How does water reach the leaves at the top of a 100-metre-tall tree with no pump?
As leaves transpire, water evaporates from their surfaces and creates a suction (negative pressure) in the xylem.
As leaves transpire, water evaporates from their surfaces and creates a suction (negative pressure) in the xylem. Water molecules cling together by cohesion (hydrogen bonding) and stick to the xylem walls by adhesion, forming one unbroken column from root to leaf. So the evaporating leaves effectively pull the whole water column upward — the cohesion-tension mechanism — without any pump, defeating gravity even in the tallest trees.
Key point
Transpiration creates suction; water's cohesion pulls an unbroken column up the xylem (cohesion-tension).
Common mistake
imagining the roots pump water all the way to the top.
Memory tip
the leaves pull, cohesion holds the column together — no pump needed.
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