Transpirational pull
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Transpirational pull is the main phenomenon driving the flow of sap in the xylem tissues of large plants.
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 pull back 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. This only works because the vessels transporting the water are very small in diameter, otherwise cavitation will break the water column.
Until recently, the negative pressure could only be measured indirectly, by applying external pressure with a Scholander bomb to counteract it. The name comes from the inventor, PF 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. This is a very complicated subject, as xylem transport is driven by a combination of transpirational pull from above and root pressure.
A common misconception is that water moves in plants 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 plants, as described above, but the relevant surface occurs at the site of evaporation within leaves, not within the xylem conduits, and that surface does not typically proceed along any conduit. Water movement within the xylem conduits is driven by a pressure gradient, not by capillary action.
More at suction.
[edit] See also
- cohesion-tension theory
- secondary xylem
- secondary growth
- soil plant atmosphere continuum
- vascular tissue
- vascular bundle
- xylem
[edit] References
- The comprehensive textbook Biology, 6th ed., by Neil A. Campbell and Jane B. Reece, published by Benjamin Cummings, has been used as a source.
- C. WEI, E. STEUDLE, M. T. TYREE3 & P. M. LINTILHAC, The essentials of direct xylem pressure measurement, Plant, Cell and Environment (2001) 24, 549?555, is the main source used for the paragraph on recent research.
- N. Michele Holbrook, Michael J. Burns, and Christopher B. Field, Negative Xylem Pressures in Plants: A Test of the Balancing Pressure Technique, Science 270 (1995), 1193, is the first published independent test showing the Scholander bomb actually does measure the tension in the xylem.
- Pockman, W.T., J.S. Sperry and J.W. O'Leary. Sustained and significant negative water pressure in xylem. Nature 378 (1995) 715-716, , is the second published independent test showing the Scholander bomb actually does measure the tension in the xylem.
- Xylem Structure and the Ascent of Sap, 2nd ed. by Melvin T. Tyree & Martin H. Zimmermann (ISBN 3-54043-354-6) recent update of the classic book on xylem transport by the late Martin Zimmermann
