in search of a plant development lab

Janice M. Glime jmglime at mtu.edu
Thu Nov 2 12:08:36 EST 2000


Jessica,
  Tropisms are always fun.  My favorite is to use a film can (ala Fred
Williams) with a bit of water in the bottom, a strip of filter paper the
height of the can and about .8 cm wide.  Grow fast plants to the cotyledon
stage with stems about 2-3 cm high - need to start about 3 days in
advance, depending on your growing conditions.  It is best to stagger over
a period of 2-3 days so you have enough the right height.  The seedlings
are then clipped at the base so they are no longer than the width of the
film can (actually a little less).  The filter paper is placed along the
side of the can with one end in the water to make a wick.  The two
cotyledons are placed against the paper so that the stem extends
horizontally across the can.  With that setup you can try various things -
a hole in the side, opaquely covered tops, transparently covered tops,
different colors of light.  The fast plants begin to respond within an
hour, but a three-hour lab would be plenty of time to see responses.  I
use this in lecture and have the students set up the experiment first (we
use only covered cans).  Then the students develop hypotheses that we list
and I get a vote for each hypothesis.  Then I show a film on
tropisms that does not get into the modern theories and mechanisms very
deeply, but covers basic principles fairly well.  Then I ask if any want
to revise their vote.
  At the end of class I ask them to look at their seedlings.  At that
time, the results are rather mixed.  I ask them to check again during the
next few hours and at the end of the day.  The next day of class I ask for
a show of hands of the results.
  The class always initially votes that the stem base will bend down.  A
few vote that it will bend up, but usually for the wrong reasons.
  This year when I asked for a show of results, all but a few had bases
that bent up.  The others reported that theirs fell off the paper.
  Since the lower side grows the most, and the cotyledons can't move, the
base curves up.  It is initially counter-intuitive and a good thinking
exercise.  In a three-hour lab, you could assess the results after 2-2.5
hours, then have students get in small groups to try to figure out why
they got the results they did.
  I have tried other kinds of plants that are less expensive, but they
mostly didn't hold well to the filter paper and their responses are
slower, generally taking nearly 24 hours to respond.
Janice
***********************************
 Janice M. Glime, Professor  
 Department of Biological Sciences
 Michigan Technological University
 Houghton, MI 49931-1295
 jmglime at mtu.edu
 906-487-2546
 FAX 906-487-3167 
***********************************



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