Ascent of Sap: Transportation of Water in Plants
What is Ascent of Sap?
Ascent of sap is the upward movement of water and dissolved minerals from roots to the stem, branches and leaves through xylem tissues.
Path of Ascent of Sap
The movement of sap occurs through the tracheary elements of xylem.
Stain Test
A freshly cut leafy shoot is placed in eosin solution. After some time, leaves and flowers become red.
Result
Microscopic examination shows coloration in xylem vessels and tracheids, proving that water moves through xylem.
Ringing Experiment
A ring of bark is removed from one shoot while xylem is removed from another.
Observation
- Leaves remain turgid when bark is removed.
- Leaves wilt when xylem is interrupted.
Result
This confirms that xylem is the main tissue responsible for ascent of sap.
Theories of Ascent of Sap
Several theories explain how water rises from roots to the tops of plants.
Vital Force Theory
Proposed by J. C. Bose, this theory suggests that living root cells actively pump water into xylem.
Limitation
Water continues to move even when living cells are destroyed, making the theory inadequate.
Root Pressure Theory
Proposed by Priestley, root pressure is the positive pressure developed in xylem due to active absorption of water.
Merits
- Osmotic mechanism supports water movement.
- Electro-osmotic potential aids transport.
- Living cells can contribute to water movement.
Demerits
- Absent in many gymnosperms.
- Occurs only under favorable conditions.
- Insufficient to explain water rise in tall trees.
- Water movement continues without roots.
Theory of Capillarity
Water rises through narrow tubes because of cohesion, adhesion and surface tension.
Demerits
- Capillarity can raise water only about 1 meter.
- Cannot explain water transport in tall trees.
- Not applicable to many tracheid-bearing plants.
Cohesion-Tension / Transpiration Pull Theory
Proposed by Dixon and Joly, this is the most accepted theory of ascent of sap.
Main Concepts
- Continuous water column in xylem.
- Cohesion between water molecules.
- Adhesion between water and xylem walls.
- Transpiration creates negative pressure.
- Tension pulls water upward.
Merits
- Explains water transport up to 130 meters.
- Supported by cohesion and tensile strength of water.
- Accounts for transpiration pull.
Demerits
- Air bubbles may break the water column.
- Observed tensions sometimes exceed calculated cohesion values.
- Some experimental results remain difficult to explain.
Frequently Asked Questions
What is ascent of sap?
The upward movement of water and minerals through xylem.
Which tissue conducts ascent of sap?
Xylem vessels and tracheids.
Who proposed the cohesion-tension theory?
Dixon and Joly.
Which theory is most accepted?
The Cohesion-Tension (Transpiration Pull) Theory.