Re: Capilary tubes.

[Larry Woolf <larry.woolf@GAT.COM>]

> To change 0.29 to 29 meters (the top of a tree) one must
> assume that r=0.5 microns. What evidence do we have for
> the existence long tubes of such radii? My guess is that the
> "tubes" are not so thin and that they work in tandem, one
> after another. Is this correct?
> Ludwik Kowalski

This point is discussed in "Life's Matrix: A Biography of Water" by Philip
Ball ( an editor at Nature) on p. 239-240.  I'll provide some quotes from
these pages.

"The second engineering challenge for a plant is to pump water up from its
roots to its leaves.  In a mature redwood tree, the water must be lifted
over 300 feet... If the tube is very narrow, the rise is accentuated by
capillary forces, but even this is not enough.
Trees solve the pumping problem by making active use of water loss from the
leaves, turning the potential problem of dehydration to their advantage.
Expulsion of water through the leaf's stomata creates an imbalance between
the water content at the bottom and the top of the tree, which draws water
up the xylem in much the same way as the imbalance in sugar concentration
inside and outside a cell draws the water inside.  As water rises up the
xylem, it experiences something that at first sounds bizarre: its pressure
falls to less than zero-less than that of a vacuum!  Yet there is nothing
particularly outlandish about the concept of a negative pressure: it is
simply a tension, an outward pull.  The water resists this pull like a
piece of stretched piece of elastic.  For this reason, water under tension
is often called stretched water.
The stretched water in the xylem of tall plants is notable for being ...
superheated.  The water is tugged up the tubes only so long as a continuous
column of liquid is maintained from top to bottom...
So plants have evolved sophisticated defenses against the formation of
bubbles.  Keeping all the tubes in the xylem narrow (no more than 0.02
inches across) rather than having wide, arterylike channels, is one such
precaution, since confinement reduces the chances of bubble nucleation in
the way ice nucleation is suppressed in tiny cloud droplets."

The end of the book has  2 fascinating chapters on the inner workings of  a
science journal and the editorial difficulties of dealing with issues such
as polywater and cold fusion.


Dr. Lawrence D. Woolf; Phone: (858)-455-4475; www.sci-ed-ga.org
General Atomics; 3550 General Atomics Court; San Diego CA 92121-1194