These instructions provide a guide, but you will
need to do much of the experimental planning on your own. You may even
want to perform the experiment more than once, to verify your results,
or to narrow the toxic dose range that you determine. Read all the instructions
first, and write your own detailed experimental plan.
- Choose a chemical
Use your imagination! Remember the famous quote from
Paracelsus. Think about the kitchen cupboard, the cleaning products, the
medicine cabinet, a local stream, and your classroom chemical shelf. The
only limitations—it must be safe for you to handle, and be water soluble.
- Plan and make dilutions.
This part is very important. (You may want to check
your plan with your teacher before beginning.) First, choose the most
concentrated dose you wish to test. This may be the full strength cleaner,
or the melted snow from a newly salted roadway. Make at least 4 "serial"
dilutions of this starting solution. (The more dilutions you use, the
more precise your results will be.) Use water to make your dilutions,
and include water (only) as a "toxin-free" control. This will give you
a total of at least 6 treatments. Examples follow.
2-fold serial dilutions
Start with the most concentrated solution. Decide
what final volume you will need for each treatment solution, and begin
with twice that. (In this example, you need 10 mL for each treatment,
so start with 20 mL of your most concentrated solution.) Take half this
amount (10 mL) and add it to the same volume of water (10 mL). This is
your first dilution, and its concentration is half that of the starting
solution. Repeat by removing half the volume of this solution, and again,
adding it to the same volume of water. Repeat until all the dilutions
are made. You will have to discard half of the volume of your last dilution
to make the final volumes all equal. (Try 5-fold or 10-fold serial dilutions
to cover a broader range of doses.)
- Set up germination chambers.
There are several options for germination chambers.
Empty plastic film canisters work great. About 10 mL final volume will
be necessary for each chamber. Other containers would work equally as
well, just be sure that they are all the same, and adjust the final volume
of solution and wick size as appropriate. Wash all containers prior to
use.
- Make wicks.
Use water mat (a felt-like material designed for use
with plants) or pellon or felt (both available at fabric stores). Pellon
and felt must be washed and double rinsed before being used. Washing removes
chemicals used in the manufacturing process which are toxic to the germinating
seed. If using film canisters as containers, wicks should be approximately
1.5 cm wide and 6 cm long.
- Put it all together.
Be sure all containers are well labeled. If you use
a wick of the size mentioned above, you can place 2 seeds on each wick.
(If you use smaller wicks and smaller containers, set up 2 per treatment.)
Place treatment solutions in the containers, and place the wicks in the
solutions. Allow the wick to absorb the solution. Once moist, seeds can
be placed on the wicks. Seal it all in an airtight container (like one
from cottage cheese), and leave it undisturbed.
- Observe.
Germination will occur over the next 3-4 days. Some
suggestions for data collection include: Germination achievement (measured
in "yes" or "no"). Time to germinate (measured in hours or days). Length
of radicle and/or hypocotyl each day (measured in millimeters). Emergence
of cotyledons and/or root hairs (measured in hours or days). Make drawings
or take pictures of the developing plants for presentation.
- Make conclusions.
What numerical conclusions can you make from your
data? (For example, "the toxic dose to inhibit germination for chemical
X is between 5 and 10 mg/mL.") Think also about other factors, besides
chemical influence, that might affect germination. Could these have had
an effect on your results? What problems did you encounter with the experiment
or its design? How would you fix these?
- Keep asking questions.
Graph your results to create a dose vs. response curve,
using the numerical data you obtained to describe germination. How might
these results relate to the toxicity of your chemical on other living
things? Design an experiment to test your chemical on other phases of
the Brassica rapa life cycle. What features of the life cycle could
you measure? How could you apply the chemical to the growing plant? Can
you isolate effects on specific stages of plant development? What related
chemicals might you test?
This laboratory exercise was adapted from materials written
by Wisconsin Fast Plants Program, College of Agriculture and Life Science,
University of Wisconsin-Madison. www.fastplants.org
Teacher's Notes for Find the Toxic
Dose
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