[Fwd: SPAC]

drobinson at bellarmine.edu drobinson at bellarmine.edu
Wed Dec 5 22:43:12 EST 2001





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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