In article ,
says...

In article ,
Broadback writes:
|
| | Just curious. As I understand it the evaporation of moisture is what
| | draws the sap from the toots up the tree. Does this mean that the sap
| | does not rise until the leaves unfurl? If so where do the leaves
| obtain
| | the energy to be "kick started"?
|
| 'Tain't so. Sounds plausible, until you think about it. If that
| were so, how would the sap reach the top of a 50' tree? Even a
| vacuum will raise water only 30'.
|
| That explanation was proposed and debunked by early in the 19th
| century - but, like all myths, cannot be killed by mere disproof.
|
| Very interesting PK, does not answer my question though. OK Nick if that
| theory is wrong what is the actual answer as to how sap gets up the tree?

Well, it appears that I was wrong! Mostly.

It seems that the plant physiologists do regard evaporation of moisture
as being the driving force, but the books I looked at gloss over the
physics so badly as to make the explanation physical nonsense. And they
were undergraduate plant physiology references, too! My daughter
confirms that corresponds with what they were taught.

Reading between the lines, it seems that the mechanism is driven as
you might expect, but with surface tension of the hydrophilic cell walls
being a critical aspect. However, the way that solar energy is translated
into potential energy is still baffling the plant physiologists - after
over a century of studying the problem!

And neither book had even a hint of an answer to your question.


Regards,
Nick Maclaren.

Trees do all sorts of clever stuff, I remember reading something about
Oak trees pumping water with their deep tap roots and pushing this out
into the soil near the surface where their feeder root system is in
prolonged dry weather. So they obviously have quite a degree of control
in the process are are not just at the mercy of the elements. I will try
and find the artical as it may shed some light on the way the system
works (which I always thought was mainly capillary, but then they must
be able to turn that off in winter?)
--
Charlie Pridham, Gardening in Cornwall
www.roselandhouse.co.uk
Holders of national collections of Clematis viticella cultivars and
Lapageria rosea