[Fwd: RE: [Fwd: SPAC]]

Jon Monroe monroejd at jmu.edu
Fri Dec 7 10:47:58 EST 2001



-------- Original Message --------
     Subject: RE: [Fwd: SPAC]
        Date: 7 Dec 2001 14:20:23 -0000
        From: drobinson at bellarmine.edu ("Robinson, Dr. David")
Organization: BIOSCI/MRC Human Genome Mapping Project Resource Centre
          To: bionet-plants-education at net.bio.net
  Newsgroups: bionet.plants.education



Katherine:  That Atmometer sounds neat. How thick is the glass tubing I
wonder? Soil potentiometers have porous ceramic bulbs on them, so maybe
they could be rigged up.

Wish I could find a place to purchase an Atmometer though. Anyone out
there know who makes/sells these?

Thanks. Dave Robinson, Bellarmine U.

-----Original Message-----
From: kschmid at butler.edu [mailto:kschmid at butler.edu]
Sent: Thursday, December 06, 2001 6:45 PM
To: plant-ed at net.bio.net
Subject: RE: [Fwd: SPAC]

I have never tried with a sponge, but we regularly do the analogous demo

with a porous ceramic bulb called an "atmometer."  The atmometer looks
rather like a hollow button mushroom.  You fit a rubber stopper with a
long
piece of glass tubing into the stalk of the mushroom.  The whole
apparatus
is filled with distilled water, and the end of the tubing is plunged
into a
beaker of a denser liquid while clamps are adjusted to hold the
apparatus
bulb end up.  We turn on the "sun" (a flood lamp on a lab stool) so that
it
shines on the atmometer bulb, and take periodic measurements of how far
the
heavy liquid has been pulled up the tube to replace the water
evaporated.  I
am told that in the old days the second liquid was mercury, but I use a
heavy sugar solution colored with kool-aid for an easily observed
interface.
Diffusion of water down into the sugar isn't fast enough to interfere;
We
usually get the liquid to be pulled almost all the way up the tube
(several
feet) in just over an hour, MUCH faster than movement in a "root
pressure"
demo in which water flows into a hollow carrot containing colored KCl
and
then moves up an equivalent glass tube.

It helps to boil the atmometer ahead of time in distilled water to
minimize
air bubbles, and to handle only the "stem" after this to avoid getting
greasy fingerprints on the porous part of the bulb.  Needless to say,
it's
also important to dismantle things before the sticky colored sugar makes
it
into the ceramic.

Does anyone know if these atmometer bulbs are still marketed?  We've
broken
a few recently, and I'd hate to lose this demo.  It would be even better
to
have enough for students to have races, doing whatever they can with
lamp
heat, wind, etc. to speed the flow before a discussion of how guard
cells
would respond to such stimuli.  I have also heard that there are/were
fancier versions of porous ceramic made in the shape of leaves.

_____________________________
Dr. Katherine M. Schmid
Dept. of Biological Sciences
Butler University
4600 Sunset Ave.
Indianapolis IN  46208
317-940-9956
kschmid at butler.edu

-----Original Message-----
From: drobinson at bellarmine.edu [mailto:drobinson at bellarmine.edu]
Sent: Wednesday, December 05, 2001 10:43 PM
To: plant-ed at net.bio.net
Subject: [Fwd: SPAC]

Does anyone have an easy demonstration to show that the Soil-Plant-Air
Continuum (the Transpiration-Cohesion Hypothesis) is a legitimate
concept for explaining how water moves up tall plants?

If you take a long glass tube full of water and sit it in a beaker of
water with a wet sponge attached at the top will the evaporation of the
sponge be replenished by water in the glass tube being pulled up, like
every botany textbook implies?  My first thought is "no".... that the
column of water would be too thick, and the sponge would evaporate too
quickly to simulate what might be going on in a plant (in other words, I

think the water column would break)....if I am correct in this
conclusion, then how CAN you demonstrate empirically that this happens?

The popular Plant Physiology lab books really don't have ways that
effectively show that the SPAC is "real". I've already seen the labs
where you cut plant stems underwater (in blue dye) and the dye can be
observed to being taken up....but that only shows that the xylem is
under negative pressure...it doesn't show that the negative pressure is
enough to pull the water all the way to the top of a tall plant.

Just curious if anyone had developed any really cool demonstrations of
this.

Thanks. Dave Robinson


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