dh321 at excite.com
dh321 at excite.com
Wed Dec 12 19:38:56 EST 2001
Some biology textbooks often seem to exaggerate the role of capillarity in
plant water movement by not explaining exactly where it occurs in the water
pathway through the plant. College Plant Physiology texts by Salisbury &
Ross and Taiz & Zeiger discuss how capillarity does not operate in
water-filled xylem. Taiz and Zeiger point out that capillarity can play a
role in leaf cell walls directly exposed to air. Capillary movement can
replace water that is lost to the air and keep the exposed cell wall surface
wet. Capillary movement of water in soils is also considered to help
replenish water at the surface of roots and root hairs. Therefore,
capillarity operates at both ends of the pathway but not in the water-filled
xylem itself. In the model under discussion here, capillarity would only
occur on the surface of the porous clay cup exposed to air.
Although there are limitations to this demonstration, if you can get a
column of mercury to rise 80 cm, that is equivalent to a water column 10.8
meters high, which is quite impressive for a classroom demonstration.
Soil Moisture Corporation has some porous ceramics rated at 15 bars for air
entry. If an artificial leaf and stem was crafted from the 15 bar rated
porous ceramic, it would theoretically support a water column higher than
the tallest tree. However, that porous ceramic would undoubtedly have a much
greater resistance to water flow than tracheids or vessels so it is
questionable whether a 100 m tall cylinder of the 15 bar rated ceramic would
make a satisfactory demonstration. Then again, maybe it would. A 100 m tall
cylinder would be impossible to use in a classroom situation however.
It seems that it would be technically very difficult and very expensive to
make a 100 meter tall column of tracheids or vessels from ceramic or some
other material with similar adhesive properties as xylem cell walls.
However, it might be possible. It also might be possible to cut an
approximately 100 meter tall core of xylem from a tree that could be
such a demonstration. An experiment like this might be possible but it would
be extremely expensive. The 100 meter column would have to be supported by
some kind of framework and raised into a vertical position with a crane.
Then the column would have to be flushed with just-boiled water to get rid
of all the air. Are there any wealthy botany enthusiasts willing to fund
such a project?
David R. Hershey
Grant Cramer <cramer at med.unr.edu> wrote in message
news:9v6akp$i80$1 at mercury.hgmp.mrc.ac.uk...> A word of caution: All of these
demonstrations are useful but they don't
> necessarily demonstrate how water moves up a TALL 100 m redwood.
> only works so far and I suspect these models have their limitations, too.
> 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
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