SPAC demonstration

Virginia Berg Virginia.Berg at uni.edu
Mon Dec 10 15:19:49 EST 2001


I have somewhere a copy of instructions for a transpiration = tension
demonstration that was absolutely convincing. It involved a .3m branch
of a conifer (hemlock, I think), a short length of tygon tubing, then a
1 m long 1 mm inner diameter glass tube (this is a standard type). The
trick was to get all the air and grease out of everything, so it
involved boiling the water and the bottom of the debarked stem to degas
them, and boiling the tygon, as well as freshly cleaning the glass tube
with chromic acid. Assembling it with the hot water wasn't fun, but we
didn't want gas to redissolve in the water. The bottom end of the glass
tube was in a beaker of water, but then after things were working well,
mercury was poured in. Only when the mercury is above 760 mm have you
shown that the water is being pulled up, not pushed up by atmospheric
pressure as in a barometer.

Practically, you need to set up a bunch of these, because inevitably
some of the water columns will break as the tension builds up from all
that weight of mercury. You need a very clean mercury
surface...everything needs to be absolutely clean. Building vibrations,
people walking by, and bumping the system can all cause the column to
break, often right where the stem starts, where you may not be able to
see it.

It was really a pain to set up. But it worked, and is absolutely
convinving. I want to do this once more some day, and videotape it.
Retirement in a few years seems like a very active proposition.

--Gini Berg


--
Dr. Virginia Berg
Professor, Plant Physiology
Biology Department
University of Northern Iowa
Cedar Falls, IA USA 50614-0421

bergv at uni.edu
http://www.uni.edu/berg
office: 319 273-2770
fax: 319 273-2893


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