Transpirational pull model:
(a) The flow of water upward through the xylem is 15 meter per hour. Most scientists agree that water
is simply 'pulled' through the plant and that the driving force for the process is transpiration pull from leaves.
(b) This is referred to as the cohesion tension transpiration pull model of water movement in the leaf. Evaporation from the leaf sets up a pressure gradient between the outside air and the air spaces
of the leaf.
The gradient is transmitted into the photosynthetic cell and on the water-filled xylem in the leaf vein