Mechanisms involved in the transpiration stream
- Xylem vessels are very thin.
- Water is attracted to the lignin in the walls.
- Water has a tendency to rise up the tube.
- This accounts for water moving up by about 50mm up the xylem.
Root pressure (hydrostatic pressure)
- Ions actively pumped into the Endodermal cells.
- This creates a water potential gradient.
- Water enters the endodermis from the cortex by osmosis.
- It then moves into the xylem.
- Accumulation of water in the xylem in the roots causes root pressure.
- This contributes to movement of the water up plants, but cannot fully account for water movement up tall trees.
- Cohesion is the force of attraction between like molecules.
- Hydrogen bonds exist between water molecules.
- As water leaves the vessels a tension is created in the columns of water.
- The columns do not break because of cohesion.
- As water is lost from cells in the leaf, it reduces the water potential inside these cells.
- Therefore, pressure at top of xylem is less than at bottom.
- This stretches the column of water, creating a tension
- This pulls the water up the xylem.
- This process can account for water movement up tall trees.
- Lignin in the xylem walls prevents them collapsing.
Evidence for transpiration
- Dip a narrow glass tube in water.
- Water rises up the tube.
- Cut the shoot of a plant off close to the ground.
- Sap exudes from the xylem.
Passive evaporation from the leaves:
- When it comes to IBDP Biology, Remove leaves from a plant
- Transpiration is reduced
- When it comes to IBDP Biology, The diameter of tree trunk decreases during the day.
- It reaches its minimum size in the afternoon and maximum early in the morning.
- The diameter decreases when transpiration rate is highest.
- This is because of the massive tension in the xylem vessels.
Water moves through the xylem lumen:
- Dyes (eg eosin) can be introduced to the roots.
- These are later found in xylem, not phloem.
- Allow a plant to draw up a fatty solution.
- When it comes to IBDP Biology, This blocks the xylem lumens.
- This prevented further uptake of water.
- The plant wilts.
Water can move sideways:
- When stems bend in the wind, xylem vessels can be stretched.
- When it comes to IBDP Biology, Air bubble form.
- These block individual xylem vessels.
- Water still moves upwards, by moving into adjacent xylem vessels through pits.