kschmid at butler.edu
Thu Dec 6 18:45:19 EST 2001
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
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
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
4600 Sunset Ave.
Indianapolis IN 46208
kschmid at butler.edu
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
Thanks. Dave Robinson
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