Xylem capillary rise

Although Equation 2.2 refers to the height of capillary rise only in a static sense, it still has important implications concerning the movement of water in plants. To be specific, let us consider a xylem vessel having a lumen radius of 20 pm. From Equation 2.2b, we calculate that water will rise in it to the following height ... 

Such a capillary rise would account for the extent of the ascent of water in small plants, although it says nothing about the rate of such movement (to be considered in Chapter 9, Section 9.4D). For water to reach the top of a 30-m-tall tree by capillary action, however, the vessel would have to be 0.5 pin in radius. This is much smaller than observed for xylem vessels, indicating that capillary rise in channels of the size of xylem cells cannot account for the extent of the water rise in tall trees. Furthermore, the lumens of the xylem vessels are not open to air at the upper end, and thus they are not really analogous to the capillary depicted in Figure 2-3. 

The numerous interstices in the cell walls of xylem vessels form a mesh-work of many small, tortuous capillaries, which can lead to an extensive capillary rise of water in a tree. A representative radius for these channels in the cell wall might be 5 nm. According to Equation 2.2b, a capillary of 5 nm radius could support a water column of 3 km—far in excess of the needs of any plant. The cell wall could thus act as a very effective wick for water rise in its numerous small interstices, although such movement up a tree is generally too slow to replace the water lost by transpiration. 

Example 7.2. Water in trees rises in capillaries which are called xylem. They are typically 5-170 //,m in radius and are completely wetted (0 = 0). What is the maximum height water can rise in such capillaries To calculate an upper limit we use a capillary radius of 5 jim. Then... 

The intermolecular attraction between like molecules in the liquid state, such as the water-water attraction based on hydrogen bonds, is called cohesion. The attractive interaction between a liquid and a solid phase, such as water and the walls of a glass capillary (a cylindrical tube with a small internal diameter), is called adhesion. When the water-wall adhesion is appreciable compared with the water-water cohesion, the walls are said to be wettable, and water then rises in such a vertical capillary. At the opposite extreme, when the intermolecular cohesive forces within the liquid are substantially greater than is the adhesion between the liquid and the wall material, the upper level of the liquid in such a capillary is lower than the surface of the solution. Capillary depression occurs for liquid mercury in glass capillaries. For water in glass capillaries or in xylem vessels, the... 

It also causes the capillary action of water, which enables water to rise up the internal tubes (xylem) within a plant to reach the top of high trees. 

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