How does water really reach the leaves of trees?

Bob Vickery vickery at mpx.com.au
Sat Mar 4 01:12:36 EST 2000


In article <89f3bb$s0a$3 at newsg1.svr.pol.co.uk>,
"Andrew Kenneth Fletcher" <gravitystudy at hotmail.com> wrote:

>Hi everyone
>
>Could we re-open this topic for discussion? I feel I have something new to
>add to this field and wander if others find the accepted explanations for
>fluid transport somewhat confusing.
>
>Has anyone developed a working model which demonstrates a lift of water
>higher that the 10 metre limit set down in the physics literature some three
>hundred years ago.
>

Not as such.  You would have to take a capillary tube full of water and
draw it out to a vertical height greater than 10 m.  The capillary would
have to be strong enough not to collapse inward under the tension generated
by the column of water.  It would also have to  be permeable to water to
allow evaporation at the top and entry of water at the bottom.  This can't
be done with synthetic materials.

What can be done is to measure the strength of water.  Plant physiology
texts describe experiments to do this.  It turns out that columns of water
in capillary tubes are strong enough to be pulled up tall trees. 
Calculations based on surface tension also show that water columns should
be very strong.


>I have read about osmosis, capillary action and root pressure, but find them
>lacking in scientific validity.
>

These phenomena are not enough to explain the ascent of sap.  The theories
behind them are valid enough, it is just that they are not relevant to
explaining how sap gets up tall trees.  Some text books are pretty
misleading on these topics.  The one you quote ( GCSE BIOLOGY, D.G.
Mackean. ISBN 0-7195-4281-2 first published in 1986.)  seems pretty
accurate.  


>Has anyone heard of alternative theories and if so could you provide us with
>the location to start this thread.

No.


Cheers






More information about the Ag-forst mailing list