The ascent of water in tall trees is mainly due to

Analyze the question: It asks for the main force that moves water up tall trees, which can be 50 m or more in height.

Explain the concept: Water moves up the xylem by the cohesion-tension (transpiration pull) theory:

1. Water evaporates from the leaves through the stomata (transpiration).
2. This lowers the water potential in the leaf mesophyll cells, creating a pull (tension) on the water in the xylem.
3. Water molecules stick to each other (cohesion) and to the xylem walls (adhesion), so the water column stays unbroken and is pulled upward as one continuous stream.
4. Water is drawn in from the roots to replace what is lost.

Evaluate each option:

• A. Adhesive forces: These help water cling to xylem walls (capillarity), but they only support the column and cannot lift water to great heights.
• B. Transpiration pull: This is the driving force. It is the suction created by water loss from the leaves, and it is strong enough to lift water to the tops of tall trees.
• C. Root pressure: It pushes water up from below, but it is weak (it can only push water a few metres). It cannot explain water reaching the top of tall trees.
• D. Cohesive forces: These hold the water column together, so they are necessary, but they are not the force that causes the movement. They transmit the pull rather than create it.

Answer: The main cause is transpiration pull, with cohesion and adhesion as supporting forces.

Exam tip: Remember "Pull from the top, not push from the bottom." For tall trees, think transpiration pull. For small herbaceous plants, or when transpiration is low (at night, for example), root pressure may contribute. If the question says "mainly" or "tall trees," go for transpiration pull.