Request for lab ideas

Ross Koning Koning at ECSUC.CTSTATEU.EDU
Mon Dec 11 14:08:34 EST 1995

At  6:28 AM 12/11/95 -0800, Mike Weber wrote:
>lies the problem, what does an interesting plant lab contain or conversely
>why are students so disinterested in photosynthesis and morphology?  The
>reasoning is to try and counter the "I really hate plants" sentiments of
>many students.  I think i remember a thread about this a while back??
>Does anyone have
>experience with an exercise students like?  Should the approach be to try
>and cater to their like of medical/economic facts regarding plants and
>build a lab around practical uses?


This is a common theme we hear in intro biology courses
in the US.  I think the problem is that elementary and
high-school biology courses have "turned off" our students
before they arrive.  I think and hope that this is changing
but the evidence is anecdotal.

The idea that plants are dead (or at least not alive) comes
from too many exercises that fail to show anything dynamic
about plants.  Morphology, anatomy, taxonomy, and facts-level
science abound in plant education and these topics are among
the least "dynamic" in the eyes of students. [Please don't
flame the messenger!!  This is student opinion, not mine!]

I would suggest the floating-disc photosynthesis exercise.
This exercise is found in many lab manuals...

Get your green-and-white leaved Dieffenbachia plant propagated
and growing some nice leaves now so you have one plant per
group.  You need two 20 mL syringes, a pliers-type paper punch
(from the stationery store), a light bulb and outlet
adapter to mount a light bulb at each station, a piece of
a bicycle inner-tube (or vial septum or etc.), and a black
35mm film can.  You also need a supply of water and baking
soda (sodium bicarbonate).

The basic idea is to punch out leaf discs with the hand punch.
These are put in the syringe body.  The syringe body is filled
with liquid.  The sheet rubber is used to seal the tip of the
syringe while the plunger is pulled back to make a partial
vacuum.  Students need to know about PV=nRT and leaf anatomy.
Maybe a prepared slide of a leaf is thrown into the mix.  Anyway
the floating discs are sunk by repeated application and release
of the partial vacuum (swirling during this process helps avoid
reintake of the air bubbles).  The volume in two syringes is then
made the same, they are stood on their plungers at the same
distance from a light source.  The light causes photosynthesis
in the discs to make oxygen and the discs refloat in a few

Be sure the lab starts with green discs in a bicarbonate solution.
Then let the fun begin.  Students can design their own variables
from there (more materials may be needed) to answer questions they
might have.  What if you make discs from white areas (role of
chlorophyll)?  What if you don't include bicarbonate (role of
CO2 in rate of light reactions)?  What if you have no light (role
of light in photolysis)?  How do you know it might be
oxygen that floats the discs (after sunk discs refloat, put back
in darkness...put on the black film can)?  It is critical to have
the control situation in one syringe and the experimental in the
other, and that these differ in only the one factor being manipulated
in the experimental syringe...the usual stuff.  The question of
heat responses and eliminating important but extraneous variables
is helpful with some of the better students.

If you have a three-hour lab, this project can be going on while
you are exposing bean plant primary leaves to negatives of the
students (see in Carol Reiss's published manual).  At the end
of class you can do the clearing of the leaves and iodine
development of the student's own images on the leaves.  They
really get excited about seeing their own image appear in a

So much "happens" during this lab that I think students get the
idea that plants are very much alive and responsive.  The fact
that they can predict outcomes and have results quickly shows
the dynamic nature of plants.  If you have a Mimosa pudica
(sensitive plant) around, this too, generates a lot of interest
as do insectivorous plants.

Good Luck!


( )______________________________________________)
 \ Ross Koning                                  \
  \ Biology Department                           \
   \ Eastern CT State University                  \
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