Re: Listen to the trees

On Wed, 12 Oct 2011 16:58:16 +0000 (UTC), Paul J Gans
<gansno@xxxxxxxxx> wrote:

r norman <r_s_norman@xxxxxxxxxxx> wrote:
On Tue, 11 Oct 2011 23:03:41 +0000 (UTC), Paul J Gans
<gansno@xxxxxxxxx> wrote:

r norman <r_s_norman@xxxxxxxxxxx> wrote:
On Tue, 11 Oct 2011 15:31:56 +0000 (UTC), Paul J Gans
<gansno@xxxxxxxxx> wrote:

r norman <r_s_norman@xxxxxxxxxxx> wrote:
On Tue, 11 Oct 2011 08:41:05 -0400, Walter Bushell <proto@xxxxxxxxx>
wrote:

In article
<6ae25bd4-d830-420b-9043-cf3c6f83de3d@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Tim Anderson <timothya1956@xxxxxxxxx> wrote:

On Oct 11, 7:26 am, "Steven L." <sdlit...@xxxxxxxxxxxxx> wrote:
"iaoua iaoua" <iaoua.ia...@xxxxxxxxx> wrote in message

news:iaoua-7c613ab7-dd99-45ce-9f80-a67767fb9782@xxxxxxxxxxxxxxxxxxxxxxxxxxx:

Ok. So I'm doing Day 3 a little early this week but I couldn't wait.

Let's assume that the world is billions of years old and that the
trees have been around for quite some time. Surely, the trees which
would win the competition of natural selection would be the tallest as
they would be the ones that win the competetive fight for sunlight.
So...

Question. If trees have been around for millions of years now then
wouldn't you expect them to be a darn sight taller?!?

Trees have this tendency to catch fire, burn, and die.

New living trees don't have to be taller than the dead trees.

And there are physical limits:  A tree that was a mile high might have
trouble transpiring moisture high enough.

Your question is like asking if predator animals like cheetahs have the
advantage of speed, then why after millions of years haven't cheetahs
evolved to move at near-supersonic speed.

"Remember:  Only YOU can prevent forest fires."
   -- Smokey The Bear

-- Steven L.

And then there is the small matter of the work required to get water
to the leaf surface from the roots. From memory, this is achieved by a
combination of capillary pressure created when water evaporates from
the leaf surface (sucking water up the stem) and active respiration by
the root cells (forcing water up the stem). Both of these processes
have physical limits that set an upper bound on tree size.

Unfortunately, there is no upper bound on ignorance. Except when
skydiving.

Last time I checked we don't understand the dynamics of water transport
up the trunk. But the tree has to do work on the water to get it up the
trunk which puts limits on how high a tree can be.

Apparently you did not see my post that says that the trunk does
absolutely no work at all on the water to lift it. The trunk is
filled with dead cells, metabolically inert, through which the water
flows. There are always details to be worked out but the
transpiration pull theory of xylem transport is well established.

Nobody ever talks about the internal pressure of fluids in
this connection. That's dU/dV at constant temperature and
amounts to a cohesive force inside a fluid that is not the same
as the surface tension but is related to it.

Actually biologists constantly talk about the internal pressure of
fluids in this connection. It is well known that the xylem fluid is
under negative absolute pressure which means tension and that the
tension is supported by the hydrostatic bonding between water
molecules, the cohesive nature of water that is of course also related
to surface tension.

The real question in biology is how the xylem bundles prevent
cavitation and there are a number of factors that act in this regard.

Sadly, thermodynamics provides no insight into how to prevent
cavitation...

I think that one result of this will be a new 15 minute segment
in my physical chemistry course on it all.

Try teaching water potential to intro biology students! That way you
can integrate osmotic pressure, hydrostatic pressure, and capillarity
into one integrated concept about how and why water moves from place
to place. Of course you have to include water potential in a gas
mixture in contact with a liquid where the partial pressure of water
vapor is less than the vapor pressure of the liquid phase. Oh, yes,
the liquid might also be salty.

It actually is quite important because active transport of ions into
cells in a special layer in the interior portion of a root acts to
pull water out of the capillary spaces in even relatively dry soil.

That's more easily done in a "light" course than a "heavy" one
since the underlying mechanisms are very different. However
the chemistry students will have already been introduced to
the chemical potential as a concept -- after which osmotic
pressure becomes easy.

Active transport is another problem entirely. We generally don't
touch non-equilibrium material much in physical chemistry any more.

What has happened is that material accumulated in pchem for years
without any of it moving downward into freshman chemistry. Especially
because everyone wants to teach quantum mechanics to freshmen these
days. As a result transport properties have basically vanished from
the curriculum.

Now that the biochemical viewpoint dominates chemistry one might
expect that the core material has shifted. And in a few schools
that may be true. But in most, nothing has happened.

Except for a greater emphasis on quantum mechanics, you could use
a 50 year old text in general chemistry or physical chemistry
in most schools today. Sad, isn't it.

Freshman courses are actually a lot easier because you can just skip
over the hard parts and essentially tell stories to make up a good
story to explain what is happening. If the students really need to
know the truth about how things work, they will get the details in
proper form in later specialty courses. But mostly they really just
need a good overall picture to get an idea about the subject matter.

In my undergrad physiology courses, I used the opportunity to
introduce hard-core quantitative notions into biology and did an awful
lot of biophysics and physical chemistry to explain cellular
phenomena. I used systems theory to explain organism level
integrative activity. Back when I taught graduate level physiology,
non-equilibrium thermodynamics was a big part of understanding the
basis.

.

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