Ascent of sap
The ascent of sap in the xylem tissue of plants is the upward movement of water and minerals from the root to the crown. Xylem is a complex tissue consisting of living and non-living cells. The conducting cells in xylem are typically non-living and include, in various groups of plants, vessels members and tracheids. Both of these cell types have thick, lignified secondary cell walls and are dead at maturity. Although several mechanisms have been proposed to explain the phenomenon, the cohesion-tension mechanism has the most evidence and support. Although cohesion-tension has received criticism, for example due to the apparent existence of large negative pressures in some living plants, experimental and observational data favor this mechanism. 
An alternative theory based on the behavior of thin films has been developed by Henri Gouin, a French professor of fluid dynamics. The theory is intended to explain how water can reach the uppermost parts of the tallest trees, where the applicability of the cohesion-tension theory is debatable.
The theory assumes that in the uppermost parts of the tallest trees, the vessels of the xylem are coated with thin films of sap. The sap interacts physically with the walls of the vessels: as a result of van der Waals forces, the density of the film varies with distance from the wall of a vessel. This variation in density, in turn, produces a "disjoining pressure", whose value varies with distance from the wall. (Disjoining pressure is a difference in pressure from that which prevails in the bulk of a liquid; it is due to the liquid's interaction with a surface. The interaction may result in a pressure at the surface that is greater or less than that which prevails in the rest of the liquid.) As a tree's leaves transpire, water is drawn from the xylem's vessels; hence, the thickness of the film of sap varies with height within a vessel. Since the disjoining pressure varies with the thickness of the film, a gradient in the disjoining pressure arises during transpiration: the disjoining pressure is greater at the bottom of the vessel (where the film is thickest) and less at the top of the vessel (where the film is thinner). This spatial difference in pressure within the film results in a net force that pushes the sap upwards towards the leaves.
- Henry H. Dixon and J. Joly (1895) "On the Ascent of Sap", Philosophical Transactions of the Royal Society of London. B, 186 : 563–576.
- Xylem Structure and the Ascent of Sap, 2nd ed. 2002. by Melvin T. Tyree and Martin H. Zimmermann (ISBN 3-540-43354-6) Springer-Verlag
- "The Cohesion-Tension Theory" by Angeles G, Bond B, Boyer JS, Brodribb T, Brooks JR, Burns MJ, Cavender-Bares J, Clearwater M, Cochard H, Comstock J, Davis SD, Domec J-C, Donovan L, Ewers F, Gartner B, Hacke U, Hinckley T, Holbrook NM, Jones HG, Kavanagh K, Law B, López-Portillo J, Lovisolo C, Martin T, Martínez-Vilalta J, Mayr S, Meinzer FC, Melcher P, Mencuccini M, Mulkey S, Nardini A, Neufeld HS, Passioura J, Pockman WT, Pratt RB, Rambal S, Richter H, Sack L, Salleo S, Schubert A, Schulte P, Sparks JP, Sperry J, Teskey R, Tyree M. New Phytologist, Vol. 163:3, pp. 451–452. (2004)
- Stiller, Volker and John S. Sperry (1999) "Canny's compensating pressure theory fails a test," American Journal of Botany, 86 : 1082–1086.
- Henri Gouin (October 2008) "A new approach for the limit to tree height using a liquid nanolayer model," Continuum Mechanics and Thermodynamics, 20 (5) : 317-329. Available on-line at: Arxiv.org
- Henri Gouin (2011) "Liquid-solid interaction at nanoscale and its application in vegetal biology," Colloids and Surfaces A, 383 : 17–22. Available on-line at: Arxiv.org
- Henri Gouin (2012) "The nanofluidics can explain ascent of water in tallest trees". Available on-line at: Arxiv.org
- Henri Gouin (2014) "The watering of trees. Embolization and recovery in xylem microtubes." Available on-line at: Arxiv.org
- Tyree M.T. (1997) "The cohesion-tension theory of sap ascent: current controversies," Journal of Experimental Botany, 48 : 1753-1765.
- Koch, W.; Sillett, S.C.; Jennings, G.M.; Davis, S.D. (2004) "The limit to tree height," Nature, 428 : 851-854.