K. Hahn, Vorrastraße 29,
D - 90482 Nürnberg.
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Water transport in trees.
An alternative model to cohesion (transpiration-pull) theory is standing below. This is a novel view of the
long-range water transport in rooted plants.
In nearly every text book of botany cohesion theory is explained. From its point of view water transport in plants
functions as a hydromechanical pulled fluid flow inside the moisture shifting tissue of plants, the xylem.
Inside the leaves the pull shall develop during transpiration. Physically this principle of transport is a mechanical
one. Poiseuille's equation shall explain the assumed fluid flow.
In past doubts about this theory never ended.
Poiseuilles equation was derived experimentally with capillaries produced from glass or metal not reacting with
the transported water. In contrast to xylem, the moved liquid is absorbed, does react with this tissue.
The moisture shifting tissue of plants, the xylem, looks like capillaries similar to that Poiseuilles equation was
derived experimentally. But indeed the long-range moisture transport of intact rooted plants is not based on a
pulled fluid flow via the xylem as assumed in cohesion- or transpiration pull -theory.
The actual transport phenomenon rests on a thermodynamical dislocation of water molecules and free matrical
binding sites for water (1) (2). A nonequilibrium between these vacancies and movable water is forced by solar
energy during transpiration with diffusional phase-transition from liuid to vapour state. The re-establishing of the
equilibrium is associated with a water uptake by the root and a mass motion of water within the xylem to the
shoot and towards the leaves, where the vacancies arose. The observed bulk effekt of moisture transfer is
counterbalanced by binding sites shifting the opposite direction.
At any height of trees the adsorptive forces of sapwood on moved water overcome gravitational tensions.
That is why extremely tensioned, metastable columns of water (the basis of cohesion theory) cannot be formed
in living, intact vascular plants.
(1) Hahn, K. (1993): Der Wasserferntransport in Bäumen. Allgemeine Forst Zeitschrift. 48, 1143-1150
(2) Hahn, K. (1997): Das 'Wasserpotential der Luft' und der Feuchtigkeitstransport in Pflanzen.
Forstarchiv 68, 180-185
last updated: 04. March 2002