How does water really reach the leaves of trees?

Bob Vickery vickery at mpx.com.au
Sat Mar 4 01:12:44 EST 2000


In article <89hhp6$ht0$1 at news7.svr.pol.co.uk>,
"Andrew Kenneth Fletcher" <gravitystudy at hotmail.com> wrote:

>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.

Concentration of sap in the leaves is an interesting effect.  It is a very
severe problem for mangroves living in sea water.  Some of them actively
excrete the excess salt.  However,  I don't think it could contribute
significantly to the movement of sap.

1) If the salt gets too concentrated, the leaf tissues will die.  This can
be observed after a period of high wind.  For the sake of argument, the
maximum concentration of sap could be about that of sea water which has a
relative density of 1.025.  So a column of seawater 10 m high could balance
a column of sap about 10.25 m high.  This is not enough to explain water
movement in trees up to 30 m high.

2) Tracking with dyes has never shown downward movement of water in xylem. 

3)  In the smallest leaf veins there is only one xylem vessel.  Movement
would have to be up and down in one vessel.

Cheers







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