Calculating Capillary Rise
We learn on page 86 of the textbook that the water properties of cohesion, adhesion, and surface tension give rise to the phenomenon of capillarity, the movement of water for small distances up a capillary tube. The smaller the tube radius, the higher the capillary rise. How far the water will move may be calculated using the following formula:
where both capillary rise and radius are expressed in meters.
For a xylem vessel with a 25 µm radius, the capillary rise is about 0.6 m. This distance is much too small to be significant for water transport up tall trees.
Fibrous materials such as cell walls can act like wicks to draw water by capillarity from nearby xylem. This capillary action ensures that cell wall surfaces that are directly exposed to the air, such as those in leaf mesophyll, remain wetted and do not dry out. Because the cell wall capillaries have a tiny radius, about 10 m–8, very large physical forces can be generated in the water just below the evaporative surfaces of cell walls.