💧 Transpiration – Full Breakdown
ICSE Class 10
Biology
June 2026
Transpiration is the loss of water vapour from plant surfaces — and ICSE exams love it because it connects plant physiology with physics (evaporation, diffusion, pressure). Almost every board paper has questions on the three types of transpiration, the potometer experiment, factors affecting transpiration, and the comparison with guttation. Written by someone who scored 93.4% in ICSE 2026.
📌 FIGURE 1 — Stomatal Transpiration
Fig: Stomatal transpiration — water vapour exits through the stomatal pore. Each stoma is bounded by two kidney-shaped guard cells that regulate opening and closing. ICSE 2019, 2021, 2023.
1) What is Transpiration?
Transpiration: The process by which water vapour is lost from the aerial parts of a plant (mainly leaves) through evaporation. It is an inevitable consequence of having stomata open for gas exchange.
Transpiration is essentially evaporation of water from inside the leaf → water vapour diffuses out through stomata (or cuticle/lenticels). The water lost is replaced by water pulled up from the roots through the xylem, creating the transpiration stream.
2) Types of Transpiration
| Type | Site | % of Total Water Loss | Details |
|---|---|---|---|
| Stomatal | Through stomata (mostly on lower leaf surface) | 80-90% (the dominant form) | Water vapour diffuses out through open stomatal pores. Occurs during daytime when stomata are open. Regulated by guard cells. |
| Cuticular | Through the cuticle (waxy layer on leaf surface) | 5-10% | Water diffuses directly through the cuticle. Very low in thick, waxy cuticles (xerophytes). Higher in young leaves with thin cuticle. |
| Lenticular | Through lenticels (pores in bark of stems & woody parts) | 1-5% | Lenticels are raised pores in the bark that allow gas exchange. Water loss is very slow and constant (occurs day and night). |
💡 Quick memory aid: S-C-L (from most water loss to least) → Stomatal (80-90%) → Cuticular (5-10%) → Lenticular (1-5%). In exam questions, "transpiration" usually means stomatal transpiration unless specified otherwise.
3) The Transpiration Stream — The Path of Water
The transpiration stream refers to the continuous flow of water from roots → stem → leaves → into the atmosphere. Here is the complete pathway:
- Soil → Root hair: Water enters root hair by osmosis (soil water has lower solute concentration than root cell sap)
- Root hair → Cortex → Endodermis → Xylem: Water moves through root cells via apoplast and symplast pathways. At the endodermis, the Casparian strip forces water into the symplast path
- Xylem (root → stem → leaf): Water moves upward through xylem vessels by transpiration pull (cohesion-tension mechanism). The continuous water column is held together by cohesion (water-water H-bonds) and adhesion (water-xylem wall attraction)
- Leaf xylem → Mesophyll cells: Water enters leaf cells from the xylem in leaf veins
- Mesophyll cells → Air spaces → Stomata → Atmosphere: Water evaporates from the moist cell walls of mesophyll cells into air spaces. Water vapour then diffuses out through the stomatal pore into the atmosphere
📌 FIGURE 2 — Potometer (Measuring Transpiration Rate)
Fig: Ganong's potometer — measures the rate of water uptake (≈ transpiration rate). As the plant transpires, the air bubble moves along the capillary tube. The distance moved per unit time gives the rate. ICSE 2018, 2020, 2022, 2024.
⚠️ Potometer — how it works:
A potometer measures the rate of water uptake by a leafy shoot, which is approximately equal to the transpiration rate (some water is used for photosynthesis and growth, but this is negligible).
Procedure:
1. Cut a leafy shoot under water (to prevent air bubbles entering the xylem)
2. Insert into the potometer under water
3. Seal all connections with petroleum jelly (airtight)
4. Introduce an air bubble into the capillary tube
5. Measure the distance the bubble moves in 1 minute → calculate the rate
Precautions: All joints must be air-tight. Leafy shoot must be cut under water. The apparatus should be dry inside before setting up (wet walls cause condensation errors). Use a fan to increase transpiration for visible results.
A potometer measures the rate of water uptake by a leafy shoot, which is approximately equal to the transpiration rate (some water is used for photosynthesis and growth, but this is negligible).
Procedure:
1. Cut a leafy shoot under water (to prevent air bubbles entering the xylem)
2. Insert into the potometer under water
3. Seal all connections with petroleum jelly (airtight)
4. Introduce an air bubble into the capillary tube
5. Measure the distance the bubble moves in 1 minute → calculate the rate
Precautions: All joints must be air-tight. Leafy shoot must be cut under water. The apparatus should be dry inside before setting up (wet walls cause condensation errors). Use a fan to increase transpiration for visible results.
4) Factors Affecting Transpiration
| Factor | Effect on Transpiration Rate | Reason |
|---|---|---|
| Light Intensity | Increases ↑ (directly proportional) | Light causes stomata to open (blue light triggers guard cell uptake of K⁺ → water enters guard cells → they become turgid and bow outwards → pore opens). More light → more stomata open → more transpiration. At night, stomata close → very little transpiration |
| Temperature | Increases ↑ (up to a point) | Higher temperature → water evaporates faster from mesophyll cell walls. Also increases the diffusion rate of water vapour. Above ~40°C, stomata may close (to conserve water) → rate drops |
| Humidity | Decreases ↓ (inversely proportional) | High humidity means the air outside the leaf is already saturated with water vapour → smaller concentration gradient → slower diffusion. Low humidity (dry air) → steeper concentration gradient → faster transpiration |
| Wind Speed | Increases ↑ | Wind removes water vapour from around the leaf → maintains a steep concentration gradient → faster diffusion. In still air, the air around the leaf becomes saturated → gradient drops → transpiration slows |
| CO₂ Concentration | Decreases ↓ | High CO₂ inside the leaf → stomata close (to prevent too much CO₂ entry). Low CO₂ → stomata open wider → more transpiration |
| Water Availability | Decreases ↓ in drought | When the soil is dry, roots can't absorb enough water → stomata close → transpiration decreases. The plant also produces abscisic acid (ABA) which triggers stomatal closure |
| Number & Distribution of Stomata | More stomata → more transpiration | Leaves with more stomata on the lower surface have higher transpiration rates. Most plants have stomata on the lower surface to reduce direct exposure to sunlight |
5) Transpiration vs Guttation
ICSE often asks students to differentiate between transpiration and guttation — they look similar but are completely different processes.
📌 FIGURE 3 — Guttation
Fig: Guttation — droplets of liquid water exuded from hydathodes at leaf margins. Caused by root pressure, NOT transpiration. Common in grasses, nasturtium, strawberry. ICSE 2020, 2023.
| Feature | Transpiration | Guttation |
|---|---|---|
| Form of water lost | Water vapour (gas) | Liquid water (droplets) |
| Site of loss | Mainly through stomata (also cuticle, lenticels) | Through hydathodes (special pores at leaf margins/vein endings) |
| Driving force | Transpiration pull (tension from evaporation in leaves) | Root pressure (positive pressure from roots pushing water up) |
| When it occurs | Daytime (when stomata are open and sunlight is available) | Early morning / night (when humidity is high and transpiration is low, so root pressure dominates) |
| Type of process | Physical evaporation (passive) | Active + Physical (root pressure is active, exudation is physical) |
| Water type | Pure water (minerals not lost) | Contains dissolved minerals (a dilute solution — not pure water) |
| Affected by environment? | Yes — strongly affected by light, temperature, humidity, wind | Minimally — mainly depends on root pressure and soil moisture |
| Plants showing this | All plants (it is unavoidable) | Only certain plants — especially herbaceous plants and grasses (strawberry, nasturtium, tomato, colocasia) |
| Can it be controlled? | Yes — stomata can close (controlled by guard cells) | No — it is an overflow mechanism when root pressure is high |
⚠️ Common board confusion — guttation is NOT dew:
Dew forms on ALL surfaces (including non-plant surfaces) from condensation of atmospheric moisture overnight. Guttation is liquid water exuded from inside the plant through hydathodes, visible only at leaf margins. Exam question: "How can you tell if droplets on a leaf are from guttation or dew?" Answer: Guttation drops appear only at leaf margins/vein endings. Dew appears randomly all over the leaf surface.
Dew forms on ALL surfaces (including non-plant surfaces) from condensation of atmospheric moisture overnight. Guttation is liquid water exuded from inside the plant through hydathodes, visible only at leaf margins. Exam question: "How can you tell if droplets on a leaf are from guttation or dew?" Answer: Guttation drops appear only at leaf margins/vein endings. Dew appears randomly all over the leaf surface.
6) Significance of Transpiration
Advantages (Why transpiration is essential):
- Ascent of sap: Transpiration pull is the main driving force for the upward movement of water and dissolved minerals from roots to leaves
- Cooling effect: Evaporation of water from leaf surfaces cools the plant (just like sweating in humans). Leaf temperature can be 5-10°C lower than the surrounding air
- Mineral transport: Minerals absorbed by roots are transported with the transpiration stream to the leaves and other parts
- Maintains cell turgidity: As long as transpiration and water absorption are balanced, cells remain turgid and the plant stays upright
- Gas exchange: Stomata must be open for transpiration, which also allows CO₂ to enter (for photosynthesis) and O₂ to exit
Disadvantages (Why excessive transpiration is harmful):
- Wilting: If transpiration exceeds water absorption, cells lose turgor → plant wilts. Temporary wilting can be reversed at night. Permanent wilting leads to death
- Reduced growth: Under water stress, stomata close → less CO₂ entry → photosynthesis reduced → less growth
- Leaf scorching: In extreme heat, leaves may get scorched or burnt due to excessive water loss
- Xylem cavitation: In intense drought, air bubbles can form in the xylem, breaking the water column (embolism) and stopping the transpiration stream
7) Adaptations to Reduce Transpiration (Xerophytes)
Plants in dry conditions (deserts, sandy soil) have evolved adaptations to minimise transpiration:
| Adaptation | How It Reduces Transpiration | Example |
|---|---|---|
| Thick, waxy cuticle | Reduces water loss through the cuticle | Cactus, aloe vera, pine needles |
| Sunken stomata | Stomata are located in pits (depressions) — creates a humid microclimate around the stoma, reducing the concentration gradient | Nerium (oleander), pine |
| Reduced leaf surface area | Leaves are reduced to spines or scales → less area for transpiration | Cactus (spines), Casuarina (scale-like leaves) |
| Rolled leaves | Leaves curl up when dry → stomata are inside the rolled portion, protected from wind and exposed to higher humidity | Marram grass (Ammophila), Stipa (esparto grass) |
| Thick, fleshy stem | Stem stores water (succulence) and also performs photosynthesis (replacing the need for large leaves) | Cactus, Euphorbia |
| Fewer stomata | Less stomatal surface area → less water loss | Many xerophytes |
| Hairy leaves (trichomes) | Hair-like structures trap a layer of humid air near the leaf surface, reducing the gradient for diffusion | Mullein, sunflower |
| Nocturnal stomatal opening (CAM plants) | Stomata open only at night (when it's cooler and less windy) to take in CO₂. CO₂ is stored as malic acid and used during the day for photosynthesis with stomata closed | Cactus, Crassula, Bryophyllum, Agave |
📌 FIGURE 4 — Xerophytic Leaf (Sunken Stomata)
Fig: T.S. of a xerophytic leaf showing thick cuticle, sunken stomata, and trichomes — all adaptations to reduce transpiration. ICSE 2019, 2022.
8) Experiments on Transpiration
Experiment 1 — Ganong's Potometer (Measuring Transpiration Rate)
Explained in detail above in the important box. This is the most frequently asked transpiration experiment in ICSE.
Experiment 2 — Weight Loss Method
- Take a potted plant and seal the pot with a plastic sheet (so no water evaporates from the soil)
- Weigh the setup at the start (time = 0)
- Leave it for a set time (e.g., 2 hours in sunlight)
- Weigh again. The loss in weight = water lost by transpiration
- This method can be used to compare transpiration rates under different conditions (with fan / with plastic bag / in shade vs sunlight)
Experiment 3 — Simple Bell Jar Experiment
- Take two potted plants (same type, similar size). Cover the pot of each with a rubber sheet or plastic to prevent soil evaporation
- Place one under a bell jar with anhydrous calcium chloride (a drying agent). Place the other similarly without the drying agent
- Observe after a few hours. The plant with CaCl₂ will show more water droplets on the inner surface of the bell jar because CaCl₂ absorbs water vapour from the air, maintaining a steep gradient → faster transpiration
- This shows that dry air (low humidity) increases transpiration rate
🧠 From my exam — potometer diagram + factors question:
In my board exam, I had a 5-mark question asking me to "Draw a labelled diagram of Ganong's potometer and explain how it works. How would the rate change if (a) a fan is placed near the shoot, (b) the leaf is covered with a plastic bag, (c) the experiment is moved to a dark room?"
Answers: (a) Fan → faster transpiration (wind removes humid air). (b) Plastic bag → slower transpiration (high humidity around leaf, gradient drops). (c) Dark → slower (stomata close in dark). Always connect each factor back to the concentration gradient of water vapour — that's the key principle.
In my board exam, I had a 5-mark question asking me to "Draw a labelled diagram of Ganong's potometer and explain how it works. How would the rate change if (a) a fan is placed near the shoot, (b) the leaf is covered with a plastic bag, (c) the experiment is moved to a dark room?"
Answers: (a) Fan → faster transpiration (wind removes humid air). (b) Plastic bag → slower transpiration (high humidity around leaf, gradient drops). (c) Dark → slower (stomata close in dark). Always connect each factor back to the concentration gradient of water vapour — that's the key principle.
9) Common ICSE Board Questions
1-Mark Definitions:
- Define: Transpiration, Guttation, Transpiration pull, Potometer, Stoma, Guard cells, Hydathode, Lenticel, Cuticle
- Name the three types of transpiration. Which one accounts for the maximum water loss?
- What is the function of the potometer?
- Differentiate between transpiration and evaporation.
- In which form is water lost during transpiration? In which form during guttation?
- What are xerophytes? Give two examples.
- Why do most plants have more stomata on the lower surface of leaves?
- What happens to the transpiration rate if wind speed increases?
- Name the force responsible for the ascent of sap in tall trees.
3-Mark / Short Answer:
- Differentiate between transpiration and guttation (any 4 points in a table). (ICSE 2020, 2023)
- Draw a labelled diagram of a stoma showing two guard cells. Label the stoma, guard cells, chloroplasts, and epidermal cells. (ICSE 2019, 2021)
- Explain the potometer experiment. Draw a labelled diagram. (ICSE 2018, 2020, 2022, 2024)
- List four factors that increase the rate of transpiration. Explain the effect of humidity on transpiration.
- What is the significance of transpiration? List 3 points.
- Describe four adaptations of xerophytes that reduce transpiration. (ICSE 2019, 2022)
- Why do stomata close at night? What happens to transpiration then?
- Differentiate between the three types of transpiration (stomatal, cuticular, lenticular).
- What is a hydathode? How is it different from a stoma?
- Explain how you can demonstrate that transpiration occurs mainly through the lower surface of a leaf using cobalt chloride paper. (ICSE 2021)
5-Mark / Long Answer:
- Draw a neat labelled diagram of Ganong's potometer. Explain the procedure to measure the rate of transpiration. What precautions must be taken? How does the rate change in (a) windy conditions, (b) high humidity, and (c) darkness? (ICSE 2018, 2020, 2022)
- Explain the path of water from the soil to the atmosphere via a plant. Name the forces involved at each step.
- What are the three types of transpiration? Write their relative contributions. Describe an experiment to demonstrate that transpiration occurs through stomata.
- Differentiate between transpiration and guttation. Draw a labelled diagram showing guttation on a leaf. Under what conditions does guttation occur? (ICSE 2020, 2023)
- Describe four adaptations in xerophytes to reduce transpiration. Draw a labelled cross-section of a xerophytic leaf showing sunken stomata. (ICSE 2019, 2022)
- Explain how environmental factors affect the rate of transpiration. Draw a graph showing the effect of light intensity on transpiration rate.
- Describe the bell jar experiment to show that transpiration rate depends on the humidity of the surrounding air.
🧠 From my exam — diagram practice for transpiration:
1. Potometer diagram — the #1 diagram you MUST know (every 2 years, guaranteed).
2. Stoma with guard cells — labelled (guard cells, stoma, chloroplasts, nucleus, epidermal cells).
3. Guttation on leaf margin — showing hydathode and water droplets.
4. Xerophytic leaf T.S. — thick cuticle, sunken stomata, trichomes.
Practise these from memory — each in under 2 minutes. The potometer diagram alone is worth 3-5 marks almost every alternate year.
1. Potometer diagram — the #1 diagram you MUST know (every 2 years, guaranteed).
2. Stoma with guard cells — labelled (guard cells, stoma, chloroplasts, nucleus, epidermal cells).
3. Guttation on leaf margin — showing hydathode and water droplets.
4. Xerophytic leaf T.S. — thick cuticle, sunken stomata, trichomes.
Practise these from memory — each in under 2 minutes. The potometer diagram alone is worth 3-5 marks almost every alternate year.
10) Quick Reference Tables (Print This)
Types of Transpiration
| Type | Site | % Loss | When Active |
|---|---|---|---|
| Stomatal | Stomata (mostly lower leaf) | 80-90% | Daytime (stomata open) |
| Cuticular | Cuticle (leaf surface) | 5-10% | Always (thin leaves lose more) |
| Lenticular | Lenticels (bark/stem) | 1-5% | Always (constant slow loss) |
Transpiration vs Guttation — One Look
| Feature | Transpiration | Guttation |
|---|---|---|
| Form of water | Water vapour (gas) | Liquid water (droplets) |
| Site | Stomata (mainly) | Hydathodes (leaf margins) |
| Force | Transpiration pull (tension) | Root pressure (positive) |
| Time | Daytime | Night / early morning |
| Contains minerals? | No (pure water) | Yes (dilute solution) |
Factors Affecting Transpiration
| Factor | ↑ Increase | ↓ Decrease |
|---|---|---|
| Light | More light → stomata open → more transpiration | Dark → stomata close → less transpiration |
| Temperature | Higher temp → faster evaporation & diffusion | Low temp → slow evaporation |
| Humidity | Low humidity → steeper gradient → more transpiration | High humidity → shallow gradient → less transpiration |
| Wind | Windy → moves humid air away → more transpiration | Still air → humid layer around leaf → less transpiration |
⚠️ Final exam strategy: Transpiration is a 8-12 mark chapter in the board exam. The biggest chunk comes from the potometer experiment (3-5 marks) and transpiration vs guttation (3 marks). Also be ready for "factors affecting transpiration" — often asked as a 3-mark question with a practical application (e.g., "why do plants in a closed car with windows up wilt faster in summer?" — answer: high temp + still air = transpiration still happens but water loss can't be replaced fast enough). Create a one-page revision sheet with: potometer diagram, transpiration types table, transpiration vs guttation table, and factors affecting transpiration. Review it twice before the exam.