Books say that water moves from places with a high water potential to places with a low water potential. The movement of water in plants are explained because water potential is lowest in leaves and highest in roots.
There is also a theory called cohesion-tension theory for explaining the movement of water in plants. This theory says that the water lost at leaves will pull up the chain of water molecules.
I want to know whether the cohesion-tension theory is a part of the water potential explanation or not. For example, if there is a tree with the trunk height of 40 m. The water at 20 m height in the xylem of this tree will move up to 21 m. This is because the water potential in xylem is lower than the water potential at 20 m (?). But I am having a difficult time explaining why it is so by using the cohesion-tension theory. For example, I want to know which water potential is lower at 21 m than at 20 m, and why? I tried to think from the matrix potential, but I was not successful.