Transpiration, Xylem and Phloem: Structure, Function and Factors (ICSE Class 8 Biology)
Updated 17 October 2026 · Fact-checked
Transpiration is the loss of water vapour from the aerial parts of a plant, mostly through stomata, and it pulls water up the xylem. Xylem carries water and minerals upward through dead vessels and tracheids. Phloem carries food made in the leaves in both directions through living sieve tubes helped by companion cells.
Understand Transpiration, Xylem and Phloem
Transpiration. The loss of water as water vapour from the aerial parts of a plant, mainly through the stomata of leaves, with a little through the cuticle and through lenticels in bark.
Why it matters.
1. Transpiration pull: evaporation from leaf cells draws more water up the xylem in an unbroken column, bringing minerals with it.
2. Cooling: evaporation takes heat from the leaf.
3. Keeps cells firm and moves minerals to every part.
Too much loss when roots cannot supply water causes wilting.
Factors that change the rate.
| Factor | Effect | Reason |
|---|---|---|
| Light | rate rises | stomata open |
| Temperature | rate rises | faster evaporation |
| Humidity | rate falls | air already holds vapour |
| Wind | rate rises | vapour is carried away |
| Soil water | less water, lower rate | stomata close |
| Leaf area, stomata | more, higher rate | more surface |
Showing transpiration. A polythene bag tied over a leafy shoot fogs with droplets (cover the soil so that evaporation from it is not mistaken for transpiration). Dry blue cobalt chloride paper on a leaf turns pink. A potometer times a moving air bubble to measure water uptake, which is close to the rate of transpiration.
Vascular bundles. In stems and leaf veins, xylem and phloem lie together in the same vascular bundle. In roots they lie on separate, alternating radii.
Xylem (water and minerals, upward only).
- Vessels: long hollow dead tubes whose end walls broke down; walls thickened with lignin.
- Tracheids: long, narrow dead cells with tapering ends; water passes through pits.
- Xylem parenchyma: the only living cells; store food.
- Xylem fibres: dead, thick-walled, give support. Wood is mostly xylem.
Phloem (food, mainly sucrose, up and down): this movement is translocation.
- Sieve tubes: living cells joined end to end; end walls are perforated sieve plates; mature sieve tubes lack a nucleus.
- Companion cells: small living cells with a nucleus beside each sieve tube, which control it.
- Phloem parenchyma: living, store food.
- Phloem fibres: dead, for support (jute fibre is of this kind).
| Xylem | Phloem | |
|---|---|---|
| Carries | water and minerals | food (sugar) |
| Direction | upward | upward and downward |
| Cells | dead vessels and tracheids | living sieve tubes |
| Extra role | support | none of note |
Ringing experiment. Remove a ring of bark, which includes the phloem, from a woody stem. After some days the bark above the ring swells because food coming down from the leaves collects there, while the leaves stay fresh for a while because the xylem inside is untouched. This shows that food travels in phloem and water in xylem.
Draw-and-label checklist: vascular bundle (stem, transverse view). Xylem towards the inside with large vessels, phloem towards the outside with sieve tubes and smaller companion cells, labelled.
Transpiration, Xylem and Phloem: key facts
- Transpiration
- Loss of water vapour
- Mainly through stomata
- Gives transpiration pull
- Cools the leaf
- Rises with light, heat and wind; falls with humidity.
- Xylem
- Water and minerals
- Upward only
- Vessels and tracheids (dead)
- Lignified, supports
- Xylem = water.
- Phloem
- Food (sucrose)
- Up and down
- Sieve tubes (living) and companion cells
- Translocation
- Phloem = food.
How to answer Transpiration, Xylem and Phloem questions
Typical items: define transpiration, state the effect of a factor with a reason, differentiate xylem and phloem, predict the result of ringing, or read a potometer.
- 1For any factor, give the direction (rises or falls) and one reason linked to stomata or evaporation.
- 2For xylem and phloem, state what is carried, the direction and whether the cells are living or dead.
- 3For the ringing experiment, say above the ring (swells) and below the ring (starves), and name phloem.
- 4For a potometer, convert distance moved by the bubble into volume: distance times bore area.
- 5Link wilting to loss of water faster than uptake.
Transpiration, Xylem and Phloem in real life
The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.
Washing dries faster in wind
Suppose two identical wet cloth strips are hung out, one in a still room and one outdoors on a warm, breezy day, and the outdoor one dries in half the time.
How does this help you explain why a potted plant wilts faster outdoors on such a day?
Try it first, then show the solutionSolution
- Warmth speeds evaporation and wind carries away the vapour near the surface.
- The same two factors raise the rate of transpiration from leaves.
- If roots cannot replace the water as fast, leaf cells lose firmness and the plant wilts.
Answer: Warm moving air raises transpiration beyond the rate of water uptake, so the plant wilts.
Memory hook: Hot and windy: leaves lose water quickly.
Sugar sap in a sugarcane field
Suppose a farmer cuts a mature sugarcane stalk and finds the inner part sweet, while the cut leaf above it, in the same plant, tastes only slightly sweet.
Which tissue would you say brought the sugar to the stalk, and in which direction did it flow?
Try it first, then show the solutionSolution
- Sugar is made in the leaves by photosynthesis.
- Food leaves the leaves through phloem and is carried to storage regions such as the stalk.
- The flow is therefore downward from the leaves to the stalk.
Answer: Phloem, by translocation downward from the leaves to the stalk.
Memory hook: Sweet stalk: sugar went down by phloem.
Tricks to remember Transpiration, Xylem and Phloem
Short memory aids. Say each one aloud once, then use it on a question the same day.
X for up
Xylem carries water Up; Phloem carries Provisions (food).
Use it when: Telling the tissues apart.
Four plus
Light, heat and wind raise transpiration; humidity lowers it.
Use it when: Factors table.
Sieve with a helper
A sieve tube has no nucleus, so its companion cell has one for both.
Use it when: Phloem structure.
Transpiration, Xylem and Phloem: solved examples
Ordered from board level to HOTS. Try each one before you open the solution.
Example 1: Companion cell · 1 mark · MCQBoard level
The cell that sits beside a sieve tube and keeps it working is the:
(a) tracheid (b) companion cell (c) xylem fibre (d) guard cell
Try it first, then show the solutionSolution
- Companion cells are living and nucleated and control the sieve tube.
Answer: (b).
Example 2: Humidity · 2 marks · Short answerBoard level
State the effect of high humidity on transpiration and give a reason.
Try it first, then show the solutionSolution
- It reduces the rate (1 mark).
- The air already holds much water vapour, so less vapour passes from the leaf to the air (1 mark).
Answer: Transpiration falls because the air is nearly saturated with vapour.
Example 3: Reading a potometer · 3 marks · Short answerHarder board level
In a potometer, a bubble moves 18 cm in 30 minutes along a capillary of bore area 0.02 cm². (a) Calculate the volume of water taken up in that time. (b) Calculate the rate in cm³ per hour. (c) Why is this only an estimate of transpiration?
Try it first, then show the solutionSolution
- (a) Volume = 18 × 0.02 = 0.36 cm³ (1 mark).
- (b) In one hour: 0.36 × 2 = 0.72 cm³ per hour (1 mark).
- (c) Some absorbed water is used in photosynthesis or kept in the plant, so uptake is slightly more than the water lost (1 mark).
Answer: (a) 0.36 cm³. (b) 0.72 cm³ per hour. (c) Not all absorbed water is transpired.
Example 4: Sieve tubes without nuclei · 3 marks · Short answerHarder board level
Mature sieve tubes lack a nucleus yet stay alive and work for a long time. Name the cell that makes this possible and state two roles of phloem.
Try it first, then show the solutionSolution
- The companion cell, which has a nucleus and controls the sieve tube (1 mark).
- Phloem carries food made in the leaves to growing tips, fruits, roots and storage organs (1 mark).
- It carries it in both directions, upward and downward (1 mark).
Answer: Companion cell; phloem conducts food (translocation) in both directions.
Example 5: Two bags · 3 marks · Case-basedHOTS / competency
Two potted shoots of the same size are covered with transparent polythene bags. Shoot A is kept in bright sunlight; shoot B in shade. After two hours A has more droplets in the bag. (a) Explain the difference. (b) What control would you use to be sure that the water came from the leaves and not from the soil? (c) Name one other factor that would give a similar result to the sunlight.
Try it first, then show the solutionSolution
- (a) Light opens stomata and warms the leaf, so A transpires more (1 mark).
- (b) Cover the soil and pot with polythene so that evaporation from soil cannot reach the bag (1 mark).
- (c) Higher temperature or moving air (1 mark).
Answer: A transpires more because light opens stomata; cover the soil; warmth or wind also raises the rate.
Where students lose marks in Transpiration, Xylem and Phloem
The slips examiners see most often, and what to write instead.
Mistake: Saying xylem carries food
Students remember that vessels are tubes and assume they carry everything.
Do this: Xylem: water and minerals; phloem: food.
Usually costs: 1 mark.
Mistake: Saying humidity increases transpiration
Humid air is mistaken for a source of water for the leaf.
Do this: High humidity lowers the rate; light, heat and wind raise it.
Usually costs: 1 mark.
Mistake: Giving only direction for phloem
Candidates write that it conducts downward only.
Do this: Write up and down, and name translocation.
Usually costs: 1 mark.
Mistake: No reason for the factor
Answers state that light increases transpiration without reasoning.
Do this: Add a reason such as stomata open in light.
Usually costs: 1 mark each.
Exam tips
- Xylem = water, upward, dead cells; phloem = food, both ways, living cells.
- Give a reason with each factor affecting transpiration.
- Cover the soil in the polythene-bag experiment.
- Ringing: swelling above the ring shows phloem carries food.
Practice questions on Transpiration, Xylem and Phloem
Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.
Q1 · 1 mark · Very short answerBoard level
Define transpiration.
Hint
Vapour, aerial parts.
Answer
The loss of water in the form of water vapour from the aerial parts of a plant, mainly through the stomata.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Water can be pulled up a tall tree even when there is no root pressure.
Reason (R): Evaporation of water from the leaves pulls a continuous column of water up the xylem.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation of (A).Hint
Transpiration pull.
Answer
(c). Transpiration pull is the main force raising water, with or without root pressure.
Q3 · 3 marks · Short answerHarder board level
Give three differences between xylem and phloem.
Hint
Substance, direction, cell type.
Answer
Xylem carries water and minerals; phloem carries food. Xylem conducts upward only; phloem conducts both ways. Xylem conducting cells (vessels, tracheids) are dead; phloem conducting cells (sieve tubes) are living.
Q4 · 3 marks · Long answerHarder board level
A leafy branch placed in water in the sun loses water faster than one in shade, and faster in dry air than in damp air. Explain both results in terms of stomata and evaporation.
Hint
Light and humidity.
Answer
In sunlight stomata open wide and the leaf warms, so more water evaporates and passes out. In dry air the difference in vapour between the leaf and the air is large, so water leaves faster; in damp air the air already holds vapour, so less leaves.
Q5 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a farmer notices that the leaves of her chilli plants droop at noon but recover by evening, although the soil is moist.
(a) Name the process that causes the noon drooping. (1)
(b) Suggest why recovery happens in the evening. (1)
(c) Name the tissue that supplies water to the leaves. (1)Hint
Water lost faster than supplied.
Answer
(a) Transpiration, which at noon exceeds the rate of water uptake. (b) In the evening it is cooler and stomata close, so water is lost more slowly than it is absorbed and the cells become firm again. (c) Xylem.
Test your recall
Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.
1.What do we call the small openings on bark that let a little vapour escape?Show answer
Lenticels.
2.Name the process of food movement in phloem.Show answer
Translocation.
3.Which substance thickens the walls of vessels?Show answer
Lignin.
4.Which cells help a sieve tube?Show answer
Companion cells.
5.What turns pink in the cobalt chloride test?Show answer
Dry blue cobalt chloride paper.
Transpiration, Xylem and Phloem: frequently asked questions
Is transpiration a waste of water?
It looks wasteful, but it pulls water and minerals up and cools the leaf.
Are lenticels and stomata the same?
No. Stomata are pores in leaves; lenticels are small openings in the bark of stems.
Why is wood mostly xylem?
Because xylem cells have thick lignified walls and are dead, so they form strong tissue.
Where are mineral deficiency diseases covered?
In the chapter page, which treats macro- and micronutrients and deficiency diseases.