[Fwd: SPAC]

Grant Cramer cramer at med.unr.edu
Thu Dec 6 17:40:11 EST 2001


Dave,
I tried your model, except I used a small rolled up piece of paper towel
surrounded by parafilm on the end with only a small exposure to the air. It
still didn't work. Eventually the water column cavitated at the top
(water/wick interface).

I think the problem may lie in the fact that the glass walls are smooth
(they don't have the intricate network of cellulose microbfibrils making
micropores to help support the water column) and the lumen is too long
(xylem cells are short and when they cavitate the water column doesn't go
very far and are then subject to refilling at night). In addition, water can
be supplied in xylem vessels from surrounding cells and other vessels and
tracheids, whereas a glass tube has only one source from the end (beaker).
Also remember, that the interior chambers of the leaf are at nearly 100%
relative humidity, unlike the atmosphere.

Here's another idea (I am not going to do this one). How about killing a
stem or branch by heating or freeze-thawing (with or without a leaf
attached) and see if it still transpires water.

I will write to a few knowledgeable people on the subject to see what they
have to say.
-- 
Grant R. Cramer
Associate Professor
Department of Biochemistry
Mail Stop 200
University of Nevada
Reno, NV 89557

Phone: (775) 784-4204
Fax: (775) 784-1650
Email: cramer at unr.edu
Web page: http://gcramer-mac.ag.unr.edu/index.html


on 12/5/01 7:43 PM, drobinson at bellarmine.edu at drobinson at bellarmine.edu
wrote:

> 
> 
> 
> 
> 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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