Transpirational pull

Transpirational pull is the main phenomenon driving the flow of water in the xylem tissues of large plants.

Mechanisms
Transpirational pull results ultimately from the evaporation of water from the surfaces of cells in the interior of the leaves. This evaporation causes the surface of the water to recess into the pores of the cell wall. Inside the pores, the water forms a concave meniscus. The high surface tension of water pulls the concavity outwards, generating enough force to lift water as high as a hundred meters from ground level to a tree's highest branches. Transpirational pull only works because the vessels transporting the water are 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 (if, for example, the soil is dry), then the gases within the solute come out of 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).

Measurement
Until recently, the negative pressure (suction) of transpirational pull could only be measured indirectly, by applying external pressure with a Scholander bomb to counteract it. The name comes from the inventor, P.F. Scholander, and from its disconcerting tendency to explode in the experimenter's face. When the technology to perform direct measurements with a pressure probe was developed, there was initially some controversy about whether the classic theory was correct, because some workers were unable to demonstrate negative pressures. More recent measurements do tend to validate the classic theory, for the most part. Xylem transport is driven by a combination of transpirational pull from above and root pressure from below, which makes the interpretation of measurements more complicated.

A common misconception is that water moves in xylem by capillary action—the movement of water along a small-diameter conduit (such as a capillary) as a result of surface tension in the meniscus at the leading surface of the moving water. Surface tension does play a critical role in water movement in xylem, as described above, but the relevant force acts at the surface site of evaporation within leaves, not within the xylem conduits. Water movement within the xylem conduits is driven by a pressure gradient created by such force, not by capillary action. Specifically, the evaporation and transpiration of water in the leaves causes water in the xylem to move from the roots, which have a higher water potential, up the stem of the plant that has a decreasing water potential along its length.

Cohesion-tension theory
The cohesion-tension theory is a theory of intermolecular attraction commonly observed in the process of water traveling upwards (against the force of gravity) through the xylem of plants, which was put forward by John Joly and Henry Horatio Dixon.

Water is a polar molecule due to the high electronegativity of the oxygen atom, which is an uncommon molecular configuration whereby the oxygen atom has two lone pairs of electrons. When two water molecules approach one other they form a hydrogen bond. The negatively charged oxygen atom of one water molecule forms a hydrogen bond with a positively charged hydrogen atom in another water molecule. This attractive force has several manifestations. Firstly, it causes water to be liquid at room temperature, while other lightweight molecules would be in a gaseous phase. Secondly, it (along with other intermolecular forces) is one of the principal factors responsible for the occurrence of surface tension in liquid water. This attractive force between molecules allows plants to draw water from the root (via osmosis) and then through the xylem to the leaf where photosynthesis converts water and carbon dioxide into glucose.

Water is constantly lost by transpiration in the leaf. When one water molecule is lost another is pulled along. Transpiration pull, utilizing capillary action and the inherent surface tension of water, is the primary mechanism of water movement in plants. However, it is not the only mechanism involved. Any use of water in leaves produces forces that causes water to move into them.