How does water really reach the leaves of trees?

Andrew Kenneth Fletcher gravitystudy at hotmail.com
Thu Mar 2 04:41:01 EST 2000


From:  Sci.bio.botany
Kaywan J. <notavailable at notavailable.na> wrote in message
news:38BCB5BC.3691B21E at notavailable.na...
> Hi. I think the cohesion-tension theory can be summed up as follows. The
> pressure differential between the air and the stomata on the leaf causes a
net
> flow of water molecules into the atmosphere. The dipoles on H2O create
> electrostatic interactions that connect neighboring water molecules and
also
> connect water molecules to the cell walls. This combination of cohesion
and
> adhesion creates a continuity of water throughout the plant. The net
result is
> a continous flow of water moving up from the roots into the atmosphere as
> evaporated water is replaced by the neighboring molecules that were
previously
> clinging.
>
> However, your idea on the differences of water potential due to solute
> concentrations is very interesting but suspiciously sounds like bulk-flow.
It
> doesn't seem likely that the lignified cell walls of xylem will facilitate
this
> flow because of their lack of elasticity; it is, IMHO, the elasticity of
phloem
> tissues that contribute greatly to the bulk-flow. Without the hydrostatic
> forces to push the water upwards your mechanism seems inefficient. While
the
> minerals may move up the water may not necessarily follow! (True?)
>

The flow rates experimentally have shown bulk flow in excess of that found
in trees.

A loop of water filled tubing was raised vertically at the centre to a
height of seventy eight feet. A small amount of salt solution was added at
the centre of the tube and both open ends of the tube were placed at the
bottom of two glass water filled bottles prior to raising the centre of the
tube.
Water flowed out of one bottle, while the level of the water in the other
bottle was observed to fall. The flow rate was observed to be very
efficient, and comparible with the fall of the saline solution, under the
influence of gravity.




> A fellow student of mine whose name I've not been told once suggested that
> oxygen dissolves into the water and this creates a pressure differential
which
> presumably would increase the rate of flow out. Perhaps the flow is really
a
> combination of mechanisms with one (the cohesion-tension hypothesis)
> dominating. Anyway, this idea about the oxygen would also in my opinion
require
> elasticity. But I don't have very many facts to work with.
>
>
> Kaywan J.
>
>
> Andrew Kenneth Fletcher wrote:
>
> > Hi Mathew
> >
> > I included the text from GCSE Biol in order to illustrate accepted
theory.
> > If we make this too complicated from the onset, it is more likely to be
> > missed or skipped.
> >
> > Would you be so kind as to sum-up the cohesion-tension theory for people
who
> > may want to follow this thread.
> >
> > I have an interesting theory of my own about the way trees and plants
lift
> > water. It also relies on some aspects of cohesion.
> >
> > The theory is very simple. Evaporation from the leaves, concentrates the
> > liquid in the leaf. Gravity then pulls the concentrated liquid down the
> > tree, which in turn draws more dilute sap up the tree.
> > This simple flow and return system, is an inevitable consequence of what
is
> > in effect, distilled water leaving a liquid which contains minerals, or
> > anything that is heavier than water.
> >
> > Matthew J. Linton <linton at botany.uga.edu> wrote in message
> > news:89gli8$5n9$1 at cronkite.cc.uga.edu...
> > > If you find that "the accepted explanations for fluid transport [are]
> > > somewhat confusing," perhaps you should do additional reading in the
> > field.
> > > The theory of water transport, which is called "the cohesion-tension
> > > theory," is well described in a number of textbooks, the best of which
> > might
> > > be "Plant Physiology" by Frank B. Salisbury and Cleon W. Ross.  Your
local
> > > college or university library probably has a copy of it.  The text
that
> > you
> > > included is VERY POOR at describing xylem and phloem transport,
osmosis,
> > and
> > > root pressure.  There are alternative theories for water transport,
> > notably
> > > one by M.J. Canny 1995 "NEW THEORY FOR THE ASCENT OF SAP-COHESION
> > SUPPORTED
> > > BY TISSUE PRESSURE" Annals Of Botany 75(4):343-357, but this theory
really
> > > hasn't survived a number of follow-up research studies, including
articles
> > > in Nature (378:715-716) and Science (270: 1193-1194).  Although it is
> > > healthy for one to question established theories in science, you can't
> > just
> > > disregard decades and decades of careful research by hundreds of
> > scientists
> > > just because "the accepted explanations...are somewhat confusing."
> > > Good luck in your quest.
> > >
> > > Matthew J. Linton
> > >
> > >
>





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