Xylem forms continuous pipe from roots to leaves.
No osmosis occurs in xylem, just mass flow.
Transpiration in leaves causes low presure in leaves and water is sucked up the stem to replace lost water.
Column of water in stem is under tension, water has a high tensile strength because water molecules stick together by H-bonds (Cohesion)
Called Cohesion-Tension Mechanism.
Very strong lignin walls stop xylem collapsing.
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