Transpiration is the loss of water from the exposed surfaces of the plants, especially through the stomata.
Water is constantly being lost from the aerial parts of the plant. This occurs mainly through the stomata of the leaves during daylight hours.
Transpiration reduces the concentration of water in the Mesophyll cells. The water potential of the Mesophyll cell sap becomes lower than the water potential in the neighbouring xylem vessels, which creates a pulling force which draws water out from the xylem vessels in the Mesophyll cells. As the leaf veins are continuous with the xylem vessels in the stem and roots, the force is transmitted through the water column all the way down the roots.
This force called transpiration pull, pulls water up the plant. It requires no energy input from the plant, yet it is the strongest force that causes water to rise up to the leaves of tall trees.
The most important cause of xylem sap flow is the evaporation of water from the surfaces of mesophyll cells to the atmosphere. This transpiration causes millions of minute menisci to form in the mesophyll cell wall. The resulting surface tension causes a negative pressure or tension in the xylem that pulls the water from the roots and soil.
Transpirational pull results from the evaporation of water from the surfaces of cells in the leaves. This evaporation causes the surface of the water to recess into the pores of the cell wall. The high surface tension of water pulls theconcavity outwards, generating enough force to lift water as high as a hundred meters from ground level to a tree's highest branches.
Transpirational pull requires the vessels transporting the water to be very small in diameter, otherwise cavitation would break the water column. And as water evaporates from leaves, more is drawn up through the plant to replace it.
When the water pressure within the xylem reaches extreme levels due to low water input from the roots, the gases then come out of the solution and form a bubble. An embolism forms, which will spread quickly to other adjacent cells, unless bordered pits are present. These have a plug-like structure called a torus, that seals off the opening between adjacent cells and stops the embolism from spreading.