The water moves inside the xylem vessel or tracheid based on the capillary rise of water in the xylem. The cohesion, adhesion and surface tension properties of water help in the capillary movement of water in the xylem. It is similar to the movement of fluid in the capillary tube to a small distance. Depending on the radius of the tube, the distance of the fluid movement also changes. As the radius is smaller, the fluid rise in the capillary tube will be more. The movement of water depending on the radius of the xylem is calculated using a formula
capillary rise = 14.9 * 10-6 m2 / radius.
The units of the capillary rise and radius are expressed in meters here.
According to this calculation, if the radius of xylem is 25um, then the distance of water movement will be 0.6 meters. This is not sufficient to explain the water rise in tall trees. So, there is yet another aspect involved other than capillary rise regarding water movement in plants.
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