By the end of this lesson
- Describe substances and direction in xylem.
- Relate vessel structure to function.
- Explain transpiration pull at an appropriate level.
BIG QUESTION
How can water rise through a tall plant without a mechanical pump?
Evaporation creates tension in continuous water columns held together by cohesion and supported by lignified vessels.
Xylem carries an upward stream
xylemPlant tissue of dead, hollow, lignified vessels that carries water and mineral ions from the roots up to the stems and leaves, and helps support the plant. transports water and dissolved mineral ions from roots to stems and leaves.
The flow supplies photosynthesis, turgor, cooling and biochemical needs.
Vessels are dead and reinforced
Vessel elements lose end walls and contents, forming long hollow tubes with low resistance. ligninA tough, waterproof substance in the walls of xylem vessels that strengthens them and the plant. thickens walls, waterproofs them and prevents collapse under tension.
Pits allow lateral movement between vessels and surrounding tissue.
Evaporation pulls a continuous column
Evaporation from mesophyll cell walls lowers leaf water potential. Water moves from xylem into leaf cells, creating tension.
Cohesion between water molecules transmits this pull down the column; adhesion and root pressure can contribute but do not replace transpiration pull.
Xylem and phloem
Vascular tissues differ in material, direction and cellular state.
| Feature | Xylem | Phloem |
|---|---|---|
| Main contents | Water and mineral ions. | Sucrose and amino acids. |
| Direction | Mainly root to shoot. | Source to sink, either direction. |
| Conducting cells | Dead, hollow vessels. | Living sieve tubes. |
| Wall/support | Lignified and supportive. | Not heavily lignified. |
| Driving process | Transpiration tension/pressure differences. | Source–sink pressure flow. |
Language that earns precision
- xylem
- Vascular tissue transporting water and mineral ions and providing support.
- lignin
- Waterproof strengthening material deposited in xylem walls.
- cohesion
- Attraction between water molecules.
- transpiration pull
- Tension generated by evaporation that draws water through xylem.
Common misconceptions
Claim: Xylem pumps water using living cells.
Correction: Mature vessels are dead; evaporation-driven tension moves the water column.
Claim: Mineral ions travel in phloem only.
Correction: Dissolved mineral ions absorbed by roots travel mainly in xylem.
Claim: Water rises because it is lighter than air.
Correction: Pressure/tension gradients and cohesive water columns drive movement.
Match the answer to the command word
State water and mineral ions, roots to other plant parts.
Use evaporation → tension → cohesion → upward movement.
Recall, apply, then justify
Explain three xylem vessel adaptations.
Hint
Hollow, joined and lignified.
Suggested marking points
- Cells are dead/hollow with no cytoplasm.
- This reduces resistance to water flow.
- End walls are absent/perforated to form continuous tubes.
- This permits long-distance flow.
- Walls are lignified.
- Lignin prevents collapse and provides support.
Explain how transpiration can move water upward.
Hint
Start at the leaf surface.
Suggested marking points
- Water evaporates from mesophyll surfaces.
- Water is drawn from leaf xylem, creating tension.
- Cohesion keeps water molecules in a continuous column.
- The tension pulls water upward from roots.
Explain why an air bubble entering a xylem vessel can reduce transport.
Planning hint
Continuity matters.
What a strong design includes
- Transpiration pull places water under tension.
- Cohesion maintains a continuous column.
- An air bubble can break that continuity.
- Tension is not transmitted effectively past the embolism.
- Pits and parallel vessels may allow bypass, limiting damage.