Explain the ascent of sap by root pressure and by transpiration pull.

1. 1 · Define in one line. Ascent of sap is the upward movement of water and dissolved minerals from the roots to the leaves through the xylem.
2. 2 · Root pressure.
   • Continuous absorption builds a pressure in the root that pushes water up.
   • Evidence: sap oozing from cut stems, and guttation in grass.
   • Limit: lifts water only a short height; cannot explain tall trees.
3. 3 · Transpiration pull.
   • Dixon and Joly (1894): the cohesion-tension theory.
   • Transpiration from the leaves creates tension in the xylem.
   • Cohesion keeps the water column unbroken; adhesion holds it to the xylem walls.
   • The pull passes down to the roots and lifts the water.
4. 4 · Draw a diagram. [A tall plant with an unbroken water column in the xylem. Arrows: water vapour leaving the stomata, the pull down the stem, water entering the root hairs]
5. 5 · Real life. Coconut and eucalyptus trees lift water many metres every day by transpiration pull.
6. 6 · Close in one line. Root pressure gives a small push; transpiration pull does most of the lifting.

Scoring tip: Write the words cohesion, adhesion and tension; they are the key words for the transpiration pull theory.