Water moves up against gravity and even for a tree of 20 m height, the ti receives water within two hours The most important Physiologica phenomenon.  which is responsible for the upward movement of water is ............ .

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 frorr Stomata, it pulls water molecules upward 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 “transpirational pull” that can lift water up to 130m high, through a xylem column.