How water climbs without a pump

A redwood lifts water 330 feet (100 m) with no machinery. Just physics, and you can drive it below.

A coast redwood moves water from its roots to its leaves more than 330 feet (100 m) up. The tallest measured in one landmark survey stood at 369.8 feet (112.7 m), and the same study put the theoretical ceiling somewhere around 400 to 427 feet (122 to 130 m) (Koch et al., 2004). No pump. No active machinery. The mechanism is cohesion-tension, and it's one of the most extraordinary things in biology.

Water molecules stick to each other (cohesion, from hydrogen bonds) and to the xylem walls (adhesion). When water evaporates from a leaf at the top, it tugs the molecule just below it. That one, stuck to all its neighbors and to the walls, gets pulled up, pulling the next, and the next. The whole column, root to leaf, behaves like a continuous chain. Pull at the top, the bottom rises (Tyree & Zimmermann, 2002). The evaporation doing the pulling has a name, transpiration, and Lesson 8 is about its budget.

This is why xylem walls are reinforced: they resist collapsing under the tension of a column being pulled upward all day (Venturas et al., 2017). (Cut a flower stem in air and water rushes in as the tension releases. That's why florists cut stems underwater, to keep the column unbroken.)

Cohesion-tension diagram: continuous water column rising through xylem.

Fig. 6. Evaporation at the leaves creates tension that pulls the whole water column up through the xylem. Molecules cohere to each other and adhere to the walls. No pump required.

Drive the system Tier 3 · simulation

The transpiration pull

Raise the light and the leaves transpire faster. Watch what that does to the column and the tension. This is the concept a static diagram can't show, so it's built, not drawn.

Transpiration rate

Column tension

Water reaching the leaves

Light on the leaves — low → high

More light, the leaf's pores open, faster evaporation, more tension, faster rise. Take the light away and the whole column slows.

From memory Tier 1

Strengthen your understanding

Try these in your head first, then open to see.

In the simulation, dropping the light to near zero slowed the column to a crawl. In one sentence, why?

Transpiration is the engine: less evaporation at the leaves means less tension pulling the column, so it rises slower. There's no pump to take over the work.

Why do florists recut flower stems underwater?

Cutting in air lets the tension release and air rush into the xylem, breaking the water column. Recutting underwater keeps it continuous so the flower can still draw water up.

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