Xylem is a compound tissue consisting of two kinds of elements,xylem vessels and tracheids which help in conducting water and minerals dissolved in water from roots to upper parts of plant.Both vessels n tracheid are non-living plant tissues and highly thick walled. The ascent of sap in the xylem tissue of plants is the upward movement of water from the root to the crown. Although several mechanisms were proposed to explain the phenomenon, none of the theories were completely satisfactory. The most plausible theory is the (cohesion-tension). The theory received criticism however, due to its failure to account for the existence of large negative pressures in some living plants. The most recently proposed CP Theory favors a version of vital theory proposed by Jagdish Chandra Bose.

Other theories include:

• Vital theory
• Root pressure theory
• CP theory (The compensating-pressure theory)

In spite of some debate on the subject, the overwhelming circumstantial evidence strongly supports the cohesion-tension theory.

References

• Xylem Structure and the Ascent of Sap, 2nd ed. by Melvin T. Tyree and Martin H. Zimmermann (ISBN 3-540-43354-6)

• "On the Ascent of Sap" by Henry H. Dixon and J. Joly. Philosophical Transactions of the Royal Society of London. B, Vol. 186, pp. 563-576. (1895)

• "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)