question_answer 34)

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

Answer:

                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 fromstomata, it pulls water molecules up ward to the leaf from the continuous chain of watermolecules carried by xylem. This transpiration from leaf creates a pressure gradient between the atmosphere andsub-stomatal cavity and intercellular spaces of leaf. This gradient is transferred tophotosynthetic 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 axylem column.