Water moves up against gravity and even for a tree of `20m`height the tip receives water within two hours . The most important physiological phenomenon which is responsible for the upward movement of water is `"______"`

Explanation

Transpiration pull is the physiological phenomenon which is responsible for the upward movement of water in trees of height up to 20m. As the water molecules transpire from stomata, it pulls water molecules up ward to the leaf from the continuous chain of water molecules carried by xylem.
This transpiration from leaf creates a pressure gradient between the atmosphere and sub-stomatal cavity and intercellular spaces of leaf. This gradient is transferred to photosynthetic cells and then on to xylem of leaf vein.
It results in a force called "transpiration pull" that can lift water upto 130m high, through a xylem column.