hours and weigh it again
- Observation: the weight decreases
- Inference: the plant has lost water by transpiration
- Control: an identical covered pot without a plant shows no significant loss
Worked example — rate of loss. A covered potted plant weighs at the start and after hours. The control pot shows no change. Find the average rate of water loss.
A leafy shoot in a test tube of water with a layer of oil on the surface, placed on a balance, shows the same result: the weight falls as the shoot transpires, while the oil stops direct evaporation from the water.
Experiment 2 — cobalt chloride paper on a dorsiventral leaf.
- Cobalt chloride paper is blue when dry and turns pink when it absorbs water; it is kept perfectly dry in a desiccator before use
- Setup: place one strip of dry blue cobalt chloride paper on the upper surface and another on the lower surface of a leaf still attached to a plant. Cover each with a glass slide and clip them in place
- Observation: the paper on the lower surface turns pink sooner than the paper on the upper surface
- Inference: more water vapour is lost from the lower surface, because a dorsiventral leaf, such as that of hibiscus, has more stomata on its lower surface
Experiment 3 — covering leaves with vaseline. Four similar leaves are treated and left on the plant or on a line:
- Leaf A — no vaseline: wilts first
- Leaf B — vaseline on the upper surface only: wilts next
- Leaf C — vaseline on the lower surface only: wilts later
- Leaf D — vaseline on both surfaces: stays fresh longest
Vaseline blocks the stomata, so the more stomata are sealed, the slower the leaf loses water — and blocking the lower surface slows it more, confirming where most stomata lie.
An everyday example. A polythene bag tied over a leafy branch of a guava tree fogs up with droplets within an hour or two in summer sunshine. Touching the droplets with dry cobalt chloride paper turns it pink, proving that the mist is water given out by the leaves.
The boundary case. The cobalt chloride experiment compares two surfaces of the same leaf at the same time, so light, temperature and humidity are equal for both. Only the number of stomata differs, which is why the result can be traced to stomata alone.
Experiment 1 — loss in weight of a potted plant.
- Setup: water a potted plant well, then cover the pot and soil completely with a polythene sheet tied round the base of the stem
- Why the cover: it prevents water evaporating from the soil and pot, so any weight lost comes from the plant
- Weigh the whole set-up, then leave it in sunlight for a few hours and weigh it again
- Observation: the weight decreases
- Inference: the plant has lost water by transpiration
- Control: an identical covered pot without a plant shows no significant loss
Worked example — rate of loss. A covered potted plant weighs at the start and after hours. The control pot shows no change. Find the average rate of water loss.
A leafy shoot in a test tube of water with a layer of oil on the surface, placed on a balance, shows the same result: the weight falls as the shoot transpires, while the oil stops direct evaporation from the water.
Experiment 2 — cobalt chloride paper on a dorsiventral leaf.
- Cobalt chloride paper is blue when dry and turns pink when it absorbs water; it is kept perfectly dry in a desiccator before use
- Setup: place one strip of dry blue cobalt chloride paper on the upper surface and another on the lower surface of a leaf still attached to a plant. Cover each with a glass slide and clip them in place
- Observation: the paper on the lower surface turns pink sooner than the paper on the upper surface
- Inference: more water vapour is lost from the lower surface, because a dorsiventral leaf, such as that of hibiscus, has more stomata on its lower surface
Experiment 3 — covering leaves with vaseline. Four similar leaves are treated and left on the plant or on a line:
- Leaf A — no vaseline: wilts first
- Leaf B — vaseline on the upper surface only: wilts next
- Leaf C — vaseline on the lower surface only: wilts later
- Leaf D — vaseline on both surfaces: stays fresh longest
Vaseline blocks the stomata, so the more stomata are sealed, the slower the leaf loses water — and blocking the lower surface slows it more, confirming where most stomata lie.
An everyday example. A polythene bag tied over a leafy branch of a guava tree fogs up with droplets within an hour or two in summer sunshine. Touching the droplets with dry cobalt chloride paper turns it pink, proving that the mist is water given out by the leaves.
The boundary case. The cobalt chloride experiment compares two surfaces of the same leaf at the same time, so light, temperature and humidity are equal for both. Only the number of stomata differs, which is why the result can be traced to stomata alone.
How is transpiration different from guttation and bleeding?
Transpiration is the loss of water vapour mainly through stomata during the day; guttation is the loss of liquid water drops from pores at leaf tips and margins, usually at night or early morning, due to root pressure; and bleeding is the flow of sap from cut or injured parts of a plant.
1. Transpiration.
- Form: water vapour
- Site: stomata, cuticle and lenticels of aerial parts
- Time: mainly during the day
- Cause: evaporation, creating transpiration pull
- Content: pure water, as vapour
- Control: regulated by the opening and closing of stomata
2. Guttation.
- Form: liquid water drops
- Site: special pores called hydathodes at the tips and margins of leaves
- Time: usually at night or early morning, when transpiration is low and the soil is moist
- Cause: root pressure pushing water out
- Content: water with dissolved minerals
- Control: not regulated
- Examples: grass, tomato and colocasia (arbi) leaves
3. Bleeding.
- Form: liquid sap
- Site: cut or injured parts of the stem or other organs
- Time: whenever the plant is cut or wounded
- Cause: pressure in the plant's tissues pushing sap out of the wound
- Content: water with dissolved sugars and minerals
- Examples: sap flowing from tapped palm trees and from cut stems of some plants
Worked example — identify the process.
- Drops of liquid appear only at the tips of grass blades at dawn — guttation
- A plastic bag over a leafy branch mists up in sunshine — transpiration
- Sweet sap drips from a cut made in the trunk of a date palm — bleeding
How to tell guttation from dew. Dew condenses from the air and covers all surfaces, including stones and soil. Guttation drops appear only at the leaf tips and edges, where the hydathodes are.
An everyday example. In parts of India, the trunks of palm trees are carefully cut so that sweet sap drips into pots tied beneath the cut, especially in the cool early morning. That flow of sap from a wound is bleeding, and the fresh sweet sap collected this way is sold as a drink.
The boundary case — the balance between root pressure and transpiration pull. During the day, transpiration pull draws water up and out as vapour, so root pressure cannot force liquid out of the leaves. At night, transpiration almost stops, and root pressure takes over — which is why guttation is seen at dawn and not at noon.
1. Transpiration.
- Form: water vapour
- Site: stomata, cuticle and lenticels of aerial parts
- Time: mainly during the day
- Cause: evaporation, creating transpiration pull
- Content: pure water, as vapour
- Control: regulated by the opening and closing of stomata
2. Guttation.
- Form: liquid water drops
- Site: special pores called hydathodes at the tips and margins of leaves
- Time: usually at night or early morning, when transpiration is low and the soil is moist
- Cause: root pressure pushing water out
- Content: water with dissolved minerals
- Control: not regulated
- Examples: grass, tomato and colocasia (arbi) leaves
3. Bleeding.
- Form: liquid sap
- Site: cut or injured parts of the stem or other organs
- Time: whenever the plant is cut or wounded
- Cause: pressure in the plant's tissues pushing sap out of the wound
- Content: water with dissolved sugars and minerals
- Examples: sap flowing from tapped palm trees and from cut stems of some plants
Worked example — identify the process.
- Drops of liquid appear only at the tips of grass blades at dawn — guttation
- A plastic bag over a leafy branch mists up in sunshine — transpiration
- Sweet sap drips from a cut made in the trunk of a date palm — bleeding
How to tell guttation from dew. Dew condenses from the air and covers all surfaces, including stones and soil. Guttation drops appear only at the leaf tips and edges, where the hydathodes are.
An everyday example. In parts of India, the trunks of palm trees are carefully cut so that sweet sap drips into pots tied beneath the cut, especially in the cool early morning. That flow of sap from a wound is bleeding, and the fresh sweet sap collected this way is sold as a drink.
The boundary case — the balance between root pressure and transpiration pull. During the day, transpiration pull draws water up and out as vapour, so root pressure cannot force liquid out of the leaves. At night, transpiration almost stops, and root pressure takes over — which is why guttation is seen at dawn and not at noon.
Exam tip
What earns full marks on potometer and transpiration experiments?
Explain each factor with its reason, describe the potometer with its principle and limitations, and present every experiment as setup, observation, inference and control.
- For each factor, state the effect and the reason — stomata opening, evaporation, diffusion gradient, removal of humid air
- Mention that very strong wind may close stomata and reduce the rate
- State the potometer principle: rate of water uptake is nearly equal to rate of transpiration
- Describe the parts: leafy shoot, water-filled vessel, reservoir with stopcock, graduated capillary tube, air bubble
- Say the shoot is cut under water and joints are sealed with vaseline
- List at least three limitations, starting with measures absorption, not transpiration
- In the weight-loss experiment, explain why the pot and soil are covered
- In the cobalt chloride experiment, state the colour change from blue to pink and which surface changes first
- Compare transpiration, guttation and bleeding on form, site, time, cause and content
- Name hydathodes as the site of guttation
The misconception to name. A potometer does not measure transpiration directly. It measures the water absorbed by the shoot, which is only approximately equal to the water lost. Stating that it measures transpiration exactly misses the limitation examiners look for.
A second trap. Confusing the colour change of cobalt chloride. Dry cobalt chloride paper is blue and turns pink with water — reversing the colours makes the whole observation wrong.
- For each factor, state the effect and the reason — stomata opening, evaporation, diffusion gradient, removal of humid air
- Mention that very strong wind may close stomata and reduce the rate
- State the potometer principle: rate of water uptake is nearly equal to rate of transpiration
- Describe the parts: leafy shoot, water-filled vessel, reservoir with stopcock, graduated capillary tube, air bubble
- Say the shoot is cut under water and joints are sealed with vaseline
- List at least three limitations, starting with measures absorption, not transpiration
- In the weight-loss experiment, explain why the pot and soil are covered
- In the cobalt chloride experiment, state the colour change from blue to pink and which surface changes first
- Compare transpiration, guttation and bleeding on form, site, time, cause and content
- Name hydathodes as the site of guttation
The misconception to name. A potometer does not measure transpiration directly. It measures the water absorbed by the shoot, which is only approximately equal to the water lost. Stating that it measures transpiration exactly misses the limitation examiners look for.
A second trap. Confusing the colour change of cobalt chloride. Dry cobalt chloride paper is blue and turns pink with water — reversing the colours makes the whole observation wrong.
Did you know
Why do farmers in dry regions plant rows of trees around their fields?
Drive through the dry farmland of western Rajasthan or parts of Gujarat and you may see long lines of trees planted along the edges of fields, sometimes in several rows. They are not there for fruit or timber alone. They are windbreaks, also called shelterbelts, and they work because of the factors in this lesson.
Wind speeds up transpiration. As the previous sections showed, moving air sweeps away the humid layer around each leaf and replaces it with dry air, so water vapour escapes faster. In an open, windswept field, crops can lose water faster than the soil can supply it, and they wilt even when irrigated.
A row of trees slows the wind. Air flowing over and through the trees loses much of its speed for some distance behind them.
- Crops in the sheltered zone lose less water by transpiration
- Less water evaporates directly from the soil, which stays moist for longer
- The sheltered air stays a little more humid, slowing water loss further
So a line of trees can save water for an entire field without a single extra litre of irrigation.
Windbreaks help in other ways too.
- They reduce soil erosion, since strong winds carry away dry topsoil in desert regions
- They protect young plants from being damaged by strong gusts
- They provide fodder, fuelwood and shade, and homes for birds and insects
The trade-off is worth noticing. The trees themselves transpire and take up water from the soil near them, and they shade the crops closest to them. Farmers choose hardy, drought-resistant trees and plant them at a sensible distance, so that the water saved across the field outweighs the water the trees use.
It is the same science as a potometer beside a table fan — switch off the wind, and water loss slows — carried out across whole landscapes to help crops survive in hot, dry, windy country.
Wind speeds up transpiration. As the previous sections showed, moving air sweeps away the humid layer around each leaf and replaces it with dry air, so water vapour escapes faster. In an open, windswept field, crops can lose water faster than the soil can supply it, and they wilt even when irrigated.
A row of trees slows the wind. Air flowing over and through the trees loses much of its speed for some distance behind them.
- Crops in the sheltered zone lose less water by transpiration
- Less water evaporates directly from the soil, which stays moist for longer
- The sheltered air stays a little more humid, slowing water loss further
So a line of trees can save water for an entire field without a single extra litre of irrigation.
Windbreaks help in other ways too.
- They reduce soil erosion, since strong winds carry away dry topsoil in desert regions
- They protect young plants from being damaged by strong gusts
- They provide fodder, fuelwood and shade, and homes for birds and insects
The trade-off is worth noticing. The trees themselves transpire and take up water from the soil near them, and they shade the crops closest to them. Farmers choose hardy, drought-resistant trees and plant them at a sensible distance, so that the water saved across the field outweighs the water the trees use.
It is the same science as a potometer beside a table fan — switch off the wind, and water loss slows — carried out across whole landscapes to help crops survive in hot, dry, windy country.
Exam relevance
How do transpiration experiments prepare you for NEET Biology?
This is foundation work for NEET Biology in Class 11 — especially Anatomy of Flowering Plants, Plant Growth and Development and the plant physiology that builds on water relations. How far transpiration is covered as a separate topic depends on the current official NEET syllabus, so check it; the skills below are used regardless.
Where the factors lead. Understanding why light, humidity, temperature and wind change the rate of water loss is the kind of cause-and-effect reasoning NEET tests in statement-based questions. Questions often describe a change in conditions and ask for its effect on stomata or water loss.
Where hydathodes and guttation lead. Class 11 Anatomy of Flowering Plants describes the epidermis, stomata and specialised structures of leaves. Distinguishing guttation through hydathodes from transpiration through stomata is a standard objective item, and the root pressure behind guttation links to the internal structure of roots.
Where experimental design leads. The covered pot, the oil layer, the four vaseline-coated leaves and the two-surface cobalt chloride test each change one factor at a time. Recognising the purpose of a control is a skill used throughout biology practical work and in questions that describe an experiment and ask what it proves.
Where stomatal regulation leads. Class 11 Plant Growth and Development links abscisic acid to stomatal closure under water stress — the reason transpiration can fall on a very hot afternoon.
Where the potometer calculation leads. Converting a distance moved along a tube into a volume uses area times length — a simple quantitative step that appears in data-interpretation questions.
Question types to expect. At this level: factors with reasons, potometer construction and limitations, experiments and comparisons. In NEET: statements on water loss, hydathodes and guttation, experimental controls, and hormone effects, often as assertion-reason or match-the-column items.
The single trap that costs marks. Treating guttation as a form of transpiration. Guttation is liquid water forced out by root pressure; transpiration is vapour lost by evaporation — and statements that blur the two are common distractors.
A second trap. Assuming wind always increases transpiration. Very strong wind can close stomata and reduce it, so an unqualified statement may be marked incorrect.
Board versus competitive emphasis. The ICSE paper marks full experimental descriptions and limitations; NEET marks the concept behind a single observation or control. The transferable habit is asking, for every experiment, which single factor was changed and what the control kept the same.
Where the factors lead. Understanding why light, humidity, temperature and wind change the rate of water loss is the kind of cause-and-effect reasoning NEET tests in statement-based questions. Questions often describe a change in conditions and ask for its effect on stomata or water loss.
Where hydathodes and guttation lead. Class 11 Anatomy of Flowering Plants describes the epidermis, stomata and specialised structures of leaves. Distinguishing guttation through hydathodes from transpiration through stomata is a standard objective item, and the root pressure behind guttation links to the internal structure of roots.
Where experimental design leads. The covered pot, the oil layer, the four vaseline-coated leaves and the two-surface cobalt chloride test each change one factor at a time. Recognising the purpose of a control is a skill used throughout biology practical work and in questions that describe an experiment and ask what it proves.
Where stomatal regulation leads. Class 11 Plant Growth and Development links abscisic acid to stomatal closure under water stress — the reason transpiration can fall on a very hot afternoon.
Where the potometer calculation leads. Converting a distance moved along a tube into a volume uses area times length — a simple quantitative step that appears in data-interpretation questions.
Question types to expect. At this level: factors with reasons, potometer construction and limitations, experiments and comparisons. In NEET: statements on water loss, hydathodes and guttation, experimental controls, and hormone effects, often as assertion-reason or match-the-column items.
The single trap that costs marks. Treating guttation as a form of transpiration. Guttation is liquid water forced out by root pressure; transpiration is vapour lost by evaporation — and statements that blur the two are common distractors.
A second trap. Assuming wind always increases transpiration. Very strong wind can close stomata and reduce it, so an unqualified statement may be marked incorrect.
Board versus competitive emphasis. The ICSE paper marks full experimental descriptions and limitations; NEET marks the concept behind a single observation or control. The transferable habit is asking, for every experiment, which single factor was changed and what the control kept the same.
Key takeaways
What must you be able to do from this part?
Four factors, one instrument, three experiments and a three-way comparison.
- Light opens stomata and warms leaves — increases transpiration
- Temperature speeds evaporation — increases transpiration
- Humidity reduces the vapour gradient — decreases transpiration
- Wind removes humid air — increases transpiration; very strong wind may close stomata
- Leaf area, stomata, cuticle and soil water also affect the rate
- Ganong's potometer: a cut leafy shoot in a sealed water-filled apparatus with a graduated capillary tube; the air bubble moves as water is absorbed
- Principle: water uptake is nearly equal to transpiration
- ** in min along a tube** is per minute
- Limitations: measures absorption not transpiration, cut shoot differs from whole plant, wilting, air leaks, temperature changes
- Precautions: cut under water, airtight joints
- Weight-loss experiment: pot and soil covered; plant loses weight — to in h is per hour
- Cobalt chloride paper: blue when dry, pink with water; lower surface of a dorsiventral leaf turns it pink sooner
- Vaseline experiment: leaf with no vaseline wilts first; both surfaces coated stays fresh longest
- Transpiration: vapour, stomata, day, evaporation, pure water, regulated
- Guttation: liquid drops, hydathodes, night or dawn, root pressure, water with minerals
- Bleeding: sap from cut or injured parts, whenever wounded
The sharpest self-test is a potometer and four conditions. Predict whether the bubble moves faster or slower when a lamp is switched on, a fan is turned on, the room is made humid and the shoot is placed in the dark — and give the reason for each prediction in one line.
- Light opens stomata and warms leaves — increases transpiration
- Temperature speeds evaporation — increases transpiration
- Humidity reduces the vapour gradient — decreases transpiration
- Wind removes humid air — increases transpiration; very strong wind may close stomata
- Leaf area, stomata, cuticle and soil water also affect the rate
- Ganong's potometer: a cut leafy shoot in a sealed water-filled apparatus with a graduated capillary tube; the air bubble moves as water is absorbed
- Principle: water uptake is nearly equal to transpiration
- ** in min along a tube** is per minute
- Limitations: measures absorption not transpiration, cut shoot differs from whole plant, wilting, air leaks, temperature changes
- Precautions: cut under water, airtight joints
- Weight-loss experiment: pot and soil covered; plant loses weight — to in h is per hour
- Cobalt chloride paper: blue when dry, pink with water; lower surface of a dorsiventral leaf turns it pink sooner
- Vaseline experiment: leaf with no vaseline wilts first; both surfaces coated stays fresh longest
- Transpiration: vapour, stomata, day, evaporation, pure water, regulated
- Guttation: liquid drops, hydathodes, night or dawn, root pressure, water with minerals
- Bleeding: sap from cut or injured parts, whenever wounded
The sharpest self-test is a potometer and four conditions. Predict whether the bubble moves faster or slower when a lamp is switched on, a fan is turned on, the room is made humid and the shoot is placed in the dark — and give the reason for each prediction in one line.