Lesson 7 ·Class 10 · Absorption by Roots — The Processes Involved · Flow Lab
Ascent of Sap
Four forces lifting water fifty metres.
A tall tree lifts water more than fifty metres, with no pump anywhere. Four forces do it together.
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⬆️ Step 1 of 6 · 1. Root pressure
The roots push water up the xylem from below. This is enough on its own for herbaceous plants, but nowhere near enough for a tall tree — and it fails entirely in rapidly transpiring plants.
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Root pressure pushes from below; transpirational pull drags from above. Cohesion and adhesion are what make a pull from the top able to move water at the bottom at all.
What this lesson covers
Challenges to clear
- Advance to the force that keeps the water column unbroken.
- Reach the force that does most of the lifting in a tall tree.
- Step through all four forces.
The task
A tall tree lifts water more than fifty metres, with no pump anywhere. Four forces do it together.
What this lesson covers
- 1. Root pressure — The roots push water up the xylem from below. This is enough on its own for herbaceous plants, but nowhere near enough for a tall tree — and it fails entirely in rapidly transpiring plants.
- 2. Capillarity — Water rises on its own in a narrow tube. The narrower the tube, the higher it climbs — and xylem vessels are very narrow indeed.
- 3. Cohesion — Water molecules cling to one another strongly enough to form an unbroken column all the way up the xylem. Break that column with an air bubble and the whole thing fails — which is why a shoot must be cut under water.
- 4. Adhesion — Water molecules also cling to the walls of the xylem vessels. This matters especially in tall trees such as pines, which do not have enough root pressure to rely on.
- The pull from the top — Evaporation at the leaves removes water from the top of the column. Because the column is cohesive, removing water at the top drags the whole column up behind it — the transpirational pull.
- Water reaches the leaves — Together these can draw water up to about 50 metres or more. Only about 2% of what is absorbed is actually used by the plant; the rest is transpired.
Hint
Root pressure pushes from below; transpirational pull drags from above. Cohesion and adhesion are what make a pull from the top able to move water at the bottom at all.